Thèses sur le sujet « Novel Oxides - Magnetic Properties »

Metal oxide nanoparticles, both magnetic and nonmagnetic, have a multitude of applications in gas sensors, catalysts and catalyst supports, airborne trapping agents, biomedicines and drug delivery systems, fuel cells, laser diodes, and magnetic microwaves. Over the past decade, an inexpensive, simple, recyclable, and environmentally friendly large, scale synthesis method for the synthesis of these metal oxide nanoparticles has been sought. Many of the current techniques in use today, while good on the small, laboratory bench scale, suffer from drawbacks that make them unsuitable for the industrial scale. The aminolytic method, developed by Dr. Man Han while working for Dr. Zhang, fits industrial scale-up requirements. The aminolytic method involves a reaction between metal carboxylate(s) and oleylamine in a non-coordinating solvent. This system was shown to produce a range of spinel ferrites. Dr. Lisa Vaughan showed that this method can be recycled multiple times without degrading the quality of the produced nanoparticles. The purpose of this thesis is to test the versatility of the aminolytic method in the production of a wide range of metal oxides as well as various core/shell systems. Chapter 2 explores the effect of precursor carboxylates chain length on the aminolytic synthesis of cobalt ferrite, and manganese ferrite nanoparticles. In Chapter 3, a series of CuxMn1-xFe₂O₄, (x ranges from 0.0 to 0.2), nanoparticles were synthesized via the aminolytic method. This series allows for the investigation of the effects of orbital Jahn-Teller distortion as well as orbital angular momentum on the magnetic properties of this ferrite. The quantum couplings of magnetic ions in spinel ferrites govern their magnetic properties and responses. An understanding of the couplings between these metal ions allows for tailoring magnetic properties to obtain the desired response needed for various applications. Chapter 4 investigates the synthesis of MnO and Mn₃O₄ nanoparticles in pure single phase with high monodispersity. To the best of our knowledge, the range of sizes produced for MnO and Mn₃O₄ is the most extensive, and therefore a magnetic study of these systems shows some intriguing size dependent properties. The final part of this chapter investigates the applicability of the aminolytic method for building a MnO shell on a CoFe₂O₄ core. Chapter 5 explores the synthesis of another metal oxide, ZrO₂ in both the cubic and monoclinic phases with no impurities. The use of the aminolytic method here removes the need for dangerous/expensive precursors or equipment and eliminates the need for extensive high temperature heat treatments that destroy monodispersity which is required for most techniques. The creation of a core/shell system between CoFe₂O₄ and ZrO₂ using the aminolytic method was also tested. This core/shell system adds magnetic manipulation which is especially useful for the recovery of zirconia based photocatalyst. Chapter 6 studies the application of the aminolytic method in the synthesis of yttrium iron garnet (YIG) and yttrium iron perovskite (YIP) nanoparticles. Current synthesis techniques used to produce YIG and YIP nanoparticles often requires high temperatures, sensitive to contamination, which could be eliminated through the use of our method

AFM materials with triangular cationic sublattices give rise to geometric magnetic frustration. The goal of this project is to study frustrated systems by designed and synthesis of more frustrated systems. For this purpose, the 5d system of osmium transition metal oxide was chosen. The osmium-based compounds are then successfully synthesized using the conventional solid state method. The crystal structures are then characterized by different techniques such as X-ray diffraction and neutron diffraction. To determine the ordering of the crystal systems, magnetic susceptibility and heat capacity measurements are carried out. By employing the spin dimer analysis, magnetic exchange interactions are calculated. These novel osmium-based ordered rock salt structure type systems are then being compared to similar crystal systems in the text.

This thesis describes the growth, structural characterisation, magnetic and magneto-optics properties of lanthanum strontium manganite (LSMO), GdMnO3 and transition metal (TM)-doped In2O3 thin films grown under different conditions. The SrTiO3 has been chosen as a substrate because its structure is suitable to grow epitaxial LSMO and GdMnO3 films. However, the absorption of SrTiO3 above its band gap at about 3.26 eV is actually a limitation in this study. The LSMO films with 30% Sr, grown on both SrTiO3 and sapphire substrates, exhibit a high Curie temperature (Tc) of 340 K. The magnetic circular dichroism (MCD) intensity follows the magnetisation for LSMO on sapphire; however, the measurements on SrTiO3 were dominated by the birefringence and magneto-optical properties of the substrate. In the GdMnO3 thin films, there are two well-known features in the optical spectrum; the charge transfer transition between Mn d states at 2 eV and the band edge transition from the oxygen p band to d states at about 3 eV; these are observed in the MCD. This has been measured at remanence as well as in a magnetic field. The optical absorption at 3 eV is much stronger than at 2 eV, however, the MCD is considerably stronger at 2 eV. The MCD at 2 eV correlates well with the Mn spin ordering and it is very notable that the same structure appears in this spectrum, as is seen in LaMnO3. The results of the investigations of Co and Fe-doped In2O3 thin films show that TM ions in the films are TM2+ and substituted for In3+. The room temperature ferromagnetism observed in TM-doped In2O3 is due to the polarised electrons in localised donor states associated with oxygen vacancies. The formation of Fe3O4 nanoparticles in some Fe-doped films is due the fact that TM-doped In2O3 thin films are extremely sensitive to the growth method and processing condition. However, the origin of the magnetisation in these films is due to both the Fe-doped host matrix and also to the nanoparticles of Fe3O4

