Dissertations / Theses on the topic 'Perovskite to post-perovskite phase transition'

The reported multiferroic perovskite series Sr1-xAxTi1/2Mn1/2O3 has been the subject of numerous structural studies, without reaching consensus. In the current work, the cubic Pm3 ̅m is confirmed for end member SrTi1/2Mn1/2O3 in the Sr1-xAxTi1/2Mn1/2O3 ( A= Ca, La; 0 ≤ x ≤ 1) series. The Pm3 ̅m  I4/mcm  Pbnm structural evolution was observed with increased doping level of Ca. A cubic Pm3 ̅m  rhombohedral R3 ̅c transition occurred when La is substituted instead of Ca. Interesting magnetic behaviours were observed and the major contribution to this was concluded to be the mixed Mn4+/Mn3+ ratio. Ru and Ir have almost identical ionic radii and behave similarly in many ways. Remarkably the structure and properties of SrRuO3 and SrIrO3 are different. The current study revealed that the divalent transition metal doped materials of the type SrR1-xMxO3 (R = Ru, Ir, and M = 3d transition metals) are isostructural. This was achieved by the synthesis of a number of new materials of the type SrIr1-xMxO3. Therefore, these two series are comparatively described in the thesis. The structure and physical properties of the iron doped series SrIr1-xFexO3 are found to be different from those of the divalent doped ones, and this was even true for Ru analogues. Therefore, Fe-doped SrRuO3 and SrIrO3, based on the results of the same level doped materials are presented in a separate chapter. In the final chapter, the impact of Cu2+ doping on the structure and electronic properties of LaCrO3 is described. In order to understand structure property relationships, all the materials structurally characterised have had magnetic and resistivity measurements conducted. Special attention is given to realise the correlations between structure, magnetism, and conductivity.

2010/2011
In questo lavoro è stata messa a punto una metodologia basata sull’analisi topologica della densità elettronica secondo la teoria di Bader che ha permesso di indagare la stabilità di fasi mineralogiche in condizioni di alta pressione. In una prima fase è stata caratterizzata la decomposizione della ringwoodite (olivina-γ) in Mg-perovskite e periclasio ( post spinel phase transition) che si ritiene essere responsabile della discontinuità sismica che si osserva a 660 Km di profondità, tra la zona di transizione del mantello ed il mantello inferiore. Lo scopo del lavoro è stato quello di ottenere informazioni sulla disposizione degli elettroni nella struttura cristallina e sulla evoluzione al variare delle condizioni di pressione. L’analisi effettuata ha mostrato l’instaurarsi di una forte instabiltà strutturale (caratterizzata da una “conflict catastrophe”) nella ringwoodite a circa 30 GPa. Tale risultato conferma il coinvolgimento della transizione di fase “post-spinel”nella discontinuità sismica a 660 Km. In una seconda fase la procedura è stata applicata alla fase Mg-perovskite allo scopo di testarne la validità. Lo studio dell’evoluzione della topologia della densità elettronica nel range di pressione da 0 a 200 GPa ha permesso di individuare una regione di stabilità della fase perovskitica (da circa 22 a circa 124 GPa) delimitata tra due “fold catastrophes”. Le due “fold catastrophes” si hanno entrambe in prossimità di discontinuità sismiche: la prima, attribuita alla transizione di fase da ringwoodite a Mg-perovskite + periclasio corrisponde alla discontinuità sismica a 660 Km e la seconda, attribuita alla transizione da Mg-perovskite a post-perovskite a circa 130 GPa, osservata a circa 2600 Km di profondità, tra il mantello profondo e il D′′-layer, poco prima della discontinuità di Gutemberg a 2900 Km.
XXIV Ciclo
1975

Bismuth ferrite (BiFeO₃) is a magneto-electric material which exhibits simultaneously ferroelectric and antiferromagnetic properties. We have used high-field electron spin resonance (ESR) as a local probe of the magnetic order in the magnetic range of 0-25 Tesla. With increasing magnetic field, an induced transition has been found between incommensurately modulated cycloidal antiferromagnetic and homogeneous magnetized spin state. The data reveal a number of interesting changes with increasing field, including: (i) significant changes in the ESR spectra; (ii) hysteresis in the spectra near the critical field. We have analyzed the changes in the ESR spectra by taking into account the magnetic anisotropy of the crystal and the homogeneous anti-symmetric Dzyaloshinsky-Moria exchange. We have also investigated phase induced transition by epitaxial constraint, and substituent and cystalline solution effects. Variously oriented BiFeO₃ epitaxial thin films have been deposited by pulsed laser deposition. Dramatically enhanced polarization has been found for (001)c, (110)c, and (111)c films, relative to that of BiFeO₃ crystals. The easy axis of spontaneous polarization lies close to (111)c for the variously oriented films. BiFeO₃ films grown on (111)c have a rhombohedral structure, identical to that of single crystals. Whereas, films grown on (110)c or (001)c are explained in terms of an epitaxially-induced transition between cycloidal and homogeneous spin states, via magneto-electric interactions. Finally, lanthanum modified BiFeO₃-xPbTiO₃ crystalline solutions have been found to have a large linear magneto-electric coefficient, ∝p. The value of ∝p (2.5x10⁻⁹ s/m or C/m²-Oe) is ∼10x greater than that of any other material (cg., Cr₂O₃ ∼2.5x10⁻¹⁰ s/m), and many order(s) of magnitude higher than unmodified BiFeO₃ crystals. The data also reveal: (i) that ∝p is due to a linear coupling between polarization and magnetization; and (ii) that ∝p is independent of dc magnetic bias and ac magnetic field. We show that the ME effect is significantly enhanced due to the breaking of the transitional invariance of a long-period spiral spin structure, via randomly distributed charged imperfections.
Master of Science

Au cours des dernières années, les perovskites hybrides organiques-inorganiques ont été employées en tant qu’absorbeurs de lumière, en raison de leurres excellentes propriétés optiques et électroniques. L’efficacité de conversion des photons des cellules solaires hybrides à base de perovskites a augmenté de 6,9% à 23,6% au cours des dernières années. Le but de cette thèse est d’étudier les propriétés optiques et électroniques des perovskites à l’aide de spectroscopie magnéto-optique. Nous avons étudié la relation entre les propriétés excitoniques et la microstructure des perovskites hybrides. Nous avons effectué des mesures de magnéto-transmission sur des couches minces polycristallines de MAPbI3 et des mesures de magnéto-réflectivité sur un monocristal de MAPbI3. Nous avons monté que, à basse température, l’énergie de liaison de l’exciton et sa masse réduite sont les mêmes pour tous les échantillons de MAPbI3 indépendamment de la taille des cristaux.Ensuite, nous avons étudié les propriétés électroniques des perovskites entièrement inorganiques, à savoir les composés CsPbX3 (X = I ou Br ou un mélange de ceux-ci). En effectuant des mesures de magnéto-transmission sur CsPbX3, nous avons déterminé les énergies de liaison de l’exciton et la masse réduite avec une grande précision. Une comparaison des valeurs de constante diélectrique des perovskites inorganiques et inorganiques montre que, à basse température lorsque les mouvements des cations organiques sont interdits, la contribution dominante à l’écrantage diélectrique estliée au mouvement relatif dans la cage à base de halogénures de plomb
Optical and electronic properties. The photon conversion efficiency of hybrid perovskite based solar cells has increased from 6.9% to 23.6% within the last few years. The aim of this thesis is to investigate the optical and electronic properties of perovskite materials using magneto-optical techniques. We have investigated the relationship between the excitonic properties and the microstructure of hybrid perovskites. We have performed magneto-transmission measurement on MAPbI3 polycrystalline thin films and magneto-reflectivity measurement on a MAPbI3 single crystal. We find that, at low temperature, the exciton binding energy and reduced mass are the same for all MAPbI3 samples with a variety of crystal grain sizes.We have also investigated the electronic properties of the fully inorganic perovskites, namely CsPbX3 compounds (X = I or Br or a mixture of those). By performing the magneto-transmission measurement on CsPbX3, we have determined the exciton binding energies and reduced mass with high accuracy. A comparison of the values of dielectric constant for the fully inorganic and the hybrid organic-inorganic perovskites indicates that, at low temperature when the organic cations are frozen, the dominant contribution to the dielectric screening is related to the relative motion within the lead halide cage

Metal organic frameworks have gained much attention due to their tunable pore sizes and very high surface areas. With the discovery many of these type materials the need has raised to look into new applications of theses porous frameworks. This thesis focuses on the synthesis of a new perovskite-type metal organic framework and measurement of its thermal conductivity in search of its applicability as a thermoelectric material. The second part of this work focuses on the synthesis of a metal organic framework incorporated with a Lewis pair for the first time. The optimum loading amount of the Lewis pair into the framework was also investigated.

Etude de la constante diélectrique, spectres Mössbauer et RMN. Les transitions de phases sont étudiées par diffraction RX et mesures diélectriques. Les systèmes étudiés sont : PbTiO3-1/2Pb2Fe2O5-CaTiO3-1/2Ca2fE2O5; Li2O-Ta2O5-MgO; Li2O-Ta2O5-ZnO; Li2O-Ta2O5-MgO.

This thesis makes contributions to the methodology of quantitative description of the tilting systems of perovskite structures and theoretical analysis of high-pressure phase transitions of representative perovskites. Chapter 1 and 2 introduce the perovskite structures, tilting classification and descriptions. The structures in each of the 15 tilt systems have been decomposed in to the amplitudes of symmetry-adapted modes in order to provide a clear and unambiguous definition of the tilt angles. A general expression in terms of tilt angles for the ratio of the volumes of the two polyhedra within the perovskite structure is derived. Chapter 3 uses the first-principles plane-wave pseudopotential calculations to investigate the high-pressure to phase transition and elasticity of LaAlO3 perovskite. This second order transition is determined to occur at ~14 GPa by the pressure variation of the squared frequency of the soft R-point mode in the structure. Elastic moduli are inverted from the calculated stress-strain data by the singular value decomposition method. The Landau parameters for this phase transition are calibrated from the calculation results. Chapter 4 uses the same method to investigate the high-pressure phase transitions and elasticity of YAlO3 perovskite. The Pnma, Imma, I4/mcm, , perovskite structures and the NH4CdCl3-, Gd2S3-, U2S3-, CaIrO3-type structures are considered. A continuous Pnma to Imma transition occurs at ~89 GPa, determined from the soft Z-point mode of the Imma structure. Then, a discontinuous Imma to I4/mcm transition occurs at ~100 GPa, suggested by the relative enthalpies and phonon dispersions. The elasticities of the Pnma, Imma and I4/mcm structures show mechanical stabilities compatible with the phase transitions. The NH4CdCl3- and CaIrO3-type structures are dynamically stable although not energetically favorable. The relative A/B site polyhedral volume ratios are found to qualitatively reproduce the relative enthalpies of the perovskite structures.
Ph. D.

Ce mémoire présente une étude de la préparation de couches minces du SmFeO3 (SFO) et du LaNiO3 (LNO) par ablation laser.
L'étude s'est d'abord focalisée sur la croissance du SFO sur silice amorphe afin de déterminer les conditions de croissance. Les épaisseurs sont mesurées in situ pendant la croissance par ellipsométrie spectroscopique. La variation thermique des indices optiques ainsi que les paramètres de maille présentent deux transitions qui semblent être corrélées aux températures de Curie (Tc) et de réorientation de spin (TRS). Une croissance épitaxiale du SFO (cube sur cube) sur STO (001) a été aussi obtenue. Les mesures magnétiques à l'ambiante (SQUID) montrent que les moments magnétiques pointent dans la direction perpendiculaire au film, c'est à dire suivant c, axe de facile aimantation à haute température (T> TRS) contrairement aux prévisions (axe a).
Une optimisation des températures de substrat et de pression d'oxygène a été ensuite réalisée lors de la croissance épitaxiale (cube sur cube) du LNO sur STO (001). La variation thermique des indices optiques, linéaire, présente un changement de pente dans la gamme [200-300°C]. L'ellipsométrie permet également la détection des transitions métal-isolant associées à la réduction et ré-oxygénation des films. La cinétique de la ré-oxygénation est détaillée ainsi que le contrôle de l'état d'oxydation.
Enfin, nous avons étudié les hétérostructures SFO/LNO/STO et SFOII/LNOII/SFOI/LNOI /STO. Nous avons montré que SFO croit de manière épitaxiale (cube sur cube) sur LNO. L'influence des épaisseurs de SFO et de LNOII sur les propriétés de transport à basse température des empilements est finalement démontrée.

Changements de phase associés aux discontinuités sismiques de 400 et 700km et conséquences sur la rhéologie du manteau. Etude des transitions polymorphiques entre les trois structures alpha , beta et gamma que peut prendre l'olivine et la décomposition de spinelle en pérovskite et magnésiowüstite. Etude réalisée en microscopie électronique en transmission sur des échantillons provenant de météorites choquées ou synthétisées à très haute pression et température dans une cellule à enclume de diamant.

Ce mémoire présente une étude de la préparation de couches minces du SmFeO3 (SFO) et du LaNiO3 (LNO) par ablation laser. L'étude s'est d'abord focalisée sur la croissance du SFO sur silice amorphe afin de déterminer les conditions de croissance. Les épaisseurs sont mesurées in situ pendant la croissance par ellipsométrie spectroscopique. La variation thermique des indices optiques ainsi que les paramètres de maille présentent deux transitions qui semblent être corrélées aux températures de Curie (Tc) et de réorientation de spin (TRS). Une croissance épitaxiale du SFO (cube sur cube) sur STO (001) a été aussi obtenue. Les mesures magnétiques à l'ambiante (SQUID) montrent que les moments magnétiques pointent dans la direction perpendiculaire au film, c'est à dire suivant c, axe de facile aimantation à haute température (T> TRS) contrairement aux prévisions (axe a). Une optimisation des températures de substrat et de pression d'oxygène a été ensuite réalisée lors de la croissance épitaxiale (cube sur cube) du LNO sur STO (001). La variation thermique des indices optiques, linéaire, présente un changement de pente dans la gamme [200-300°C]. L'ellipsométrie permet également la détection des transitions métal-isolant associées à la réduction et ré-oxygénation des films. La cinétique de la ré-oxygénation est détaillée ainsi que le contrôle de l'état d'oxydation. Enfin, nous avons étudié les hétérostructures SFO/LNO/STO et SFOII/LNOII/SFOI/LNOI /STO. Nous avons montré que SFO croit de manière épitaxiale (cube sur cube) sur LNO. L'influence des épaisseurs de SFO et de LNOII sur les propriétés de transport à basse température des empilements est finalement démontrée
This work presents the growth of SmFeO3 (SFO) and LaNiO3 (LNO) thin films by pulsed laser deposition. First, we have focused on deposition conditions of SFO growth on fused quartz substrate. Thicknesses have been calculated from in situ ellipsometric measurements during the growth. Furthermore, optical indexes and lattice parameters show two transition temperatures in correlation with the Curie (Tc) and spin reorientation (TRS) temperatures. Epitaxial cube on cube growth has been obtained on SrTiO3 (STO) (001). In this case, the easy magnetization axis, as measured by SQUID magnetometry, is perpendicular to the film plane at room temperature whereas it is expected to be along the a direction. Second, substrate temperatures and oxygen pressures have been varied to determine the optimal deposition conditions for epitaxial cube on cube growth of LNO films on STO (001) substrate. The thermal variation of optical indexes is linear with a change in the slope around [200-300°]. In situ spectroscopic ellipsometry is used, as well, for the detection of the metal-insulator transition associated with the reduction and re-oxygenation of thin films. The kinetics of this last transformation are detailed and we show that it's possible to tune of the oxidation state. Finally, SFO/LNO/STO and SFOII/LNOII/SFOI/LNOI/STO heterostructures have been made. Again, the growth of SFO on LNO is epitaxial (cube on cube). Transport measurements have revealed an effect of both SFO and LNOII thicknesses at low temperature and are discussed here

Aux conditions de pression et température de la couche D'', située à 2700 km de profondeur sous la surface terrestre, la Bridgmanite (Pv), le minéral le plus abondant dans le manteau profond, se transforme en sa phase de haute pression, (Mg,Fe)SiO3 post-perovskite (pPv). Cette transformation de phase est souvent évoquée pour expliquer les différentes anomalies et discontinuités des ondes sismiques au sein de la couche D''. Toutefois, nous manquons d'information sur les détails de cette transformation. L'objectif de cette thèse fut d'améliorer notre compréhension du mécanisme de transformation Pv / pPv et d'en étudier la cinétique. Pour cela, j'ai utilisé la cristallographie multigrains, une méthode qui permet de caractériser des centaines de cristaux in situ dans un matériau polycristallin. Le manuscrit commence par une démonstration de la fiabilité de cette méthode pour des expériences aux conditions extrêmes de pression. Je décris ensuite l'étude expérimentale du mécanisme de transformation Pv/pPv avec l'analogue structural NaCoF3. Je trouve que ce mécanisme est martensitique pour le sens Pv vers pPv et reconstructif au retour. Je discute également leurs impacts sur la microstructure au sein de la couche D''. Par la suite, je présente l'étude cinétique de cette transformation dans la composition (Mg0,86,Fe0,14)SiO3 et j'extrapole nos données afin de contraindre la dynamique et la cinétique de cette transformation aux conditions P/T de la couche D'', en tenant compte de la pression, de la température, et de la taille de grains
The radial seismic structure of the earth is marked by a sharp transition about 200 km above the core-mantle boundary. This defines the top of the region called the D'' layer. Moreover, at these P/T conditions, Bridgmanite (Pv), the main lower mantle mineral, transforms into its high-pressure phase, (Mg,Fe)SiO3 post-perovskite (pPv). This phase transition has received considerable interest due to its thermodynamic properties, the induced textures and microstructures that seem to explain many of the seismic anomalies of the D'' layer. However, its thermodynamic properties and transformation mechanisms are not very well known. The main purpose of this thesis was to investigate the Pv/pPv phase transition and its kinetics. To do so, we used a novel method, called Multigrain Crystallography, to characterize in-situ hundreds of crystals in a polycrystalline material. The reliability of the method for experiments under extreme conditions is tested in the first part of this manuscript. I then focus on the Pv/pPv phase transition mechanism on a structural analog of composition NaCoF3. I determine that the Pv to pPv transformation is martensitic and that the reverse transformation is reconstructive. Their impacts on the D'' layer microstructure are also discussed. Finally, I explore the kinetics of the (Mg0,86,Fe0,14)SiO3 Pv to pPv transition by time-series experiments. Moreover, based on our data, I present two possible kinetic models that include the effect of pressure, temperature, and grain size. These models have important implications to constrain the dynamics and kinetics of the Bridgmanite to pPv transition at the D'' layer P/T conditions

Etude de petrologie experimentale sur les transformations a hautes pressions et hautes temperatures, de silicates et de germanates a structure olicine et pyroxene. Il a ete determine en fonction de la pression et de la temperature, la distribution du magnesium, du fer et du calcium entre mineraux mantelliques. Un modele thermodynamique coherent est construit et a permis de determiner le champ de stabilite de la perovskite silicatee et de proposer une interpretation de la discontinuite sismique des 670 km. Des mecanismes microscopiques de transformations de phase de l'olivine sont proposes. De quelques composes etudies par spectroscopie raman, a ete tire des parametres thermodynamiques harmoniques et anharmoniques de ces phases

Ceramics in the solid solution series, Ag(NbxTal-x)03 were fabricated to study their phase transitions and functional properties. As Ta concentration increased, the onset temperature of the M-phase transitions decreased in agreement with the Pawelcyck phase diagram. Electron diffraction and Raman spectroscopy revealed that the reduction in onset temperature may be attributed to a decrease in the correlation length and magnitude of cation displacements as the less polarisable Ta replaced the highly polarisable Nb ion in the B06 octahedra. For compositions with x = 0.75, a previously unreported phase transition was observed at ~ 3 l5K. Further studies were also performed on Li doped AgNb03, AgNbl/2 Ta 1/203 and AgTa03 compositions to determine whether their functional properties could be modified or enhanced. For > 0.05% Li doped AgNb03, the structure transformed from orthorhombic, Pbcm (√2ap,√2ap,4ap) to a new structure with a unit cell, √2ap√2ap,6ap, where ap is the pseudocubic perovskite lattice parameter. The new cell may be envisaged as arising from a complex modulated tilt system in which blocks of 3 in- phase tilted octahedra are rotated in antiphase with respect to each other around the c- axis. The onset of this transition resulted in asymmetry of the M3-M2 transition but there was no discernible improvement in ferroelectric properties at room temperature. For Li doped AgNbl/2 Ta1/2O3, the correlation length of cation displacements and to a lesser extent octahedral tilting decreased according to electron diffraction and Raman spectroscopy as Li concentration increased. The decrease in correlation length manifested itself as a highly perturbed domain structure at the solubility limit (~ 15 mol% Li) and resulted in convergence of the M-phase transitions to give a single broad dielectric response at the solubility limit. For Li doped AgTaO3, there were no apparent changes in the structure at room temperature. However, temperature dependent dielectric measurements clearly indicated that Li substitution had induced the onset of a ferroelectric phase transition in contrast to the incipient behaviour of AgTaO3. The temperature of the ferroelectric transitions increased with increase in Li concentration up to the solubility limit (~ 15 mol %.). The onset of the ferroelectric transition was also observed by Raman spectroscopy but further work is required to determine its precise nature.

Metal oxides, such as perovskites, are flexible materials in which a large range of properties can be accessed and tuned. This flexibility results in ideal materials for the study of structure and properties. Understanding structure-property relationships underpins the understanding of functional materials - and provides the fundamental motivation for this work. The work described in the first part of this thesis focuses solely on the evaluation of the ambient structures of some uranium double perovskites, and their behaviour at elevated temperatures. The results nicely complement, as well as contribute to, the current literature. The bulk of this work, however, involves the study of Tc-containing oxides. Due to the challenges associated with the synthesis and characterisation of active materials, the perovskite systems described, although simple, have never before been studied. Therefore, this thesis describes the first known work on the tertiary systems Ba(x)Sr(1-x)TcO3 and SrTc(x)Ru(1-x)O3. Both systems exhibit an intimate relationship between the structure and magnetism. Finally, the unusual transition in a Ru-containing hexagonal perovskite Ba3BiRu2O9 is studied. Total scattering neutron data is analysed using the reverse Monte Carlo (RMC) modelling technique, providing information on the local structure otherwise inaccessible using traditional diffraction analysis.

This thesis set out to investigate lead free ferroic materials with perovskite and tungsten bronze type structures, primarily focussing on the relationship between composition, temperature and crystal structure. A combination of diffraction techniques were employed to investigate the crystal structures. Additionally, other techniques including XANES, SEM, TGA, DSC and ferroic peroperty measurements were also employed to further illuminate these compounds. The first system investigated was the defect perovskite Sr0.8Ti0.6-yZryNb0.4O3, 0.0 ≤ y ≤ 0.6. It was found that neutron powder diffraction data were essential for determining the phase boundary composition. Second order compositional and temperature phase transitions were observed. Increasing the zirconium content increased octahedral tilting and led to higher transition temperatures. Local ordering was determined to be highly probable and it was found that the presence of vacancies extended the range of the high symmetry phase. The second system investigated was the BaxSr3-xTi1-yZryNb4O15, 0.0 ≤ x ≤ 3.0, 0.0 ≤ y ≤ 1.0, tungsten bronze type system. Barium rich compositions were found to adopt a tetragonal structure, while strontium rich compositions adopted an orthorhombic structure. Increasing the zirconium content of samples was seen to make the orthorhombic phase persist further. A large focus was placed on determining the structure of Sr3TiNb4O15 as a model for all orthorhombic compounds. A new structural model was proposed for Sr3TiNb4O15 distinct from those previously published. All orthorhombic compositions were observed to undergo first order phase transitions to the tetragonal structure on heating. The barium and strontium atoms were found to order onto two crystallographically distinct A sites. It was found that the tungsten bronze tolerance factor could be used as a predictive tool for the crystal symmetry of these materials. All compounds in this system for which ferroelectric measurements were performed displayed ferroelectric hysteresis behaviour.

Etude de la constante diélectrique, spectres Mössbauer et RMN. Les transitions de phases sont étudiées par diffraction RX et mesures diélectriques. Les systèmes étudiés sont : PbTiO₃-1/2Pb2Fe2O5-CaTiO₃-1/2Ca2fE2O5; Li2O-Ta2O5-MgO; Li2O-Ta2O5-ZnO; Li2O-Ta2O5-MgO.