The magnetic properties of the first row transition metal oxides are wide and varied and have been studied extensively since the 1930’s. Observations that the magnetic properties of these material types change with the dimension of the sample have stimulated many theoretical and experimental studies of the systems involved. As sample sizes decrease towards the nanoscale long range crystallographic order is no longer possible. However, the application of mesoporous silica samples as hard exo-templates to direct the formation of mesoporous metal oxides has provided a new opportunity to explore the influence of scale of crystallographic order further. These types of samples have pore systems running through the material on the mesoscale (diameter between 2nm to 50nm) with pore walls truly in the nanoscale region (7nm to 9nm thick) crystallographically ordered over large scale distances. The work presented in this thesis presents magnetic and crystallographic studies of a variety of the first row transition metal oxides from chromium to nickel in three dimensional mesoporous forms predominantly using SQUID magnetometry and neutron powder diffraction. Rietveld refinements of diffraction data from hematite and eskolaite (®-Fe2O3 and Cr2O3) show that the samples have space groups identical to their bulk counterparts, however slight differences in lattice parameters are observed. Refinement of magnetic properties has also been performed and compared to magnetic property measurements. Of particular interest are results from a mesoporous hematite which show suppression of a well defined first-order magnetic phase transition (the Morin transition). This suppression has been studied extensively with neutron powder diffraction and preliminary inelastic neutron spectroscopic measurements. Comparisons with hematite nanoparticles which also show the suppression of the Morin transition can be drawn. Parametric neutron powder diffraction studies on Co3O4 and NiO samples shows that the Néel ordering temperatures are lowered as the mesoporous structure is imposed. This too was observed in eskolaite. Other studies have been carried out on mesoporous alpha-MnO2 (magnetometry) and nanoscale Li1+xMn2–xO4 (X-ray photo electron spectroscopy) with comparisons to their bulk counterparts and finally nanoparticulate hausmannite Mn3O4 (magnetometry and muon spin relaxation) which exhibits spin-glass type behaviour.

Complex lanthanide oxide systems are known to host novel phases of matter, while also providing functionality for practical applications. In this dissertation, the structural, magnetic and magnetocaloric properties of three families of lanthanide oxides have been studied with the dual aims of investigating the magnetic behaviour and identifying promising magnetic refrigerants for cooling to temperatures currently accessible using non-renewable liquid He. The thesis presents a two-part study of the magnetic and magnetocaloric properties of the geometrically frustrated lanthanide garnets, where the magnetic $Ln^{3+}$ form corner-sharing triangles. First, the family of garnets $Ln_3A_2X_3$O$_{12}$, $Ln$ = Gd, Tb, Dy, Ho, $A$ = Ga, Sc, In, Te, $X$ = Ga, Al, Li are investigated. Changes to the single-ion anisotropy of the magnetic ion as well as variations in the chemical pressure radically alters the nature of magnetic ordering, the degree of frustration and the magnetocaloric performance. In the second part, the garnets $Ln_3A$Ga$_4$O$_{12}$, $Ln$ = Gd, Tb, Dy, Ho, $A$ = Cr, Mn, are studied. Introducing additional spins significantly reduces the frustration in the garnet lattice. Low temperature powder neutron diffraction of Ho$_3$MnGa$_4$O$_{12}$ reveals concomitant ordering of Ho$^{3+}$ and Mn$^{3+}$ moments below the ordering temperature, $T_N$ = 5.8 K. The magnetocaloric performance of $Ln$_3CrGa$_4$O$_{12}$, $Ln$ = Gd, Dy, Ho, greatly surpasses that of the parent $Ln_3$Ga$_5$O$_{12}$ at $T$ = 2 K. The final results chapters in the thesis describe the magnetism and magnetocaloric effect in the lanthanide orthoborates, $Ln$BO$_3$ , $Ln$ = Eu, Gd, Tb, Dy, Ho, Er, Yb and the lanthanide metaborates, $Ln$(BO$_2$)$_3$, $Ln$ = Pr, Nd, Gd, Tb. The magnetic $Ln^{3+}$ form slightly distorted edge-sharing triangular layers in $Ln$BO$_3$. Unique magnetic features are observed, including short-range ordering and spin reorientation transitions depending on the single-ion anisotropy of the $Ln^{3+}$. The $Ln$BO$_3$ are also efficient magnetocalorics in the liquid helium temperature range. The lanthanide metaborates contain one-dimensional chains of magnetic lanthanide ions. Bulk magnetic measurements show features consistent with low-dimensional magnetism, such as magnetisation plateaux at one-third of the saturation magnetisation for Nd(BO$_2$)$_3$ and Tb(BO$_2$)$_3$ in a field of 14 T. This thesis provides insight into the fundamental magnetic properties of complex lanthanide oxide systems and also demonstrates strategies for identifying new magnetocaloric materials: both through chemical control of the structure of well-known magnetocalorics and by studying materials that have not been explored previously. The results pave the way for further in-depth investigations and finding new magnetic coolants based on complex lanthanide oxide systems.