Le but de la présente thèse est d'étudier la dynamique des réseaux de ferrites Bi1-xAxFeO3(A= Sr, Ca) et de manganites La1-хСахМnОЗ et Pr1-хCaхMnO3. Dans ces systèmes les paramètres d'ordres électrique et magnétique sont en compétition. Afin d'identifier les mécanismes microscopiques de l'ordre magnétique et électrique dans ces systèmes, une étude de diffraction des rayons X a été menée pour mettre en évidence l'effet de la substitution aliovalente sur les changements de la structure. L'évolution de la symétrie cristalline dépend non seulement de la concentration de l'élément de substitution, mais aussi de sa nature (Sr ou Ca). Les spectroscopies Raman, infrarouge, Mössbauer et XPS ont été utilisées comme des techniques appropriées pour étudier les excitations de réseau dans les systèmes étudiés où les ordres polaires et magnétiques devraient coexister. Afin de mieux comprendre les effets de contrainte et de taille sur les mécanismes microscopiques de l'ordre magnétique et électrique dans la même phase, des films épitaxiés de certaines phases sélectionnées ont été élaborés par la technique d'ablation laser pulsé sur des substrats de SrTiO3 (STO), MgO et MgO tamponné par STO. Nos résultats confirment la coexistence de l'état antiferromagnétique de charge-ordonné et de phases ferromagnétiques métalliques avec la compétition entre les interactions de super échange et de double échange. Dans ce travail, les conditions de coexistence des structures magnétiques et électriques se sont avérées dépendre des substitutions de cations ainsi que de la méthode de synthèse des échantillons sous forme céramique ou films minces
The aim of the present thesis is to study the lattice dynamics of ferrites Bi1-xAxFeO3 (A= Sr, Ca) and manganites La1-хСахМnОЗ and Pr1-хCaхMnO3 systems subject of competitive electric and magnetic order parameters. To identify the microscopic mechanisms of both magnetic and electric ordering in these systems, usual x-ray diffraction study has been conducted to highlight the effect of aliovalent substitution on the structural changes. The evolution of crystal symmetry was found to depend not only to the concentration of substituting element but also to its nature (Sr or Ca). Raman and infrared and of sub-THz, Mössbauer, and XPS were used as appropriate spectroscopic techniques for studying the lattice excitations in the studied systems where polar and magnetic orders expected to coexist. In order to shed more light on strain- and size effects on the microscopic mechanisms of both magnetic and electric ordering in the same phase, epitaxial films of some selected phase were grown by pulse laser deposition on SrTiO3(STO), MgO, and MgO buffered STO substrates. Our results confirm the coexistence of charge–ordered antiferromagnetic state and ferromagnetic metallic phases with the competition between super-exchange and double-exchange interactions. In this work, the conditions of coexistence of magnetic and electric structures were found to depend on the cation substitutions as well as on the method of sample elaboration either in ceramics or in thin films forms

Non-stoichiometric strontium iron oxide is described by an abbreviated formula SrFeOx (2.5 ≤ x ≤ 3.0) exhibits a variety of interesting physical and chemical properties over a broad range of temperatures and in different gaseous environments. The oxide contains a mixture of iron in the trivalent and the rare tetravalent state. The material at elevated temperature is a mixed oxygen conductor and it, or its derivatives,can have practical applications in oxygen conducting devices such as pressure driven oxygen generators, partial oxidation reactors in electrodes for solid oxide fuel cells (SOFC). ¶ This thesis examines the behaviour of the material at ambient and elevated temperatures using a broad spectrum of solid state experimental techniques such as: x-ray and neutron powder diffraction,thermogravimetric and calorimetric methods,scanning electron microscopy and Mossbauer spectroscopy. Changes in the oxide were induced using conventional thermal treatment in various atmospheres as well as mechanical energy (ball milling). The first experimental chapter examines the formation of the ferrite from a mixture of reactants.It describes the chemical reactions and phase transitions that lead to the formation of the oxide. Ball milling of the reactants prior to annealing was found to eliminate transient phases from the reaction route and to increase the kinetics of the reaction at lower temperatures. Examination of the thermodynamics of iron oxide (hematite) used for the reactions led to a new route of synthesis of the ferrite frommagnetite and strontium carbonate.This chapter also explores the possibility of synthesis of the material at room temperature using ball milling. ¶ The ferrite strongly interacts with the gas phase so its behaviour was studied under different pressures of oxygen and in carbon dioxide.The changes in ferrite composition have an equilibrium character and depend on temperature and oxygen concentration in the atmosphere. Variations of the oxygen content x were described as a function of temperature and oxygen partial pressure, the results were used to plot an equilibrium composition diagram. The heat of oxidation was also measured as a function of temperature and oxygen partial pressure. ¶ Interaction of the ferrite with carbon dioxide below a critical temperature causes decomposition of the material to strontium carbonate and SrFe12O19 . The critical temperature depends on the partial pressure of CO2 and above the critical temperature the carbonate and SrFe12O19 are converted back into the ferrite.The resulting SrFe12O19 is very resistant towards carbonation and the thermal carbonation reaction does not lead to a complete decomposition of SrFeOx to hematite and strontium carbonate. ¶ The thermally induced oxidation and carbonation reactions cease at room temperature due to sluggish kinetics however,they can be carried out at ambient temperature using ball milling.The reaction routes for these processes are different from the thermal routes.The mechanical oxidation induces two or more concurrent reactions which lead to samples containing two or more phases. The mechanical carbonation on the other hand produces an unknown metastable iron carbonate and leads a complete decomposition of the ferrite to strontiumcarbonate and hematite. ¶ Thermally and mechanically oxidized samples were studied using Mossbauer spectroscopy. The author proposes a new interpretation of the Sr4Fe4O11 (x=2.75) and Sr8Fe8O23 (x=2.875)spectra.The interpretation is based on the chemistry of the compounds and provides a simpler explanation of the observed absorption lines.The Mossbauer results froma range of compositions revealed the roomtemperature phase behaviour of the ferrite also examined using x-ray diffraction. ¶ The high-temperature crystal structure of the ferrite was examined using neutron powder diffraction.The measurements were done at temperatures up to 1273K in argon and air atmospheres.The former atmosphere protects Sr2Fe2O5 (x=2.5) against oxidation and the measurements in air allowed variation of the composition of the oxide in the range 2.56 ≤ x ≤ 2.81. Sr2Fe2O5 is an antiferromagnet and undergoes phase transitions to the paramagnetic state at 692K and from the orthorhombic to the cubic structure around 1140K.The oxidized formof the ferrite also undergoes a transition to the high-temperature cubic form.The author proposes a new structural model for the cubic phase based on a unit cell with the Fm3c symmetry. The new model allows a description of the high-temperature cubic form of the ferrite as a solid solution of the composition end members.The results were used to draw a phase diagramfor the SrFeOx system. ¶ The last chapter summarizes the findings and suggests directions for further research.

CASTRO JUNIOR, Manoel Carvalho. Transições de fase em perovskitas politípicas. 2013. 153 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2013.
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In this thesis, vibrational spectroscopy was used to investigate the structural transitions of polytypic perovskites. Representative compounds of two polytypes were studied: the multiferroic 3C-type perovskite Pb(Fe1/2Nb1/2)O3 (PFN) and the 6H-type perovskites Ba3B2+Sb2O9 (B = Mg, Ca e Sr). Based on anomalies or deviations from normal behavior of the adjustable parameters of the temperature dependence of the phonon spectra of these compounds, it was possible to identify structural transformations. In PFN, it was observed, evidences of two transitions between ferroelectric phases, in addition to a transition between different magnetic ordering states (paramagnetic → antiferromagnetic) and different polar states (paraelectric → ferroelectric). These results provide stronger evidences than previous Raman Spectroscopy reports in literature due to a better resolution, which allowed us to observe, for example, a new phonon at the Néel temperature. In the case of the 6H perovskites, besides to the usual observation of temperature dependence of the phonon spectra, which provides evidences of the transition of a hexagonal to monoclinic phase at 140 K in the Ba3MgSb2O9 (BMS) and of a monoclinic to triclinic phase at 240 K in the Ba3MgSb2O9 (BCS), we classify the phonons of the three compounds using two methods. First we applied the Hartree-Fock ab-initio method to calculate the normal modes of the Sb2O9 dimer; and assuming that the unit cell of BMS is constituted of Sb2O9 dimers and isolated ions of Ba and Mg, we correlate, using group theory methods, the calculated modes of Sb2O9 with the internal modes of BMS. Later we used the FG Wilson method to confirm the previous results, being obtained a good agreement between the two methods.
Nesta tese a espectroscopia vibracional foi empregada para investigar as transições estruturais de perovskitas politípicas. Compostos representativos de dois polítipos foram estudados: a perovskita multiferróica tipo 3C Pb(Fe1/2Nb1/2)O3 (PFN) e as perovskitas tipo 6H Ba3B2+Sb2O9 (B = Mg, Ca e Sr). A partir de anomalias ou desvios do comportamento normal dos parâmetros ajustáveis com a temperatura do espectro de fônons destes compostos foram identificadas transformações estruturais. No PFN foram observados indícios de duas transições entre fases ferroelétricas, além de uma transição entre diferentes fases de ordenamento magnético (estado paramagnético → estado antiferromagnético) e uma transição entre um estado paraelétrico e um ferroelétrico. Estes resultados são expostos de forma bem mais clara do que em trabalhos de espectroscopia Raman anteriormente informados na literatura, devido a melhor resolução obtida, o que propiciou até a observação da origem de um novo modo no entorno da temperatura de Néel. Nas perovskitas 6H, além da observação usual do comportamento dos fônons em função da temperatura, o que rendeu a observação da transição de uma fase hexagonal para uma fase monoclínica em 140K no Ba3MgSb2O9 (BMS) e de uma fase monoclínica para uma fase triclínica em 240 K no Ba3MgSb2O9 (BCS), procurou-se classificar os fônons dos três compostos utilizando dois métodos computacionais. Primeiramente utilizou- se o método ab initio de Hartree-Fock para calcular os modos normais do dímero Sb2O9; e assumindo que a célula unitária do BMS é formada de dímeros Sb2O9 e íons isolados de Ba e Mg, conseguiu-se correlacionar, utilizando teoria de grupos, os modos calculados do Sb2O9 com os modos internos do BMS. Posteriormente utilizou-se o método FG de Wilson para corroborar os resultados anteriormente obtidos, sendo observado um bom acordo entre ambos métodos.

An extensive study of the crystal structure and phase transitions in Na0.5Bi0.5TiO3 has been carried out using neutron/x-ray powder, and single-crystal x-ray, diffraction techniques. This has been complemented with optical and Raman studies. The sequence of phase transitions from the high-temperature prototypic cubic structure (above 813 K), to one of tetragonal (673-773 K) and then rhombohedral structure (5-528 K) has been established in Na0.5Bi0.5TiO3. Coexisting tetragonal/cubic (773-813 K) and rhombohedral/tetragonal (528-673 K) phases have also been observed. Rietveld refinements have revealed the rhombohedral phase, space group Ric, exhibits an antiphase, a-a-a- oxygen octahedra tilt system, with parallel cation displacements. The tetragonal phase, space group P4bm, possesses an unusual combination of in-phase, a0a0c+ oxygen octahedra tilts and antiparallel cation displacements along the polar axis. This structure type is unprecedented amongst the perovskites. A high-pressure study using synchrotron and neutron diffraction has revealed the ambient pressure rhombohedral structure undergoes a phase transition to an orthorhombic structure with Pnma symmetry and the a-b+a- oxygen octahedra tilt system together with antiparallel A-cation ordering along [010]. The pure rhombohedral structure persists up to 8 kbar; between 10-19.4 kbar a rhombohedral/orthorhombic coexistence region was observed and above 26.2 kbar the purely orthorhombic phase was seen. Structural variations as a function of temperature, pressure and doping across the (Na1-xKx)0.5Bi0.5TiO3 series are also presented. With increasing potassium doping across this series, phase transitions from the rhombohedral structure with octahedral tilting to a non-tilted rhombohedral structure (space group, Ram) and then to a tetragonal structure (space group P4mm) was observed. A basic phase diagram for this series has been assembled outlining phase boundaries across the series and as a function of temperature. In this study, the optimum conditions used for the fabrication of powders, ceramics and single crystals of the perovskite compound Na0.5Bi0.5TiO3 and the solid solution across the (Na1-xKx)0.5Bi0.5TiO3 series have been established. A detailed analysis of this A-site substituted distorted perovskite compound is made with structural variations, cation displacements and octahedral distortions for the different phases being reported. The A-site substituted compounds are thoroughly discussed in the context of modem perovskite science.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico
Nesta tese a espectroscopia vibracional foi empregada para investigar as transiÃÃes estruturais de perovskitas politÃpicas. Compostos representativos de dois polÃtipos foram estudados: a perovskita multiferrÃica tipo 3C Pb(Fe1/2Nb1/2)O3 (PFN) e as perovskitas tipo 6H Ba3B2+Sb2O9 (B = Mg, Ca e Sr). A partir de anomalias ou desvios do comportamento normal dos parÃmetros ajustÃveis com a temperatura do espectro de fÃnons destes compostos foram identificadas transformaÃÃes estruturais. No PFN foram observados indÃcios de duas transiÃÃes entre fases ferroelÃtricas, alÃm de uma transiÃÃo entre diferentes fases de ordenamento magnÃtico (estado paramagnÃtico → estado antiferromagnÃtico) e uma transiÃÃo entre um estado paraelÃtrico e um ferroelÃtrico. Estes resultados sÃo expostos de forma bem mais clara do que em trabalhos de espectroscopia Raman anteriormente informados na literatura, devido a melhor resoluÃÃo obtida, o que propiciou atà a observaÃÃo da origem de um novo modo no entorno da temperatura de NÃel. Nas perovskitas 6H, alÃm da observaÃÃo usual do comportamento dos fÃnons em funÃÃo da temperatura, o que rendeu a observaÃÃo da transiÃÃo de uma fase hexagonal para uma fase monoclÃnica em 140K no Ba3MgSb2O9 (BMS) e de uma fase monoclÃnica para uma fase triclÃnica em 240 K no Ba3MgSb2O9 (BCS), procurou-se classificar os fÃnons dos trÃs compostos utilizando dois mÃtodos computacionais. Primeiramente utilizou- se o mÃtodo ab initio de Hartree-Fock para calcular os modos normais do dÃmero Sb2O9; e assumindo que a cÃlula unitÃria do BMS à formada de dÃmeros Sb2O9 e Ãons isolados de Ba e Mg, conseguiu-se correlacionar, utilizando teoria de grupos, os modos calculados do Sb2O9 com os modos internos do BMS. Posteriormente utilizou- se o mÃtodo FG de Wilson para corroborar os resultados anteriormente obtidos, sendo observado um bom acordo entre ambos mÃtodos.
In this thesis, vibrational spectroscopy was used to investigate the structural transitions of polytypic perovskites. Representative compounds of two polytypes were studied: the multiferroic 3C-type perovskite Pb(Fe1/2Nb1/2)O3 (PFN) and the 6H-type perovskites Ba3B2+Sb2O9 (B = Mg, Ca e Sr). Based on anomalies or deviations from normal behavior of the adjustable parameters of the temperature dependence of the phonon spectra of these compounds, it was possible to identify structural transformations. In PFN, it was observed, evidences of two transitions between ferroelectric phases, in addition to a transition between different magnetic ordering states (paramagnetic → antiferromagnetic) and different polar states (paraelectric → ferroelectric). These results provide stronger evidences than previous Raman Spectroscopy reports in literature due to a better resolution, which allowed us to observe, for example, a new phonon at the NÃel temperature. In the case of the 6H perovskites, besides to the usual observation of temperature dependence of the phonon spectra, which provides evidences of the transition of a hexagonal to monoclinic phase at 140 K in the Ba3MgSb2O9 (BMS) and of a monoclinic to triclinic phase at 240 K in the Ba3MgSb2O9 (BCS), we classify the phonons of the three compounds using two methods. First we applied the Hartree-Fock ab-initio method to calculate the normal modes of the Sb2O9 dimer; and assuming that the unit cell of BMS is constituted of Sb2O9 dimers and isolated ions of Ba and Mg, we correlate, using group theory methods, the calculated modes of Sb2O9 with the internal modes of BMS. Later we used the FG Wilson method to confirm the previous results, being obtained a good agreement between the two methods.

Les solutions solides (1-x)Na0,5Bi0,5TiO3 (NBT) – xCaTiO3 (CT) ont été étudiées par diffraction des rayons X, spectroscopie Raman, microscopie électronique à balayage, spectroscopie d’impédance et DSC. Ce sont des matériaux présentant la structure cristalline pérovskite. L’étude révèle la complexité mais aussi la richesse des phénomènes physiques dans cette famille de composés : les séquences des transitions de phases, l’influence du dopant Ca2+ sur les propriétés physico-chimiques du matériau, la relation étroite entre propriétés diélectriques et caractéristiques structurales. Des solutions solides (1-x)NBT – xCT, avec 0 ≤ x ≤ 1,00, ont été synthétisées par voie solide classique puis frittées selon une procédure spécifique dans un milieu confiné pour éviter toute perte de sodium et de bismuth. Les caractéristiques cristallines des solides obtenus imposent clairement de distinguer trois domaines suivant les valeurs de x. En effet, pour les valeurs croissantes de x et à la température ambiante, on observe un premier domaine (Région I, pour x ≤ 0,07) dans lequel le solide obtenu est une solution solide de structure cristalline, de groupe d’espace R3c, identique à celle de NBT pur. Pour les valeurs les plus élevées de x (Région II, pour x ≥ 0,15), le solide obtenu est une solution solide de structure cristalline, de groupe d’espace Pnma, identique à celle de CT pur. Enfin, entre ces deux domaines (Région III, 0,09 ≤ x ≤ 0,13), les solides obtenus sont biphasés, R3c + Pnma, en se limitant aux appellations des groupes d’espacé des phases formées. Dans la région I, lors du chauffage, la séquence des transitions de phases R3c → P4bm → Pm3m est mise en évidence; les températures des transitions se déplacent vers les plus basses températures quand la concentration en Ca2+ augmente. Les solides sont ferroélectriques à l’ambiante puis développent un caractère relaxeur, par coexistence de deux phases, avec l’augmentation de la température. Dans la région II, les solides révèlent un comportement relaxeur dès l’ambiante. Une transition de phase diffuse au sein de la phase orthorhombique Pnma est toutefois mise en évidence ; le solide passe d’un état relaxeur à un état paraélectrique tout en conservant, a priori, la même structure cristalline. Le phénomène de relaxation dans ces composés est expliqué par la formation de micros ou nanorégions polaires. La région III, quant à elle, est caractérisée par l’apparition d’une hystérésis thermique mise en évidence pour la première fois ; elle est expliquée par la relation entre la microstructure cristalline et les propriétés diélectriques. Enfin, l’ensemble de nos résultats a été regroupé dans un diagramme de phase original en composition et en température
The solid solutions (1-x)Na0,5Bi0,5TiO3 (NBT) – xCaTiO3 (CT) were studied by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, impedance spectroscopy and DSC. These materials have a perovskite crystalline structure. This study reveals not only the complexity but also the richness of physical phenomena in these compounds: phases transitions sequences, the Ca2+ effect on the physical-chemistry properties and the relation between dielectric properties and crystalline structure. Thereby, (1-x)NBT – xCT solid solutions (0 ≤ x ≤ 1.00) were synthesized by chemical solid route, then they were sintered by a particular procedure in order to avoid sodium and bismuth volatilization. The solid crystalline characteristics obtained prove clearly the necessity to distinguish three fields as a function of x values. First of all, for increasing x at room temperature, there is a first region so called region I (x ≤ 0.07), wherein the crystalline structure of solid solutions obtained has a space group R3c identical to that of pure NBT. For the highest values of x, (Region II, x ≥ 0.15), the solid obtained has a space group Pnma, identical to that of pure CT. Finally, between these two regions, (0.09 ≤ x ≤ 0.13), the solid solutions obtained are biphasic, R3c + Pnma, limited to appellations of the space groups formed phases. In region I, upon heating, phase transition sequence R3c → R3c + Pnma → Pnma was determined; the corresponding transition temperatures move to low values with increasing Ca2+ concentration. These solids are ferroelectric at room temperature and then develop a relaxor character, by coexistence of two phases, with increasing temperature. In region II, these solids reveal a relaxor behavior at room temperature. However, a diffuse phase transition within the orthorhombic phase Pnma has been identified; the solid changes from relaxor to paraelectric while maintaining the same crystal structure. This phenomenon was explained by the formation of micro or nano-polar regions. Region III, demonstrated for the first time, is characterized by thermal hysteresis, and explained by the relation between crystalline microstructure and dielectric properties. Finally, all our results were assembled in an original phase diagram as a function of concentration of Ca2+ dopant and temperature

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In this work, the lead-free ceramic powders of xBi0,5Na0,5TiO3 - (0,7186 - 0,7143x) Bi0,5K0,5TiO3 - (2814 - 2857x) BaTiO with x = 0,8200; 0,8625; 0,8792, 0,9126 and 0,9300 (BNBK1000x) were prepared by the solid state reaction method, followed by conventional densification. The ceramic bodies showed high density, which was higher than 95% in obtained samples. For the all ceramics were performed structural, micro structural, electric and anelastic characterizations. The X-ray diffraction (XRD) analysis indicated the formation of the complex perovskite type crystaline structure for all compositions analyzed without the presence of spurious phases. By the structural refinement by the Rietveld method of XRD data and by Raman spectroscopy were observed that for BNBK912 and BNBK930 compositions, at room temperature, the predominant symmetry is rhombohedral (R3c), while the BNBK820 composition exhibits a tetragonal crystalline symmetry (P4mm). For the BNBK879 and BNBK826 compositions presented a mixture of phases, possibly with rhombohedral and tetragonal symmetry, reaveling the morphotropic phase boundary (MPB) of this system. The microstructure of BNBK1000x ceramics was investigated by scanning electron microscopy (SEM), where the morphology grains with irregular sizes and shapes, where the increased levels of K+ and Ba2+ ions suppressed the growth of the grains. The characterizations by Raman spectroscopy at room temperature showed broad Raman modes, due to the chemical and/or structural disorder related to the substitution of elements Bi and Na for Ba and K. The ferroelectric characterizations at room temperature of the BNBK1000x ceramics showed that all compositions studied are ferroelectric. Through the comparison among the measurements of electrical impedance and mechanical spectroscopy, was possible to identify the different structural and electric phase transitions that were employed in the construction of a pseudodiagram of phases for the BNBK1000x compositions.
Neste trabalho, os pós-cerâmicos livres de chumbo xBi0,5Na0,5TiO3 - (0,7186 - 0,7143x) Bi0,5K0,5TiO3 - (2814 - 2857x) BaTiO com x = 0,8200; 0,8625; 0,8792, 0,9126 e 0,9300 (BNBK1000x) foram obtidos através do método de reação de estado sólido, seguido por densificação convencional. Os corpos cerâmicos obtidos apresentaram elevada densificação, sendo maior que 95% nas amostras produzidas. Para todas as cerâmicas foram realizadas caracterizações estruturais, microestruturais, elétricas e anelásticas. As análises por difração de raios-X (DRX) indicaram a formação da estrutura cristalina tipo perovkista complexa para todas as composições analisadas, sem a presença de fases espúrias. O refinamento estrutural, pelo método de Rietveld, dos resultados de DRX e os resultados de espectroscopia Raman apontam que para as composições BNBK930 e BNBK912 a simetria predominante em temperatura ambiente é romboédrica (R3c), enquanto que a composição BNBK820 apresenta a simetria cristalina tetragonal (P4mm). Já as composições BNBK879 e BNBK862 apresentaram uma mistura de fases, possivelmente com simetrias romboédrica e tetragonal, evidenciando o contorno de fase morfotrópico (CFM) deste sistema. A microestrutura das cerâmicas de BNBK1000x foi investigada por microscopia eletrônica de varredura (MEV), onde a morfologia apresentada pelas diferentes composições estudadas possuíam grãos com tamanhos e formatos irregulares, sendo que o aumento dos teores dos íons de K+ e Ba2+ inibiram o crescimento dos grãos. A caracterização por espectroscopia Raman, em temperatura ambiente, apresentou modos Raman amplos, mostrando um elevado grau de desordem química e/ou estrutural devido à substituição dos elementos Bi e Na por Ba e K. As caracterizações ferroelétricas, também em temperatura ambiente das cerâmicas de BNBK1000x mostraram que todas as composições estudadas possuem propriedades ferroelétricas. Através da comparação entre as medidas de impedância elétrica e anelástica, levando em consideração as caracterizações estruturais e ferroelétricas com temperatura, foi possível identificar diferentes transições de fase estruturais e elétricas, que foram empregadas na construção de um pseudo-diagrama de fases para as composições de BNBKx.