Thin films (5nm ± 2nm thick) of ZrO2, Al2O3, TiO2, Sm, Gd, Zr, Ni, and Pt were deposited onto TbFeCo films (100nm ± 20nm thick) on silicon and graphite substrates and analyzed with XPS as prospective candidates for TbFeCo diffusion barriers. Metals were chosen primarily according to electronegativity. Samples were typically heated to 272°C in UHV for 20 hours to enhance diffusion. Experiments with the metals were performed in a more consistent manner than with the oxides. The Sm, Gd, and Zr were reactively oxidized during the deposition. The Sm/Sm-oxide and Gd/Gd-oxide appeared to be favorable candidates for TbFeCo diffusion barriers. TbFeCo was not detected near the surface before or after heating the samples to 272°C for 20 hours and depth profiles indicated oxygen contamination decreased steadily as the barrier/TbFeCo interface was approached. For the other materials examined, either the oxides were reduced (at least partially) during heating to 272°C (381°C for Al₂O₃) or diffusion of TbFeCo was detected after heating, indicating that they would not be favorable candidates for TbFeCo diffusion barriers.

2009 - 2010
This thesis work deals with the magnetic, orbital and transport properties of (LaMnO3)2N(SrMnO3)N digital superlattices and of LaxMnO3-d thin films, both deposited by molecular beam epitaxy (MBE) technique on SrTiO3 substrates. The (LaMnO3)2N(SrMnO3)N digital superlattices represent the new kind of heterostructure, actually under intensive study, composed by an integer number of unit cells, where electronic reconstruction effects at the (clean) interface are expected. In the first part of the thesis work, optical lithography techniques and different deposition techniques are employed in order to perform transport measurement in current perpendicular to plane (CPP) configuration and field effect measurements. The CPP technique would allow to gain more information respect to the in-plane measurements as the ones used in the first part of the thesis. Field effect has been widely investigated in manganite (especially LMO-based) systems, as it represents a method to tune the carrier density, and in order to engineer all oxide field effect devices. The results of the optimization of such techniques, together with the optimization of the materials needed as base electrode, side and gate insulators and top electrode is reported. In the second part of the thesis, both LaMnO3-based systems were studied by traditional techniques (transport measurements, SQUID magnetometer) and soft X-ray absorption and emission techniques by synchrotron radiation. Collecting the data from different measurement techniques, precious information about ferromagnetism, antiferromagnetism and orbital ordering is obtained. [edited by author]
IX n.s.

Magnetic properties of powders, thin films and single crystals have been investigated using magnetometry methods. This thesis provides analysis and conclusions that are supported by the results obtained from spectroscopic and diffraction measurements as well as from theoretical calculations. First, the magnetic behavior of transition metal (TM) doped ZnO with respect to doping, growth conditions and post annealing has been studied. Our findings indicate that the magnetic behavior stems from small clusters or precipitates of the dopant, with ferromagnetic or antiferromagnetic interactions. At the lowest dopant concentrations, the estimated cluster sizes are too small for high resolution imaging. Still, the clusters may be sufficiently large to generate a finite spontaneous magnetization even at room temperature and could easily be misinterpreted as an intrinsic ferromagnetic state of the TM:ZnO compound. Second, influence of lattice strain on both magnetic moment and anisotropy has been investigated for epitaxial MnAs thin films grown on GaAs substrates. The obtained magnetic moments and anisotropy values are higher than for bulk MnAs. The enhanced values are caused by highly strained local areas that have a stronger dependence on the in-plane axis strain than out-of plane axis strain. Finally, spin glass behavior in Li-layered oxides, used for battery applications, and a double perovskite material has been investigated. For both Li(NiCoMn)O2 and (Sr,La)MnWO6, a mixed-valence of one of the transition metal ions creates competing ferromagnetic and antiferromagnetic interactions resulting in a low temperature three-dimensional (3D) spin glass state. Additionally, Li(NiCoMn)O2 with large cationic mixing exhibits a percolating ferrimagnetic spin order in the high temperature region and coexists with a two-dimensional (2D) frustrated spin state in the mid temperature region. This is one of the rare observations where a dimensional crossover from 2D to 3D spin frustration appears in a reentrant material.
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 720

Studies to compare and elucidate some of the surface chemical characteristics of chromium dioxide and barium ferrite magnetic particles have been performed. These investigations primarily involved using the technique of flow microcalorimetry to look at the interaction of molecular probes (pyridine, 4-nitrophenol) and various components present in a magnetic ink (dispersants and model binder compounds) with the particles. The interactions were measured in terms of heats of adsorption and adsorption density. Ancillary experiments using FTIR and XPS were also performed. Both electrophoretic measurements and calorimetric studies showed that stabilized CrO₂ was less acidic than un-stabilized CrO₂. Similar measurements showed that un-doped barium ferrite was more acidic than Co and Ti doped barium ferrite. The interactions of dispersants and model binder components with barium ferrite were found to be very exothermic; heats of interaction of greater than -20 kcal/mole were not uncommon. Subsequent analysis of barium ferrite particles treated with dispersants and binder compounds using FTIR and XPS suggested that these compounds formed chemical complexes on the surface of the ferrite particles.