Dans cette thèse, nous présentons des études théoriques de matériaux pérovskites sous con-trainte mécanique uniaxiale en combinant les calculs de premier principe DFT ainsi quela théorie phénoménologique de type Landau. Les pérovskites ABO3 forment une classetrès importante de matériaux fonctionnels, qui peuvent présenter un large éventail de pro-priétés (e.g., supraconductivité, magnétisme, ferroélectricité, multiferroïcité, transitionsmétal-isolant. . . ) grâce aux petites distorsions d’ une même structure prototype cubique.Bien que ces composés aient été largement étudiés expérimentalement et théoriquement, ilreste encore des questions importantes et non résolues concernant les effets de contraintesuniaxiales. Au cours de ces dernières années, l’ ingénierie de contrainte a été décrite commeune approche originale pour ajuster les propriétés ferroélectriques pérovskites ABO3. Alorsque les effets de tension épitaxié-biaxiale et pression la hydrostatique, sont plutôt bien com-pris dans cette classe de matériaux, très peu est connu en ce qui concerne l’ effet des con-traintes mécaniques uniaxiales. Notre étude est motivée par ce manque de compréhensionactuelle de l’ effet de tension et compression uniaxiale, qui a été jusqu’à présent presquetotalement négligé. Deux composés prototypes sont étudiés dans le détail: PbTiO3 etSrTiO3. Après une introduction générale sur les composés ABO3 et les calculs techniques(ab initio et modèle phénoménologique de Landau), nous avons étudié l’ effet de contraintesmécaniques sur ces matériaux dans notre thèse.PbTiO3 est un composé ferroélectrique prototypique et également l’ un des composantsmère de la solution solide Pb(Zr,Ti)O3 (PZT), qui est le piézoélectrique le plus largementutilisé dans des applications. Pour PbTiO3, nous avons montré que indépendammentde la contrainte mécanique uniaxiale appliquée, le système conserve un état fondamentalpurement ferroélectrique avec la polarisation alignée, soit le long de la direction de lacontrainte (en phase FEz) ou bien le long d’ un des axes pseudo-cubique, qui lui estperpendiculaire (phase de FEx). Cela contraste avec les cas de contraintes mécaniquesisotropes ou bi-axial, pour qui de nouvelles phases combinant des modes ferroélectriqueset antiferrodistortives ont déjà été décrites. Sous contrainte uniaxiale, PbTiO3 passe d’unétat fondamental FEx sous compression à un état fondamental FEz en tension au-delà d’une tension critique !czz! +1%. Sous contrainte uniaxiale, PbTiO3 présente soit un étatfondamental FEx sous compression ("zz < 0) ou un état fondamental de FEz sous tension("zz > 0). Cependant, ici, un brusque saut des paramètres structuraux est prévu sousdes contraintes de compression et de traction à des valeurs critiques "zz! +2 GPa et −8GPa. Ce comportement semble similaire à celui pré-prédit sous pression isotrope négativeet pourrait se révéler utile en pratique pour améliorer la réponse piézoélectrique dans lesnano-composants.Le deuxième composé intéressant est SrTiO3. Il a été largement étudié au cours desdernières décennies, en raison de ses propriétés exceptionnelles à basse température. Dansce travail, nous avons élargi nos précédentes études de PbTiO3, en explorant théorique-ment les effets de pression sur la perovskite SrTiO3, combinant les premiers principes decalculs et un modèle phénoménologique de type Landau. Nous avons discuté de l’évolutiondes fréquences des phonons de SrTiO3 des trois cas de contraintes isotrope, uniaxial ettensions biaxiaux en utilisant les calculs de premier principe. Nous confirmons des travauxexpérimentaux précédents sur SrTiO3 que ça soit en contrainte épitaxiée ou sous pressionhydrostatique. Enfin, nous avons calculé de diagramme de phase de SrTiO3 sous contrainteuniaxiale, obtenue à partir de la théorie de Landau que nous avons comparé aux calculsde premier principe
In the present thesis we present theoretical studies of perovskite compounds under uniax-ial mechanical constraints combining first-principles DFT calculations approach and phe-nomenological Landau theory. ABO3 perovskites form a very important class of functionalmaterials that can exhibit a broad range of properties (e.g., superconductivity, magnetism,ferroelectricity, multiferroism, metal-insulator transitions. . . ) within small distortions ofthe same simple prototype cubic structure. Though these compounds have been exten-sively studied both experimentally and computationally, there are still unresolved issuesregarding the effect of pressure. In recent years, strain engineering has reported to bean original approach to tune the ferroelectric properties of perovskite ABO3 compounds.While the effect of epitaxial biaxial strain and hydrostatic strain is rather well understoodin this class of materials, very little is yet known regarding the effect of uniaxial mechanicalconstraints. Our study is motivated by the little existing understanding of the effect ofuniaxial strain and stress, that has been up to now almost totally neglected. Two proto-type compounds are studied in detail: PbTiO3 and SrTiO3. After a general introductionon ABO3 compounds and calculations techniques (ab initio and phenomenological Landaumodel), we studied the effect of mechanical constraints in these compounds in our thesis.PbTiO3 is a prototypical ferroelectric compound and also one of the parent components ofthe Pb(Zr,Ti)O3 solid solution (PZT), which is the most widely used piezoelectrics. ForPbTiO3, we have shown that irrespectively of the uniaxial mechanical constraint applied,the system keeps a purely ferroelectric ground-state, with the polarization aligned eitheralong the constraint direction (FEz phase) or along one of the pseudocubic axis perpen-dicular to it (FEx phase). This contrasts with the case of isotropic or biaxial mechanicalconstraints for which novel phases combining ferroelectric and antiferrodistortive motionshave been previously reported. Under uniaxial strain, PbTiO3 switches from a FEx groundstate under compressive strain to FEz ground-state under tensile strain, beyond a critical strain !czz! +1%. Under uniaxial stress, PbTiO3 exhibits either a FEx ground state undercompression ("zz < 0) or a FEz ground state under tension ("zz > 0). Here, however, anabrupt jump of the structural parameters is also predicted under both compressive andtensile stresses at critical values "zz! +2 GPa and −8 GPa. This behavior appears similarto that predicted under negative isotropic pressure and might reveal practically useful toenhance the piezoelectric response in nanodevices.The second compound of interest is SrTiO3. It has been widely studied in the past decadesdue to its unusual properties at low temperature. In this work, we have extended ourprevious investigations on PbTiO3 by exploring theoretically the pressure effects on per-ovskite SrTiO3 combining the first-principles calculations and a phenomenological Landaumodel. We have discussed the evolution of phonon frequencies of SrTiO3 with the threeisotropic, uniaxial and biaxial strains using first-principles calculations. We also reproducethe previous work done in SrTiO3 with epitaxial strain and hydrostatic strain. Finally,we have calculated the phase diagram of SrTiO3 under uniaxial strain, as obtained fromLandau theory and discussed how it compares with the first-principles calculations

Dans cette thèse, nous avons développé l'utilisation de l'activation microondes pour fonctionnaliser des pérovskites lamellaires et notamment la phase d'Aurivillius Bi2SrTa2O9 (BST), connue pour ses propriétés ferroélectriques. Nous sommes parvenus à protoner cette phase (HST) et à la fonctionnaliser par diverses amines et polyamines, avec des temps de réaction considérablement réduits par rapport aux fonctionnalisations en conditions classiques. Cette approche nous a permis de fonctionnaliser HST par des amines plus encombrées et plus complexes. Cette stratégie a ensuite été étendue au greffage d'alcools et de polyols. Nous avons également établi une stratégie de modification post-synthèse, pour synthétiser in situ la molécule désirée, en utilisant la chimie "click" et l'activation microondes. Enfin, nous sommes parvenus à insérer des ions métalliques et des complexes de métaux de transition, ce qui constitue une première étape vers la synthèse de nouveaux hybrides multiferroïques
During this PhD thesis, we have developed the use of microwave activation to functionalize layered perovskites, among which the Aurivillius phase Bi2SrTa2O9 (BST), known for its ferroelectric properties. We managed to protonate this phase (leading to HST) and to functionalize it by various amines and polyamines, with reaction times much shorter than using classical conditions. This approach allowed us to functionalize HST by bulkier and more complex amines. This strategy has further been extended to the grafting of alcohols and polyols. We have also established a postsynthesis modification strategy, in order to synthesize the desired molecule in situ, within the interlamellar space, using "click" chemistry and microwave activation. Finally, we managed to insert transition metal ions and complexes, which constitutes a promising step towards the synthesis of new multiferroic hybrid materials

Parmi la grande diversité de composés formant la famille des perovskites hexagonales, les systèmes à base de cobalt sont largement étudiés à cause de leurs propriétés électroniques et magnétiques complexes. Ainsi, l'investigation des systèmes Ba-Co-O-X avec X=F, Cl ou Br a permis de synthétiser de nouvelles phases dont les structures sont caractérisées par des groupements trimères Co3O12 (trois octaèdres CoO6 reliés par une face) ou tétramères Co4O15 (quatre octaèdres CoO6 reliés par une face). De fortes relations structurales ont été mises en évidence entre les formes trimères et tétramères des composés oxydes, oxyfluorures, oxychlorures et oxybromures, notamment l'existence de désordres des atomes d'oxygène ou des transformations de phases à haute température (tétramères -> trimères). Dans tous ces matériaux, la couche d'interface entre blocs élémentaires joue un rôle prépondérant sur la dimensionnalité des structures créées. Par ailleurs, des mesures de susceptibilité magnétique combinées à des expériences de diffraction des neutrons ont permis d'établir l'existence, en l'absence de champ magnétique, d'une mise en ordre antiferromagnétique selon l'axe c à basse température dans les matériaux halogéno-cobaltites. L'évolution des courbes d'aimantation en fonction du champ appliqué montre un comportement magnétique plus complexe pour les matériaux bromés, avec notamment la possibilité d'aligner les moments magnétiques dans le plan (ab) par application d'un champ magnétique. Finalement, ce travail de thèse présente les relations entre structure cristalline, dimensionnalité et propriétés magnétiques de ces nouvelles phases halogéno-cobaltites.

Crystal growth and characterization of few multifunctional materials with perovskite (ABX3) structure are discussed in this thesis. Efforts were made to modify the magnetic and electric behaviour of these materials by selective tuning of A, B and X components. Structural, magnetic and dielectric characterization are detailed in various chapters for doped (A and B site) rare-earth manganites and organometallic compounds with different (Chloride or formate) anions. The relevant aspects of crystal structure and its relationship with ordered ground states are discussed in the introductory chapter. A detailed review of prominent theories pertaining to magnetic and ferroelectric ordering in the literature is provided. Growth of various inorganic compounds by solid-state reaction and floating zone method as well as use of solvothermal techniques for growing organometallic compounds are discussed. Material preparation, optimization of crystal growth processes and results of characterization are addressed in various chapters. The effect of Yttrium doping on structural, magnetic and dielectric properties of rare-earth manganites (RMnO3 where R = Nd, Pr) has been investigated. Neutron diffraction studies (Pr compounds) confirm A-type antiferromagnetic structure and fall in transition temperature as the Yttrium doping level increases. Diffraction experiments in conjunction with dc magnetization and ac susceptibility studies reveal magnetic frustration in excess Yttrium dopedcompounds. When mutliglass properties of 50% B-site doped Nd2NiMnO6 were investigated, evidence of re-entrant cluster glass phase was seen probably due to presence of anti-site disorder. The relaxor-like dielectric behaviour arises from crossover of relaxation time in grain and grain boundary regions. Multiferroic behaviour of the organometallic compound (C2H5NH3)2CuCl4 as well as the ferroelectric transition were investigated in detail. The role of Hydrogen bond ordering in driving structural transitions is elucidated by low temperature dielectric and Raman studies in (C2H5NH3)2CdCl4. It was found possible to tune the magnetic and ferroelectric properties in metal formate compounds (general formula AB(HCOO)3) by selectively choosing organic cations [(CH3)2NH2+; C(NH3)3+] and transition metal ion [B = Mn, Co and Cu]. The nature of magnetic ordering and transition temperature could be altered by the transition metal ion. The effect of reorientation of organic cations which leads to ferroelectric nature is discussed using dielectric and pyroelectric data. Significant results are summarized in the chapter outlining general conclusions. Future prospects of work based on these observations are also provided. The conclusions are corroborated by detailed analysis of experimental data.

Thesis (Ph. D.)--University of Alberta, 2009.
Title from pdf file main screen (viewed on July 13, 2009). "A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Department of Physics, University of Alberta." Includes bibliographical references.

Oxides of ABO3 composition (A = alkali, alkaline earth or rare earth metal in general, B = transition metal) constitute a large family of metal oxides of current interest to solid state and materials chemistry. Among the several structure types exhibited by ABO3 oxides (ilmenite, LiNbO3, perovskite, YAIO3/YMnO3, KSbO3, pyrochlore, among others), the perovskite structure is probably the most well known and widely investigated. The ideal perovskite structure consists of a three-dimensional (3D) framework of corner-sharing BO6 octahedra in which the A cation resides in the dodecahedral site surrounded by twelve oxide ions. The ideal cubic structure occurs when the Gold Schmidt’s tolerance factor, t = (rA + ro)/{V2 (rB + ro)}, adopts a value of unity and the A-O and B-O bond distances are perfectly matched. The flexibility of the perovskite structure towards a wide variety of substitutions at both A and B sites gives rise to a very large number (several hundreds) of perovskite derivatives with subtle variations in structure. The perovskite structure can also tolerate vacancies at both the A and O sites giving ordered superstructures. Members of y4BO3 oxides have numerous properties that find technological application, such as nonlinear optical response (LiNbO3), Ferro electricity (BaTiO3), piezoelectricity (PbZn_xTixO3), magneto ferroelectricity (YMnO3), superconductivity (Bai_xKxBi03)5 colossal magnetoresistance (La^xCaxMnO3) and ionic conductivity [(Lil_a)TiO3] Ordering of cations at the A and B sites of the perovskite structure is an important phenomenon. Ordering of B site cations in double (/42BB'O6) and multiple (/43BB'2Og) perovskites gives rise to newer and interesting materials properties For example, 1*1 ordered Sr2FeMoO6 and Sr2FeReO6 are half-metallic ferrimagnets; Pb3MgNb2O9 is a relaxor ferroelectric; Ba3ZnTa2O9 is a low loss dielectric used in telecommunication and, last but not least, Ba3CoNb2O9 is a visible light driven photocatalyst. Realization of these properties in these materials depends crucially on the ordering/or otherwise of the B site cat ions in the perovskite structure. Furthermore, ordering of not only the metal atoms but also the oxygen/oxygen vacancies in the perovskite structure is equally important for the occurrence of superconductivity in the cuprate superconductor, YBa2Cu3O7. The ideal perovskite structure gives way to hexagonal YMnO3/YAIO3 structure for smaller A cations (tolerance factor, t < 1). Oxides of this structure are attracting current attention for the realization of multiple magnetoferroic properties. On the other hand, for larger A cations (tolerance factor, t > 1), various perovskite polytypic structures are formed. For example, BaNiO3 forms a 2H polytypic structure, SrMnO3 and BaRuO3 adopts a 4H and 9R structures respectively, where the SO6 octahedra share faces or faces and corners. Besides the foregoing 3D perovskites, a number of layered variants of the perovskite structure are also known. The most common layered perovskites are the Aurivillius phases, (Bi2O2)[A»-iBnO3n+iL the Ruddlesden-Popper phases, /4'2|7ln_iBnO3n+1], and the Dion-Jacobson phases, A[An^BnOzn+-\]' The two-dimensional (2D) perovskite unit, [^n-iBnOsn+i], which could be visualized as formed by slicing the 3D perovskite structure along <001>p is common for all the three layered perovskite series. The perovskite slabs are stacked alternately with various charge-balancing units, for example, with [Bi2O2]2+ in the Aurivillius phases and two alkali/alkaline earth cations (A+JA2+) in the Ruddlesden-Popper phases etc. Members of the layered perovskites are also important from the point of view of materials properties. For example, 2D magnetism (K2NiF4), superconductivity (La2-xSrxCuO4), ion exchange, Bronsted acidity, intercalation, exfoliation (K2La2Ti3Oio and CsCa2Nb3O10), photo catalysis (Rb2La2Ti30io) are some of the important materials properties found in layered perovskites. The high Tc-superconductors, Bi2Sr2CaCu2O8+XJ TI2Ba2Ca2Cu3Oi0, TIBa2Ca2Cu3O9 and HgBa2Ca2Cu3O8+x, also belong to the family of layered perovskites where the defective perovskite cuprate sheets are interleaved by other 2D entities like (Bi2O2), (TI2O2), (TIO) or (HgOx). In addition, Aurivillius phases, such as Bi2SrTa209 and Bi325Lao75Ti3Oi2, in thin film geometry are candidate materials for non-volatile ferroelectric memory devices. Synthesis plays a key role in realizing new structures and materials properties for ABO3 oxides. The conventional synthetic methods (ceramic method) involve mixing and heating of solid reactants at elevated temperatures. Although this approach continues to be employed to synthesize new materials, it is often limited by the fact that it yields thermodynamically stable phases. Since many of the perovskite oxides showing useful materials properties are metastable in nature and are required in the form of fine particles (free-standing / monodisperse / submicron or nanometer dimensions) for application, the ceramic methods are of no avail for this purpose. Therefore, materials chemists constantly endeavor to develop alternate synthetic routes that enable them to synthesize novel oxides under mild conditions. Typical examples of metastable perovskites are: the super conducting cuprates (e.g. TlosPbosS^CaC^Og) and perovskite based lithium ion conductors (La2/3-xLi3XDi/3-2xTiO3). Also the control of oxidation states in double perovskites, such as Sr2FeMoO6 and Sr2FeRe06 and pyrochlores such as Pb2MnReC>6, cannot be achieved by conventional means. Therefore, the synthesis of such metastable phases requires special synthetic strategies that involve soft chemistry (chimie douce) methods where mild reactions/reaction conditions are employed to access metastable phases. The present thesis is mainly devoted to an investigation of perovskite related oxides towards developing new synthetic strategies and materials as well as exploring hydrogen insertion - a novel materials property - in certain members of this family. Solid-state metathesis (SSM) reactions provide a convenient route for the synthesis of a wide variety of non-oxide ceramic materials such as, bondes, carbides, silicides, pnictides and chalcogenides. A typical metathesis reaction, for example, M0CI5 + 5/2 Na2S -» MoS2 + 5 NaCI + 1/2 S (1) involves exchange of atoms/ions between the reactants and is accompanied by a large enthalpy change (AHm = - 890 kJ mol"1) and high adiabatic reaction temperature (Tm = 1413 °C). The reactions are often self-propagating and believed to be driven by the formation of stable salt byproducts such as alkali halides with high lattice energy. In our laboratory we have developed a different kind of metathesis reaction for the synthesis of perovskite related oxides, a typical example being, K2La2Ti30io + 2 BiOCI -* [Bi2O2]La2Ti3O10 + 2 KCI. A major difference between metathesis reactions (1) and (2) is that unlike (1), reaction (2) is not self-propagating, requiring longer duration. In this study, we have investigated metathesis reactions of the second kind at some length for the synthesis of perovskite related oxides. We found that rocksalt oxides such as UMO2 (M = Mn, Co) and Li2TiO3 constitute convenient precursors for the formation of v4BO3 perovskite oxides in metathesis reactions with appropriate reaction partners such as halides, oxyhalides or sulphates, LiCoO2 + LaOCl -» LaCoO3 + LiCt (3) LiMnO2 + LaOCl + x/2 O2 -> LaMnO3+x + LiCI (4) Li2TiO3 + PbSO4 -» PbTiO3 + Li2SO4. (5) We could synthesize not only well known ABO3 oxides but also functional perovskites such as PbZr0 4sTio 52O3 (PZT), La2/3Cai/3MnO3 as well as superconducting BaPbo75Bio2s03 by this method. We could also synthesize La2CuO4 and its superconducting analogues, La185^oi5Cu04 (A = Sr, Ba), by the same method using Li2CuO2 and LaOCl. For the synthesis of double perovskites A2BB%OQ by this method however, appropriate lithium containing rocksalt precursor oxides are not known in the literature. Therefore, we first synthesized rocksalt precursor oxides of the general formula Li4MWO6 (M = Mg, Mn, Fe, Ni) and established their identity. Using these precursor oxides, we could synthesize the double perovskite oxides Sr2MWO6 (M = Mg, Mn, Fe, Ni) in the metathesis reaction Li4MWO6 + 2 SrCI2 -» Sr2MWO6 + 4 LiC Significantly, the double perovskites are formed with an ordered structure at relatively low temperatures (750 - 800 °C) as compared to the high temperatures (up to 1400 °C) usually employed for the synthesis of these materials by conventional ceramic approach. Next, we investigated ABO$ compositions corresponding to the formula for 6 = Cu and Ni, where we could obtain a YAIO3 superstructure consisting of triangular Cu clusters for 6 = Cu, whereas a perovskite phase for B = Ni. Moreover, the Cu-phase appears to be a unique line phase formed around LasCi^VOg composition, whereas a continuous series of GdFeO3-like perovskite oxides are formed for LaNii»xVxO3 (0 < x < 1/3)forS = Ni. Considering the current interest in bringing different transition metal ions (d°/dn electronic configuration) in the same perovskite related structure towards developing multiferroic materials, we investigated the substitution of aliovalent cations in a typical Aurivillius phase, Bi2Sr2Nb2TiOi2. We have characterized new aliovalent cation substituted Aurivillius phases, Bi2SrNaNb2TaOi2, Bi2Sr2Nb2Zr012J Bi2Sr2Nb2 5Feo50i2 and Bi2Sr2Nb2 ezZno 33O12. Lastly, we investigated the interaction of hydrogen with perovskite oxides, /\MnO3 (A = Ca, Sr, Ba) in an attempt to characterize possible existence of hydrogen-inserted oxide materials. An oxide-hydride of the formula LaSrCoO3H07 has recently been reported in the literature. Conventionally, the interaction of hydrogen with perovskite related oxides is known to result in either anion deficient phases (e.g. CaMnO3 -> Ca2Mn205), or hydrogen inserted materials, 'hydrogen bronzes', (e. g. HXWO3, HxBaRuO3), where hydrogen acts as an electron donor (H -^ H+ + e). We have characterized a new mode of hydrogen incorporation in Pt dispersed BaMnO3 and SrMnO3. Detailed investigation of the hydrogen sorption behaviour of 1 atom % Pt dispersed materials showed that about 1.25 mass % of hydrogen is inserted per mole of BaMnO3/Pt, corresponding to an insertion of - 3 hydrogen atoms giving 'BaMnOsHs'. While the exact nature of inserted hydrogen is yet to be established unambiguously, our results suggest that the inserted hydrogen is unlikely to be protonic (H+) in the hydrogen insertion product, BaMnO3H3. The results of these investigations are presented in the thesis consisting of seven chapters. Chapter 1 gives an overview of perovskite related oxides - structure, properties and synthesis. Chapter 2 presents metathesis as a general route for the synthesis of ABO3 oxides and illustrates the method by transforming several rocksalt oxides such as LiCoO2, Li2Mn03 and Li2Ti03 to corresponding ABO3 oxides, LaCoO3, /\MnO3 and ATiO3 (A = Ca, Sr, Ba). Uniformly in all the cases, the perovskite oxides are obtained in the form of loosely connected submicron sized particles at considerably lower temperatures than those usually employed for their synthesis by ceramic methods. Thermodynamic calculations have also been carried out to probe into the driving force of metathesis reactions involved in the synthesis. Chapter 3 describes an extension of the metathesis route for the synthesis of double perovskites, Sr2MWO6 (M = Mg, Mn, Fe, Ni). For this purpose, first we synthesized new rocksalt oxides of the general formula, Li4MWO6 (M = Mg, Mn, Fe, Ni). The oxides adopt rocksalt superstructures related to Li4MgReO6 (for M = Mg, Mn, Ni) and U4WO5 (for M = Fe). Metathesis reaction between Li4MWO6 and SrCi2 at 750 - 800 °C yields the corresponding double perovskites where the octahedral site M and W are ordered in the long range. Formation of ordered perovskite oxides at relatively low temperatures (750 - 800 °C) by the metathesis route is a significant result, considering that synthesis of these oxides by conventional ceramic method requires much higher temperatures (1300 - 1400 °C) and prolonged annealing. Synthesis of La2CuO4, Nd2CuO4 and super conducting La-j 85>4oi5Cu04 (A = Sr, Ba) by the metathesis route is described in Chapter 4. Chapter 5 deals with synthesis, structure and magnetic properties of mixed-metal oxides of ABO3 composition in the La-6-V-O (6 = Ni, Cu) systems. While the B = Ni oxides adopt GdFeO3-like perovskite structure containing disordered nickel and vanadium at the octahedral B site, La3Cu2VO9 crystallizes in a YAIO3-type structure. A detailed investigation of the superstructure of nominal La3Cu2VO9 by WDS analysis and Rietveld refinement of powder XRD data reveals that the likely composition of the phase is Lai3Cu9V4O38 5, where the Cu and V atoms are ordered in a Vi3ah (ah = hexagonal a parameter of YAlCMike subcell) superstructure. Magnetic susceptibility data support the proposed superstructure consisting of triangular Cu3 clusters. The present work reveals the contrasting behaviour of La-Cu-V-O and La-Ni-V-0 systems, while a unique line-phase related to YAIO3 structure is formed around La3Cu2VO9 composition in the copper system, a continuous series of perovskite-GdFeO3 solid solutions, LaNi1.0CVxO3 for 0 < x < 1/3 seems to obtain in the nickel system. The chapter also describes the formation of a new transparent Cu(l) oxide, Lai4V6CuO365, and its characterization. This oxide was obtained during attempts to grow single crystals of LasC^VOg. Single crystal structure determination of Lai4V6CuO36 5 showed that the structure contains isolated VO43" tetrahedra and [OCuO]3" sticks dispersed in a lanthanum oxide network. Films of Lai4V6CuO36 5 were grown on R-plane sapphire by using pulsed laser deposition. Rutherford backscattering spectroscopic and X-ray diffraction analyses of the films showed oriented growth of the title phase, with an optical band gap of -~ 5 eV and n-type conductivity Chapter 6 presents the work on the flexibility of the Aurivillius structures for substitution of aliovalent/isovalent cations at both A and 6 sites of the perovskite slabs. For example, in a typical n = 3 member, Bi2Sr2Nb2TiOi2, substitution of both Sr and Na at the A site and Ta at the B site has enabled us to synthesize a new n = 3 member, Bi2SrNaNb2Ta0i2, where we see a preference of Nb for the terminal octahedral sheets. Similarly, aliovalent substitution only at the B site of the perovskite slabs of Bi2Sr2Nb2TiOi2 has yielded new members for specific compositions, Bi2Sr2Nb2ZrOi2, Bi2Sr2Nb2 5Feo50i2 and Bi2Sr2Nb2 67Zno33012 that tend to be oxygen-stoichiometric. The latter phases again show a preference of Nb for the terminal octahedral sites that are strongly distorted as compared to the middle octahedral site. This chapter also describes substitution of La3+ for Bi3+ in the perovskite slabs of Bi4Nb30i5 stabilizing a new series of n = 1/ n = 2 intergrowth Aurivillius phases of the formulas, Bi4LnNb3Oi5 (Ln = La, Pr, Nd) and Bi4LaTa30i5. The present work suggests that replacement of Bi3+: 6s2 lone pair ion by non-6s2 cations such as Sr2"* and La3+ in the perovskite slabs of Aurivillius phases tends to render the structure Centro symmetric and the materials lose NLOSHG response. Chapter 7 describes our investigation of the interaction of hydrogen with alkaline earth manganites (IV) >AMnO3 (>A = Ca, Sr, Ba) dispersed with 1 atom % Pt. The result shows an unprecedented uptake of hydrogen by BaMnO3/Pt to the extent of - 1.25 mass % at moderate temperatures (190 - 260 °C) and ambient pressure. Gravimetric sorption isotherms and mass spectrometric analysis of the desorption products indicate that approximately three hydrogen atoms per mole of BaMnCVPt is inserted reversibly. The nature of hydrogen in the insertion product, BaMnO3H3, is discussed in the light of the structure of BaMnC>3. The work presented in the thesis is carried out by the candidate as a part of the Ph. D. training programme and most of it has been published in the literature. He hopes that the studies reported here will constitute a worthwhile contribution to the materials chemistry of ABO3 oxides in general.