The materials synthesised in this study were designed to have novel morphology coupled with a tightly controlled surface composition that could be varied depending on a application. Preparations with simple single metal oxides (i.e. TiO2, SiO2, Al2O3 and ZrO2) were used as the starting point with the latter stages involving multi-metal oxide coatings and materials. The research was divided into three interconnected areas; i) biotemplating, ii) alternative synthetic morphologies to biotemplating and iii) the synergy between microparticles and insecticides. For the investigation into biotemplating pollen was chosen as the main example due to its ubiquity. Here, good replication of its structure with metal oxides can be achieved by two-dimensional solgel chemistry. Such materials can be further modified to have tunable surface chemistry through dopants and optical properties (i.e. fluorescence) through the use of dyes. Materials were extensively characterised using primarily spectroscopy (UV and IR) and microscopy (i.e. SEM coupled with EDX elemental analysis). These were considered for several applications and examples investigated here included as a taggant technology and photocatalytic removal of methyl orange in an aqueous environment (TiO2-pollen only). For the latter, results have been compared with those of a commercially available alternative (P25) where the preliminary results are very promising. The method of overcoating was also shown to be transferrable to other flora and fauna biotemplates. Synthetic alternatives for the biotemplated pollen were considered in the second investigative area where solution sol-gel processes such as the Stöber method were considered in addition to other suspension based precipitation methods (i.e. refluxes and microemulsions). Processes developed in the biotemplating research were applied here and analysed again using spectroscopy and microscopy as the main techniques. As part of this aspect, a novel fast-drying water-in-oil microemulsion delivery and preparative system was also developed using low boiling point solvents such as isopentane and ethanol and low toxicity sucrose ester surfactants. Hollow oxide shells could be prepared in these using a novel low-temperature route that were comparable in thickness (but significantly smaller in size) to hollow pollen replicas. In this second area attention was shifted to more focus on oxides of Si and Al (as opposed to TiO2 that used extensively in biotemplating) to broaden the scope of the research and investigate other potential applications, such as nanoabrasives (surface roughness and ability to cleave DNA). The third and final area of interest used the materials from the previous two aspects in coatings that were applied to investigating the knockdown (KD) and total mortality (TM) of selected arthropods. Here mosquitoes of the A. Gambiae and S. Aegypti genus were considered with particular focus on synergistic effects with existing commercial insecticides (using mainly CDC bottle tests). Microscopy was used as the primary characterisation technique here to determine particle transfer after each assay. In these tests %TM suggested SiO2 microspheres were particularly effective at in enhancing mortality of the commercial l-cyhalothrin insecticide. Additionally, novel methods of recording mosquito behaviour was investigated through optical and thermographic stills and videos.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.
Includes bibliographical references (p. 119-124).
Transition metal oxides comprise a fascinating class of materials displaying a variety of magnetic and electronic properties, ranging from half-metallic ferromagnets like CrO2, ferrimagnetic semiconductors like Fey's, and antiferromagnetic insulators like rocksalt-structured FeO. The accessibility of multiple electronic configurations and coordination of cations in these oxides enables the control of magnetism by external stimuli. One such stimulus is the insertion of Li+, as occurs during the discharge cycle of a lithium battery. This can lead to the change in valence and locations of the metal cations within the structure therefore a change in magnetic moment. Fey's and CrO2 are of considerable interest, primarily because they demonstrate room-temperature magnetism and high spin polarization.Previous studies focussed on use of these materials as cathodes and characterization of lithiated compounds made through solid state chemical synthesis or via chemical lithiation. In this work, changes in magnetization and structure of pulsed laser deposition (PLD)-grown Fey's (magnetite) thin films, Fe3O4 nanoparticles, and CrO2 nanoparticles have been investigated upon electrochemical lithiation. The reasonable electrical conductivity of magnetite opens the possibility of modifying the saturation magnetization by inserting Li+ ions into thin films grown on conducting substrates. A substantial decrease in M8 (up to 30%) was observed in PLD-grown thin films. Significantly larger reduction in moment (up to 75%) was observed in commercially available nanoparticles upon addition of 2 moles of Li per formula unit, along with changes in remanence and coercivity. The smaller drop in M8 observed in thin films is attributed to a kinetic effect due to high density and greater diffusion lengths in PLD-grown films.
(cont.) The electrochemical lithiation process has also been applied to needle-shaped particles of chromium dioxide and a model has been proposed to explain the observations. The effects of cycling and discharge-charge rate on these CrO2 particles have been studied. It has been shown that the process may be partially reversible for low Li contents. The effects of increasing the temperature of cycling and decreasing the length of the CrO2 particles have been explored. These changes in magnetic moment may be rendered useful in magnetomechanical or magnetoelectronic applications.
by Vikram Sivakumar.
Ph.D.