Materials chemistry is essentially concerned with the design/synthesis of new solids endowed with functional properties that could be of relevance to today’s materials technology. Among the large variety of solid materials that attract attention, metal oxides continue to contribute significantly to current materials chemistry. A wide variety of oxide materials (based on rocksalt, spinel, corundum, perovskite, garnet, pyrochlore and other structures) and their properties have been investigated over the years. Most of these oxides are derived from the transition metals. Transition metal oxides with structures derived from metal-oxygen (MO6) octahedra, in particular, display an array of exotic properties with potential or proven technological application. While it is traditionally believed that the partially filled d shell (dn : 0 < n < 10) of the transition metal atoms plays a crucial role in deciding the electronic properties, the significance of d0 metal atoms for the properties (and structure) of transition metal oxides is not fully recognized. Magnetism (SrRuO3, Fe3O4), metallicity (ReO3, LaNiO3), colossal magnetoresistance (La1-xCaxMnO3) and superconductivity (La2xSrxCuO4, Sr2RuO4) are some of the properties that can be traced to the presence of partially filled d shell, while properties like ferroelectricity (BaTiO3), piezoelectricity (PbZr1-xTixO3) and nonlinear optical response (LiNbO3) could be traced to the presence of transition metals (TiIV, ZrIV, NbV) with d0 electronic configuration. The empty d orbitals on the metal atoms constitute the low lying unoccupied states (LUMO) that mix with the highest occupied states (HOMO) of the ligand atoms (oxygen) through special chemical bonding effects (second order Jahn-Teller effect, SOJT). This mixing results, among others, in out-of-centre distortion(s) of the MO6 octahedra and this distortion is at the heart of several properties mentioned above. Among the transition metal oxide structures based on MO6 octahedra, three structures are noteworthy: the perovskite, the pyrochlore and the rutile. The AMO3 perovskite structure consists of a three-dimensional framework of corner sharing MO6 octahedra in which the A cation occupies the dodecahedral site surrounded by twelve oxide ions. The perovskite structure can accommodate a large variety of substitutions at both the A and the M sites as well as vacancies at the A/O sites, giving a large number of derivatives. Several variants of the perovskite structure are also known, for instance, the layered perovskites and ordered perovskites. Many nonperovskite structures are also known for the composition AMO3 : hexagonal YMnO3 is an alternative structure for AMO3 composition where manganese exists as MnO5 trigonal bipyramids. The A2M2O7 pyrochlore structure is also based on a corner-connected network of MO6 octahedra which interpenetrates an A2O network. The rutile (TiO2) is a well-known structure consisting of chains of edge-sharing MO6 octahedra, which are connected through corners to adjacent chains. A large number of oxide materials based on the above three structure types have been reported : for example, perovskite [Ba3ZnTa2O9 (microwave telecommunication ceramic), Pb3MgNb2O9 (relaxor ferroelectric), Bi4Ti3O12 (high temperature ferroelectric)], pyrochlore [Nd2Mo2O7 (metallic ferromagnet), AOs2O6 for A = K, Rb, Cs (superconductor)] and rutile [TiO2 (photocatalyst), CrO2 (metallic ferromagnet), VO2 (insulator-metal transition)]. Considering the current interest in oxide materials of these three structure types which continue to generate new variants and novel properties, we undertook the present research project to synthesize new derivatives of these structure types, and characterize their structures and relevant electronic properties. In doing so, we recognized that synthesis based on an understanding of the reactivity of the constituents and crystal chemistry of the expected products plays a crucial role in this effort. Accordingly, we tailored several new compositions of AMO3, A2M2O7 and MO2 stoichiometries and adopted appropriate methodologies for their synthesis. We have characterized the structures and properties of the solid products by means of state-of-the-art methods available to us. There are two main approaches to the synthesis of nonmolecular inorganic solids: conventional ceramic route and chimie douce / soft chemistry routes. In the ceramic route, solid reactants are heated at elevated temperatures for long durations with intermittent mixing/grinding until the reaction is complete. Chimie douce routes, on the other hand, utilize gentle reactions such as dehydration, decomposition, intercalation, ion exchange, and so on to synthesize the desired phases. The ceramic route generally provides access to the thermodynamically controlled product(s), while chimie douce routes allow access to metastable phases (kinetically controlled product(s)). Disadvantages notwithstanding, the ceramic route has been the mainstay of materials chemistry and several important materials continue to be discovered / synthesized by this route. The choice of the synthetic route based on an understanding of the crystal chemical preferences and the reactivities of the constituents involved is often crucial to achieve the desired final products. The present thesis is devoted to the synthesis and investigation of MO6 octahedra-based oxides belonging to the perovskite, pyrochlore and rutile structure types wherein we have explored alternate synthetic strategies (perovskite-based Ba3MM'2O9 telecommunication ceramics and a solution route for the synthesis of ruthenium-based pyrochlores) and probed structure-property relations of perovskite oxides (Ba3MM'M''O9 oxides for various M/M'/M'' atoms) as well as formation of new derivatives of layered Aurivillius phases. In addition, we have also synthesized new noncentrosymmetric oxides possessing the YMnO3 structure. Our investigation of rutile based oxides has resulted in the discovery of a new lead-free relaxor ferroelectric material, FeTiTaO6. Given that the lone pair PbII:6s2 plays a crucial role in the ferroelectric properties of Pb-based perovskite oxides, we have also investigated members of the Pb1-xLix/2Lax/2TiO3 system for their structure and dielectric response. The present thesis describes the results of these investigations in eight chapters. Chapter 1 provides a general introduction to oxides of the perovskite, pyrochlore and rutile structures. In Chapter 2, we describe a new one-pot metathesis strategy for the synthesis of dielectric ceramics Ba3MM'2O9 (M = Mg, Ni, Zn; M' = Nb, Ta). Rietveld refinement of X-ray diffraction data shows near-complete ordering of M-site ions in many cases. The dielectric properties of the products synthesized are found to be in reasonable agreement with reported data. The synthesis of ordered materials at lower temperatures (~1100 °C) than that employed in the conventional ceramic route (~1500 °C) is a significant result of this work. Chapter 3 presents a study of Ba3MIIMIVWO9 (MII = Ca, Zn; MIV = Ti, Zr) perovskite oxides for the purpose of synthesizing new dielectric ceramic materials and to gain understanding of the factors that stabilize 3C vs. 6H structures. In general, a 1:2-ordered 6H perovskite structure is stabilized at high temperatures (1300 °C) for all of the Ba3MIITiWO9 oxides investigated. An intermediate phase possessing a partially ordered 1:1 double perovskite (3C) structure with the cation distribution, Ba2(Zn2/3Ti1/3)(W2/3Ti1/3)O6, is obtained at 1200 °C for Ba3ZnTiWO9. A metastable perovskite, Ba3CaZrWO9, that adopts the 1:1 3C structure has also been synthesized by a low-temperature metathesis route. Besides yielding several new perovskite oxides that may be useful as dielectric ceramics, the investigation provides new insights into the complex interplay of crystal chemistry (tolerance factor) and chemical bonding (anion polarization and d0-induced distortion of metaloxygen octahedra) in the stabilization of 6H versus 3C perovskite structures for the Ba3MIIMIVWO9 series. In Chapter 4, we describe the synthesis and investigation of the structure and dielectric properties of Ba3MIIITiMVO9 (MIII = Fe, Ga, Y, Lu; MV = Nb, Ta, Sb) perovskite oxides. The MV = Nb, Ta oxides adopt disordered/partially ordered 3C perovskite structures, where all the MIII/Ti/MV metal-oxygen octahedra are corner-connected. In contrast, the MV = Sb oxides show a distinct preference for the 6H structure, where SbV/TiIV metal-oxygen octahedra share a common face, forming (Sb,Ti)O9 dimers, that are corner-connected to the MIIIO6 octahedra. Investigation of dielectric properties of MIII = Y/Lu, MV = Nb/Ta oxides reveals a normal low loss dielectric behaviour with ε = 30 – 50 in the temperature range 50 – 350 °C. The MIII = Fe, MV = Nb/Ta members show a dielectric behaviour similar to relaxor ferroelectric materials. Chapter 5 deals with a study of isomorphous substitution of several metal atoms in two Aurivillius structures, Bi5TiNbWO15 and Bi4Ti3O12, in an effort to probe structure-property correlations. These investigations have led to the synthesis of new derivatives, Bi4LnTiMWO15 (Ln, = La, Pr; M = Nb, Ta), as well as Bi4PbNb2WO15 and Bi3LaPbNb2WO15, that largely retain the Aurivillius intergrowth structure of the parent oxide Bi5TiNbWO15, but characteristically tend toward a centrosymmetric / tetragonal structure for the Ln-substituted derivatives. On the other hand, coupled substitution, 2TiIV Æ MV + FeIII in Bi4Ti3O12, yields new Aurivillius phases, Bi4Ti3-2xNbxFexO12 (x = 0.25, 0.50) and Bi4Ti3-2xTaxFexO12 (x = 0.25) that retain the orthorhombic noncentrosymmetric structure of the parent Bi4Ti3O12. Chapter 6 describes the design and synthesis of a new series of noncentrosymmetric oxides, R3Mn1.5CuV0.5O9 (R = Y, Ho, Er, Tm, Yb, Lu) possessing the YMnO3 structure. Investigation of the Lu-Mn-Cu-V-O system revealed the existence of an isostructural solid solution series, Lu3Mn3-3xCu2xVxO9 for 0 < x ≤ 0.75. Magnetic and dielectric properties of the oxides are consistent with a random distribution of Mn3+, Cu2+ and V5+ atoms that preserves the noncentrosymmetric RMnO3 structure. An exploratory investigation of the synthesis, structure and electronic properties of new ruthenium(IV) pyrochlore oxides and their manganese-substituted derivatives is presented in Chapter 7. The richness of the electronic properties of ruthenium-based metal oxides is affirmed by the results which revealed several novel electronic ground states : a metallic and Pauli paramagnetic state for BiPbRu2O6.5 that turns into a semiconducting ferromagnetic spin-glass state at 50 K for BiPbRuMnO6.5 ; a metallic state that likely shows a charge density wave (CDW) instability at 50-225 K for Bi1.50Zn0.50Ru2O6.75, that is suppressed by manganese substitution in Bi1.50Zn0.50Ru1.75Mn0.25O6.50, and a metallic ferromagnetic spin-glass-like state for Pb2Ru1.75Mn0.25O6.15. We describe the investigation of the structure and dielectric properties of rutile-based MTiTaO6 (M = Al, Cr, Fe) in Chapter 8. All the oxides possess disordered rutile structure. FeTiTaO6 shows a strong relaxor ferroelectric effect, while CrTiTaO6 shows a weaker relaxor ferroelectric behaviour. This work is significant for two reasons: the new material is lead-free and it is based on the rutile structure, unlike the conventional relaxors which are mostly derived from the perovskite structure. The work presented in the thesis is carried out by the candidate as a part of the Ph.D. training programme and most of it has been published in the literature. She hopes that the studies reported here will constitute a worthwhile contribution to materials chemistry in general.

This thesis focuses on the phase transitions, including ferro-magnetic, anti-ferromagnetic, metal to "Mott" insulator and structural transitions in perovskite and Ruddlesden-Popper ruthenates. The thesis is mainly composed of two parts. The first half presents Density Functional Theory (DFT)+Dynamical Mean Field Theory (DMFT) studies of the electronically driven phase transitions in various ruthenate materials. We study cubic perovskite BaRuO$_3$ via DFT add DMFT method using interaction parameters which were found in previous studies to be appropriate for the related materials, CaRuO$_3$ and SrRuO$_3$. The calculated variation in transition temperature between the Ba and Sr compounds is consistent with experiment, confirming the assignment of the compounds to the Hund's metal family of materials, the appropriateness of the single-site dynamical mean field approximation for these materials as well as confirming the appropriateness of the values for the interaction parameters. The results provide insights into the origin of magnetism and the role of the van Hove singularity in the physics of Hund's metals. We also study the metal-insulator transition (MIT) and magnetic transitions in Ca$_2$RuO$_4$. The Ru-O bonds lengths are found to be the most important control parameters for the metal-insulator transitions and rotations are found to be less important. The calculation successfully captures the important features of the para-magnetic (PM) "Mott" insulating state, including the orbital occupancy disproportionation and the orbitally resolved electron spectral function. It shows the advantage of single set DFT+DMFT in dealing with strongly correlated multi-orbital systems without the assumption of spin symmetry breaking. In the second half, we present a Landau free energy model that incorporates the electronic energetics, the coupling of the electronic state to local distortions and the coupling of local distortions to long-wavelength strains. The model is used to elucidate important experimental features in thermal and current-induced MIT in Ca$_2$RuO$_4$ and Ca$_3$Ru$_{2-x}$Ti$_x$O$_7$ materials. The investigation of lattice and electronic energetics and determination of parameters using DFT+DMFT methods is explained. The change in lattice energy across the metal-insulator transition is shown to be comparable to the change in electronic energy. Important consequences are a strongly first order transition, a sensitive dependence of the phase boundary on pressure and that the geometrical constraints on in-plane lattice parameter associated with epitaxial growth on a substrate typically change the lattice energetics enough to eliminate the metal-insulator transition entirely. The change in elasto-resistance across the MIT is determined. The DFT+U relaxation study shows the octahedron relaxation with respect to uniaxial strain on a and b axes are very different. This sensitive a and b axes dependence is observed in calculations on both Ca$_2$RuO$_4$ and Ca$_3$Ru$_2$O$_7$. The theory model is also generalized to investigate spatially non-homogeneous solutions. Important features of the stripe patterns at the domain boundaries of metallic and insulating phases are discussed and compared with experiments.

Non-stoichiometric strontium iron oxide is described by an abbreviated formula SrFeOx (2.5 ≤ x ≤ 3.0) exhibits a variety of interesting physical and chemical properties over a broad range of temperatures and in different gaseous environments. The oxide contains a mixture of iron in the trivalent and the rare tetravalent state. The material at elevated temperature is a mixed oxygen conductor and it, or its derivatives,can have practical applications in oxygen conducting devices such as pressure driven oxygen generators, partial oxidation reactors in electrodes for solid oxide fuel cells (SOFC). ¶ This thesis examines the behaviour of the material at ambient and elevated temperatures using a broad spectrum of solid state experimental techniques such as: x-ray and neutron powder diffraction,thermogravimetric and calorimetric methods,scanning electron microscopy and Mossbauer spectroscopy. Changes in the oxide were induced using conventional thermal treatment in various atmospheres as well as mechanical energy (ball milling). ¶ ...

Ferroelectric materials are very promising for a variety of applications such as high-permittivity capacitors, ferroelectric memories, pyroelctric sensors, piezoelectric and electrostrictive transducers and electro-optic devices, etc. In the area of ferroelectric ceramics, lead-based compounds, which include lead zirconatetitanate (PZT) solid solutions, occupy an important place because of their superior physical properties. However, due to the toxicity of lead, there is an increasing concern over recycling and disposing of the devices made out of these compounds, which has compelled the researchers around the globe to search for lead-free compounds with promising piezo and ferroelectric properties. Ferroelectric materials that belong to Aurivillius family of oxides have become increasingly important from the perspective of industrial applications because of their high Curie-temperatures, high resistivity, superior polarization fatigue resistanceand stable piezoelectric properties at high temperatures. These bismuth layer-structured ferroelectrics (BLSF) comprise an intergrowth of [Bi2O2]2+ layers and [An+1Bn O3n+1]2- pseudo-perovskite units, where ‘n’ represents the number of perovskite-like layers stacked along the c-axis. ‘A’ stands for a mono-, di- or trivalent ions or a combination of them, ‘B’ represents a small ion with high valencysuch as Ti4+, Nb5+, Ta5+or a combination of them.Ferroelectricity in the orthorhombic phase of these compounds was generally attributed to the cationic displacement along the polar a-axis and the tilting of octahedra around the a- and c-axes. Sr2Bi4Ti5O18(SBT) is ann = 5 member of the Aurivillius family and possess promising ferroelectric and piezoelectric properties that could be exploited for a wide range of applications, including ferroelectric random access memories (FeRAM), piezoelectric actuators, transducers and transformers. Reports in the literaturereveal that the ferroelectricand piezoelectric properties of these oxides can be tuned depending on synthesis routes vis-a-vis micro-structural aspects (texture, grain size) and site specific dopant substitutions.In the present study, textured SBT ceramics were fabricated using pre-reacted precursors and their anisotropic dielectric, piezoelectric and ferroelectric properties were demonstrated. Grain size tunability with regard to their physical properties was accomplished in the ceramics, fabricated using fine powders obtained from citrate assisted sol-gel synthesis. The grain size dependent second harmonic generation activity of SBT ceramics was investigated. Enhancement in the piezoelectric and ferroelectric properties of SBT ceramics was achieved by substituting A site ions (Sr2+) with a combination of Na+ and Bi3+. From the perspective of non-linear optical device applications, physical properties associated with the SBT crystallized in a transparent lithium borate glass matrix were studied. The results obtained in the present investigations are organized as follows, Chapter 1 gives a brief exposure to the field of ferroelectrics. The emphasis has been on the ferroelectric oxides belonging to the Aurivillius family. Structural aspects and the underlying phenomena associated with ferroelectricity in these compounds are discussed. A brief introduction to the glasses, thermodynamic aspects of glass formation and fabrication of glass- ceramics are included. Basic principles involved in the non-linear optical activities are highlighted. Chapter 2 describes the various experimental techniques that were employed to synthesize and characterize the materials under investigation. The experimental details pertaining to the measurement of various physical properties are included. Chapter 3 deals with the fabrication of Sr2Bi4Ti5O18 ceramics using the pre-reacted Bi4Ti3O12 and SrTiO3 powders viasolid-state reaction route. These in stoichiometric ratio were uniaxially pressed and sintered at 1130oC for 3 h resulting in textured Sr2Bi4Ti5O18 ceramics. The obtained dense ceramics exhibited crystallographic anisotropy with prominent c-axis oriented grains (Lotgering factor of 0.62) parallel to the uniaxially pressed direction. The resultant anisotropy in the ceramics was attributed to the reactive template-like behavior of Bi4Ti3O12 that was used as a precursor to fabricate Sr2Bi4Ti5O18 ceramics. Dielectric, ferro and piezoelectric properties measured on the ceramics in the direction perpendicular to the uniaxially pressed axis were found to be superior to that measured in the parallel direction. Chapter 4 reports the details pertaining to the synthesis of strontium bismuth titanate (Sr2Bi4Ti5O18) powders comprising crystallites of average sizes in the range of 94–1400 nm via citrate-assisted sol-gel route. X-ray powder diffraction, Transmission Electron Microscopy (TEM) and Raman spectroscopy were employed for the structural studies. A crystallite size-dependent variation in the lattice parameters and the shift in the Raman vibration modes were observed. Second harmonic signal (532 nm) intensity of the Sr2Bi4Ti5O18 powders increased with the increase in the average crystallite size and the maximum intensity obtained in the reflection mode was 1.4 times as high as that of the powdered KH2PO4. Piezo force microscopic analyses carried out on an isolated crystallite of size 74 nm, established its single domain nature with the coercive field as high as 347 kV/cm. There was a systematic increase in the d33 value with an increase in the size of the crystallite and a high piezoelectric coefficient of ~27 pm/V was obtained from an isolated crystallite of size 480 nm. Chapter 5 illustrates the details concerning the fabrication of Sr2Bi4Ti5O18(SBT) ceramics with different grain sizes (93 nm–1.42 μm) using nano-crystalline powders synthesized via citrate assisted sol-gel method. The grain growth in these powder compacts was found to be controlled via the grain boundary curvature mechanism, associated with anactivation energy of 181.9 kJ/mol. Interestingly with a decrease in grain size there was an increase in the structural distortion which resulted in a shift of Curie-temperature (phase transition) towards higher temperatures than that of conventional bulk ceramics. Extended Landau phenomenological theory for the ferroelectric particles was invoked to explain experimentally observed size dependent phase transition temperature and the critical size for SBT is predicted to be 11.3 nm. Grain size dependent dielectric, ferroelectric and piezoelectric properties of the SBT ceramics were studied and the samples comprising average grain size of 645 nm exhibited superior physical properties that include remnant polarization (2Pr) = 16.4 μC cm-2, coercive field (Ec) = 38 kV cm-1, piezoelectric coefficient (d33) = 22 pC N-1 and planar electromechanical coupling coefficient (kp) = 14.8 %. In Chapter 6, the studies pertaining to the fabrication of Sr(2-x)(Na0.5Bi0.5)xBi4Ti5O18 (SNBT) ceramics for various x values (0, 0.1, 0.25, 0.3, 0.4 and 0.5), using fine powders synthesized via sol-gel route are dealt with. X-ray powder diffraction, transmission electron microscopy and Raman spectroscopic studies were carried out to confirm composition dependent structural changes taking place in the SNBT ceramics. Scanning electron microscopic studies carried out on ceramics revealed that dopants played an important role in inhibiting the grain growth. Dielectric constants of the ceramics were found to decrease with an increase in ‘x’. The increase in Curie temperature with increase in ‘x’ is attributed to the decrease in the tolerance factor. Particularly,x = 0.3 composition of the SNBT ceramics exhibited better piezo and ferroelectric properties with a higher Curie-temperature (569 K). The piezoelectric coefficient (d33) and the planar electromechanical coupling coefficient (kp) of SNBT(x = 0.3) were enhanced by 25% and 42% respectively as compared to that of the undoped ceramics. Chapter 7 deals with the glasses in the system (100 –x) {Li2O + 2B2O3} ─x {2SrO + 2Bi2O3 +5TiO2} (where, x = 10, 25 and 35) fabricated via conventional melt-quenching technique. The amorphous and glassy characteristics of the samples were confirmed respectively using X-ray diffraction (XRD) and differential scanning calorimetric (DSC) methods. All the compositions under investigation exhibited two distinct crystallization peaks (exothermic peaks in the DSC traces): the first peak at ~ 545 °C and the second at ~610 °C that were found to be associated with the crystallization of the phases (as confirmed from the XRD studies) Sr2Bi4Ti5O18 (SBT)and Li2B4O7 (LBO) respectively. Non-isothermal crystallization kinetics (using modified Ozawa-type plots) for SBT crystallization in the LBO glass matrix for the compositions x = 10 and 35, indicated three dimensional growth of the crystallites from pre-existing nuclei present in the as-quenched samples and their effective activation energies for crystallization were found to be around 686 ± 85 kJ/mol and 365 ± 53 kJ/mol, respectively. The optical band gap of the as-quenched glasses for the composition x = 35 was 2.52 eV, is less than that of the composition x = 10 (2.91 eV). The Urbach energies for the as-quenched glasses of compositions x = 10, 25 and 35 were found to be 118 ± 2 meV, 119 ± 2 meV and 192 ± 1 meV respectively.The glasses associated with the composition x = 35, on controlled heat-treatment at 515 °C for various durations (1―20 h), yielded glass-ceramics comprising SBT nano-crystals (18―28 nm) embedded in the LBO glass matrix. Compressive strain in the nano-crystallites of SBT, analyzed using Williamson-Hall method was found to decrease with an increase in the crystallite size. The second harmonic generation signal (532 nm) intensity emanating from glass-nanocrystal composites comprising 22.1 nm SBT crystallites was nearly 0.3 times that of a KDP single crystal. Although each chapter is provided with conclusions and a list of references, thesis ends with a separate summary and conclusions.

Zur Messung pyrelektrischer Koeffzienten wurde ein Messplatz nach einem erweiterten SHARP-GARN-Verfahren entwickelt und zur Untersuchung von Phasenumwandlungen in Pyroelektrika eingesetzt. Einerseits konnten pyroelektrische Messungen im elektrischen Feld die Pyroelektrizität einer neuen durch elektrisch angetriebene Defektmigration erzeugten Phase in Strontiumtitanat nachweisen. Andererseits gelang es, Ferroelektrizität in der Hochtemperaturphase von Poly(Vinylidenfluorid-Trifluorethylen), mittels phasenreiner Präparation der Hochtemperaturphase unterhalb der CURIEtemperatur und anschließender Polarisierung, nachzuweisen. Ferner ließen sich mittels thermisch angeregter Pyroelektrika Redoxprozesse antreiben, was durch Desinfektion von Escherichia coli Bakterien in wässriger Lösung mittels Lithiumniobat und -tantalat gezeigt wurde. Die Hypothese der Desinfektion durch reaktive Sauerstoffspezies konnte durch spektroskopisch nachgewiesene OH-Radikale - erzeugt mittels thermisch angeregter Bariumtitanatnanopartikel - belegt werden.