Transition metal compounds, especially the oxides, containing dn (0 ≤ n ≤ 10) electronic configuration, constitute the backbone of solid state/materials chemistry aimed at realization of novel materials properties of technological importance. Some of the significant materials properties of current interest are spin-polarized metallic ferromagnetism, negative thermal expansion, second harmonic nonlinear optical (NLO) susceptibility, fast ionic and mixed electronic/ionic conductivity for application in solid state batteries, and last but not the least, high-temperature superconductivity. Typical examples for each one of these properties could be found among transition metal oxides. Thus, alkaline-earth metal (A) substituted rare-earth (Ln) manganites, Lnı.xAxMnΟ3, are currently important examples for spin-polarized magnetotransport, ZrV2O7 and ZrW2O8 for negative thermal expansion coefficient, KTiOPO4 and LiNbO3 for second harmonic NLO susceptibility, (Li, La) TiO3 and LiMn2O4 for fast-ionic and mixed electronic/ionic conductivity respectively, and the whole host of cuprates typified by YBa2Cu3O7 for high Tc superconductivity. Solid state chemists constantly endeavour to obtain structure-property relations of solids so as to be able to design better materials towards desired properties. Synthesis coupled with characterization of structure and measurement of relevant properties is a common strategy that chemists adopt for this task. The work described in this thesis is based on such a broad-based chemists' approach towards understanding and realization of novel materials properties among the family of metal oxides. A search for metallic ferro/ferrimagnetism among the transition metal perovskite oxides, metallicity and possibility of superconductivity among transition-metal substituted cuprates and second order NLO susceptibility among metal oxides containing d° cations such as Ti(IV), V(V) and Nb(V) - constitute the main focus of the present thesis. New synthetic strategies that combine the conventional ceramic approach with the chemistry-based 'soft1 methods have been employed wherever possible to prepare the materials. The structures and electronic properties of the new materials have been probed by state-of-the art techniques that include powder X-ray diffraction (XRD) together with Rietveld refinement, electron diffraction, thermogravimetry, measurement of magnetic susceptibility (including magnetoresistance), Mossbauer spectroscopy and SHG response (towards 1064 nm laser radiation), besides conventional analytical techniques for determination of chemical compositions. Some of the highlights of the present thesis are: (i) synthesis of new mixed valent [Mn(III)/Mn(IV)] perovskite-type manganites, ALaMn2O6-y (A = K, Rb) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr) that exhibit ferromagnetism and magnetoresistance; (ii) investigation of a variety of ferrimagnetic double-perovskites that include ALaMnRuO6 (A = Ca, Sr, Ba) and ALaFeVO6 (A = Ca, Sr) and A2FeReO6 (A = Ca, Sr, Ba) providing new insights into the occurrence of metallic and nonmetallic ferrimagnetic behaviour among this family of oxides; (iii) synthesis of new K2NiF4-type oxides, La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe, Ru) and investigation of Cu-O-M interaction in two dimension and (iv) identification of the structural rnotif(s) that gives rise to efficient second order NLO optical (SHG) response among d° oxides containing Ti(IV), V(V), Nb(V) etc., and synthesis of a new SHG material, Ba2-xVOSi2O7 having the fresnoite structure. The thesis consists of five chapters and an appendix, describing the results of the investigations carried out by the candidate. A brief introduction to transition metaloxides, perovskite oxides in particular, is presented in Chapter 1. Attention is focused on the structure and properties of these materials. Chapter 2 describes the synthesis and investigation of two series of anion-deficient perovskite oxides, ALaMn2O6-y (A = K, Rb, Cs) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr). ALaMn2O6-y (A = K, Rb, Cs) series of oxides adopt 2 ap x 2 ap superstructure for K and Rb phases and √2 av x √2 ap x 2 ap superstructure (ap = perovskite subcell) for the Cs phase. Among ALaBMn3O9-y phases, the A = Na members adopt a new kind of perovskite superstructure, ap x 3 ap, while the A = K phases do not reveal an obvious superstructure of the perovskite. All these oxides are ferromagnetic (Tc ~ 260-325 K) and metallic exhibiting a giant magnetoresistance behaviour similar to alkaline earth metal substituted lanthanum manganites, Lai_xAxMnO3. However, unlike the latter, the resistivity peak temperature Tp for all the anion-deficient manganites is significantly lower than Tc. In Chapter 3, we have investigated structure and electronic properties of double-perovskite oxides, A2FeReO6 (A = Ca, Sr and Ba). The A = Sr, Ba phases are cubic (Fm3m) and metallic, while the A = Ca phase is monoclinic (P2yn) and nonmetallic. All the three oxides are ferrimagnetic with Tcs 315-385 K as reported earlier. A = Sr, Ba phases show a negative magnetoresistance (MR) (10-25 % at 5 T), while the Ca member does not show an MR effect. 57Fe Mossbauer spectroscopy shows that iron is present in the high-spin Fe3+ (S = 5/2) state in Ca compound, while it occurs in an intermediate state between high-spin Fe2+ and Fe3+ in the Ba compound. Monoclinic distortion and high covalency of Ca-O bonds appear to freeze the oxidation states at Fe+3/Re5+ in Ca2FeRe O6, while the symmetric structure and ionic Ba-O bonds render the FeReO6 array highly covalent and Ba2FeReO6 metallic. Mossbauer data for Sr2FeReO6 shows that the valence state of iron in this compound is intermediate between that in Ba and Ca compounds. It is likely that Sr2FeReO6 which lies at the boundary between metallic and insulating states is metastable, phase-seperating into a percolating mixture of different electronic states at the microscopic level. In an effort to understand the occurrence of metallicity and ferrimagnetism among double perovskites, we have synthesized several new members : ALaMnFeO6 (A = Ca, Sr, Ba), ALaMnRuO6 (A = Ca, Sr, Ba) and ALaVFeO6 (A = Ca, Sr) (Chapter 3). Electron diffraction reveals an ordering of Mn and Ru in ALaMnRuO6 showing a doubling of the primitive cubic perovskite cell, while ALaVFeO6 do not show an ordering. ALaMnRuOs are ferrimagnetic (Tcs ~ 200-250 K) semiconductors, but ALaVFeO6 oxides do not show a long range magnetic ordering . The present work together with the previous work on double perovskites shows that only a very few of them exhibit both metallicity and ferrimagnetism, although several of them are ferrimagnetic. For example, among the series Ba2MReO6 (M = Mn, Fe, Co, Ni), only the M = Fe oxide is both metallic and ferrimagnetic, while M = Mn and Ni oxides are ferrimagnetic semiconductors. Similarly, A2CrMoO6 (A = Ca, Sr), A2CrRe06 (A = Ca, Sr), and ALaMnRuO6 (A = Ca, Sr, Ba) are all ferrimagnetic but not metallic. While ferrimagnetism of double perovskites arise from an antiferromagnetic coupling of B and B' spins through the B-O-B' bridges, the occurrence of metallicity seems to require precise matching of the energies of d-states of B and B' cations and a high covalency in the BB'O6 array that allows a facile electron-transfer between B and B', Bn++B’m+↔B(n+1)++B’(m-1)+ without an energy cost, just as occurs in ReO3 and other metallic ABO3 perovskites. In an effort to understand the Cu-O-M (M = Ti, Mn, Fe, Ru) electronic interaction in two dimension, we have investigated K2N1F4 oxides of the general formula La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe or Ru). These investigations are described in Chapter 4. For M = Ti, only the x = 0.5 member could be prepared, while for M = Mn and Fe, the composition range is 0 < x < 1.0, and for M = Ru, the composition range is 0 < x ≤ 0.5. There is no evidence for ordering of Cu(II) and M(IV) in the x = 0.5 members. While the members of the M = Ti, Mn and Ru series are semiconducting/insulating, the members of the M = Fe series are metallic, showing a broad metal-semiconductor transition around 100 K for 0 < x ≤ 0.15 that is possibly related to a Cu(II)-O-Fe(IV) < > Cu(III)-O-Fe(III) valence degeneracy. Increasing the strontium content at the expense of lanthanum in La2-2xSr2XCui.xFexO4 for x ≤ 0.20 renders the samples metallic but not superconducting. In a search for inorganic oxide materials showing second order nonlinear optical (NLO) susceptibility, we have investigated several borates, silicates and phosphates containing /ram-connected MO6 octahedral chains or MO5 square-pyramids, where M = d°: Ti(IV), Nb(V) or Ta(V). Our investigations, which are described in Chapter 5, have identified two new NLO structures: batisite, Na2Ba(TiO)2Si4O12, containing trans-connectd TiO6 octahedral chains, and fresnoite, Ba2TiOSi2O7, containing square-pyramidal T1O5. Investigation of two other materials containing square-pyramidal TiO5, viz., Cs2TiOP2O7 and Na4Ti2Si8O22. 4H2O, revealed that isolated TiO5 square-pyramids alone do not cause a second harmonic generation (SHG) response; rather, the orientation of T1O5 units to produce -Ti-O-Ti-O- chains with alternating long and short Ti-0 distances in the fresnoite structure is most likely the origin of a strong SHG response in fresnoite. Indeed, we have been able to prepare a new fresnoite type oxide, Ba2.xVOSi2O7 (x ~ 0.5) that shows a strong SHG response, confirming this hypothesis. In the Appendix, we have described three synthetic strategies that enabled us to prepare magnetic and NLO materials. We have shown that the reaction CrO3 + 2 NH4X > CrO2 + 2 NH3 + H2O + X2 (X = Br, I), which occurs quantitatively at 120-150 °C, provides a convenient method for the synthesis of CrO2. Unlike conventional methods, the method described here does not require the use of high pressure for the synthesis of this technologically important material. For the synthesis of magnetic double perovskites, we have developed a method that involves reaction of basic alkali metal carbonates with the acidic oxides (e.g. Re2O7) first, followed by reaction of this precursor oxide with the required transition metal/transition metal oxide (e.g. Fe/Fe2O3). By this method we have successfully prepared single-phase perovskite oxides, A2FeReO6, ACrMoO6 and ALaFeVO6. We have prepared the new NLO material Ba2_xV0Si207 from Ba2VOSi2O7 by a soft chemical redox reaction involving the oxidation of V(IV) to V(V) using Br2 in CH3CN/CHCI3. Ba2V0Si207 + 1/2 Br2 > Bai.5V0Si207 + 1/2 BaBr2. The work presented in this thesis was carried out by the candidate as part of the Ph.D. training programme. He hopes that the studies reported here will constitute a worthwhile contribution to the solid state chemistry of transition metal oxides and related materials.

Transition metal compounds, especially the oxides, containing dn (0 ≤ n ≤ 10) electronic configuration, constitute the backbone of solid state/materials chemistry aimed at realization of novel materials properties of technological importance. Some of the significant materials properties of current interest are spin-polarized metallic ferromagnetism, negative thermal expansion, second harmonic nonlinear optical (NLO) susceptibility, fast ionic and mixed electronic/ionic conductivity for application in solid state batteries, and last but not the least, high-temperature superconductivity. Typical examples for each one of these properties could be found among transition metal oxides. Thus, alkaline-earth metal (A) substituted rare-earth (Ln) manganites, Lnı.xAxMnΟ3, are currently important examples for spin-polarized magnetotransport, ZrV2O7 and ZrW2O8 for negative thermal expansion coefficient, KTiOPO4 and LiNbO3 for second harmonic NLO susceptibility, (Li, La) TiO3 and LiMn2O4 for fast-ionic and mixed electronic/ionic conductivity respectively, and the whole host of cuprates typified by YBa2Cu3O7 for high Tc superconductivity. Solid state chemists constantly endeavour to obtain structure-property relations of solids so as to be able to design better materials towards desired properties. Synthesis coupled with characterization of structure and measurement of relevant properties is a common strategy that chemists adopt for this task. The work described in this thesis is based on such a broad-based chemists' approach towards understanding and realization of novel materials properties among the family of metal oxides. A search for metallic ferro/ferrimagnetism among the transition metal perovskite oxides, metallicity and possibility of superconductivity among transition-metal substituted cuprates and second order NLO susceptibility among metal oxides containing d° cations such as Ti(IV), V(V) and Nb(V) - constitute the main focus of the present thesis. New synthetic strategies that combine the conventional ceramic approach with the chemistry-based 'soft1 methods have been employed wherever possible to prepare the materials. The structures and electronic properties of the new materials have been probed by state-of-the art techniques that include powder X-ray diffraction (XRD) together with Rietveld refinement, electron diffraction, thermogravimetry, measurement of magnetic susceptibility (including magnetoresistance), Mossbauer spectroscopy and SHG response (towards 1064 nm laser radiation), besides conventional analytical techniques for determination of chemical compositions. Some of the highlights of the present thesis are: (i) synthesis of new mixed valent [Mn(III)/Mn(IV)] perovskite-type manganites, ALaMn2O6-y (A = K, Rb) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr) that exhibit ferromagnetism and magnetoresistance; (ii) investigation of a variety of ferrimagnetic double-perovskites that include ALaMnRuO6 (A = Ca, Sr, Ba) and ALaFeVO6 (A = Ca, Sr) and A2FeReO6 (A = Ca, Sr, Ba) providing new insights into the occurrence of metallic and nonmetallic ferrimagnetic behaviour among this family of oxides; (iii) synthesis of new K2NiF4-type oxides, La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe, Ru) and investigation of Cu-O-M interaction in two dimension and (iv) identification of the structural rnotif(s) that gives rise to efficient second order NLO optical (SHG) response among d° oxides containing Ti(IV), V(V), Nb(V) etc., and synthesis of a new SHG material, Ba2-xVOSi2O7 having the fresnoite structure. The thesis consists of five chapters and an appendix, describing the results of the investigations carried out by the candidate. A brief introduction to transition metaloxides, perovskite oxides in particular, is presented in Chapter 1. Attention is focused on the structure and properties of these materials. Chapter 2 describes the synthesis and investigation of two series of anion-deficient perovskite oxides, ALaMn2O6-y (A = K, Rb, Cs) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr). ALaMn2O6-y (A = K, Rb, Cs) series of oxides adopt 2 ap x 2 ap superstructure for K and Rb phases and √2 av x √2 ap x 2 ap superstructure (ap = perovskite subcell) for the Cs phase. Among ALaBMn3O9-y phases, the A = Na members adopt a new kind of perovskite superstructure, ap x 3 ap, while the A = K phases do not reveal an obvious superstructure of the perovskite. All these oxides are ferromagnetic (Tc ~ 260-325 K) and metallic exhibiting a giant magnetoresistance behaviour similar to alkaline earth metal substituted lanthanum manganites, Lai_xAxMnO3. However, unlike the latter, the resistivity peak temperature Tp for all the anion-deficient manganites is significantly lower than Tc. In Chapter 3, we have investigated structure and electronic properties of double-perovskite oxides, A2FeReO6 (A = Ca, Sr and Ba). The A = Sr, Ba phases are cubic (Fm3m) and metallic, while the A = Ca phase is monoclinic (P2yn) and nonmetallic. All the three oxides are ferrimagnetic with Tcs 315-385 K as reported earlier. A = Sr, Ba phases show a negative magnetoresistance (MR) (10-25 % at 5 T), while the Ca member does not show an MR effect. 57Fe Mossbauer spectroscopy shows that iron is present in the high-spin Fe3+ (S = 5/2) state in Ca compound, while it occurs in an intermediate state between high-spin Fe2+ and Fe3+ in the Ba compound. Monoclinic distortion and high covalency of Ca-O bonds appear to freeze the oxidation states at Fe+3/Re5+ in Ca2FeRe O6, while the symmetric structure and ionic Ba-O bonds render the FeReO6 array highly covalent and Ba2FeReO6 metallic. Mossbauer data for Sr2FeReO6 shows that the valence state of iron in this compound is intermediate between that in Ba and Ca compounds. It is likely that Sr2FeReO6 which lies at the boundary between metallic and insulating states is metastable, phase-seperating into a percolating mixture of different electronic states at the microscopic level. In an effort to understand the occurrence of metallicity and ferrimagnetism among double perovskites, we have synthesized several new members : ALaMnFeO6 (A = Ca, Sr, Ba), ALaMnRuO6 (A = Ca, Sr, Ba) and ALaVFeO6 (A = Ca, Sr) (Chapter 3). Electron diffraction reveals an ordering of Mn and Ru in ALaMnRuO6 showing a doubling of the primitive cubic perovskite cell, while ALaVFeO6 do not show an ordering. ALaMnRuOs are ferrimagnetic (Tcs ~ 200-250 K) semiconductors, but ALaVFeO6 oxides do not show a long range magnetic ordering . The present work together with the previous work on double perovskites shows that only a very few of them exhibit both metallicity and ferrimagnetism, although several of them are ferrimagnetic. For example, among the series Ba2MReO6 (M = Mn, Fe, Co, Ni), only the M = Fe oxide is both metallic and ferrimagnetic, while M = Mn and Ni oxides are ferrimagnetic semiconductors. Similarly, A2CrMoO6 (A = Ca, Sr), A2CrRe06 (A = Ca, Sr), and ALaMnRuO6 (A = Ca, Sr, Ba) are all ferrimagnetic but not metallic. While ferrimagnetism of double perovskites arise from an antiferromagnetic coupling of B and B' spins through the B-O-B' bridges, the occurrence of metallicity seems to require precise matching of the energies of d-states of B and B' cations and a high covalency in the BB'O6 array that allows a facile electron-transfer between B and B', Bn++B’m+↔B(n+1)++B’(m-1)+ without an energy cost, just as occurs in ReO3 and other metallic ABO3 perovskites. In an effort to understand the Cu-O-M (M = Ti, Mn, Fe, Ru) electronic interaction in two dimension, we have investigated K2N1F4 oxides of the general formula La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe or Ru). These investigations are described in Chapter 4. For M = Ti, only the x = 0.5 member could be prepared, while for M = Mn and Fe, the composition range is 0 < x < 1.0, and for M = Ru, the composition range is 0 < x ≤ 0.5. There is no evidence for ordering of Cu(II) and M(IV) in the x = 0.5 members. While the members of the M = Ti, Mn and Ru series are semiconducting/insulating, the members of the M = Fe series are metallic, showing a broad metal-semiconductor transition around 100 K for 0 < x ≤ 0.15 that is possibly related to a Cu(II)-O-Fe(IV) < > Cu(III)-O-Fe(III) valence degeneracy. Increasing the strontium content at the expense of lanthanum in La2-2xSr2XCui.xFexO4 for x ≤ 0.20 renders the samples metallic but not superconducting. In a search for inorganic oxide materials showing second order nonlinear optical (NLO) susceptibility, we have investigated several borates, silicates and phosphates containing /ram-connected MO6 octahedral chains or MO5 square-pyramids, where M = d°: Ti(IV), Nb(V) or Ta(V). Our investigations, which are described in Chapter 5, have identified two new NLO structures: batisite, Na2Ba(TiO)2Si4O12, containing trans-connectd TiO6 octahedral chains, and fresnoite, Ba2TiOSi2O7, containing square-pyramidal T1O5. Investigation of two other materials containing square-pyramidal TiO5, viz., Cs2TiOP2O7 and Na4Ti2Si8O22. 4H2O, revealed that isolated TiO5 square-pyramids alone do not cause a second harmonic generation (SHG) response; rather, the orientation of T1O5 units to produce -Ti-O-Ti-O- chains with alternating long and short Ti-0 distances in the fresnoite structure is most likely the origin of a strong SHG response in fresnoite. Indeed, we have been able to prepare a new fresnoite type oxide, Ba2.xVOSi2O7 (x ~ 0.5) that shows a strong SHG response, confirming this hypothesis. In the Appendix, we have described three synthetic strategies that enabled us to prepare magnetic and NLO materials. We have shown that the reaction CrO3 + 2 NH4X > CrO2 + 2 NH3 + H2O + X2 (X = Br, I), which occurs quantitatively at 120-150 °C, provides a convenient method for the synthesis of CrO2. Unlike conventional methods, the method described here does not require the use of high pressure for the synthesis of this technologically important material. For the synthesis of magnetic double perovskites, we have developed a method that involves reaction of basic alkali metal carbonates with the acidic oxides (e.g. Re2O7) first, followed by reaction of this precursor oxide with the required transition metal/transition metal oxide (e.g. Fe/Fe2O3). By this method we have successfully prepared single-phase perovskite oxides, A2FeReO6, ACrMoO6 and ALaFeVO6. We have prepared the new NLO material Ba2_xV0Si207 from Ba2VOSi2O7 by a soft chemical redox reaction involving the oxidation of V(IV) to V(V) using Br2 in CH3CN/CHCI3. Ba2V0Si207 + 1/2 Br2 > Bai.5V0Si207 + 1/2 BaBr2. The work presented in this thesis was carried out by the candidate as part of the Ph.D. training programme. He hopes that the studies reported here will constitute a worthwhile contribution to the solid state chemistry of transition metal oxides and related materials.

The crystallographic and magnetic properties of manganese oxides belonging to the Ruddlesden Popper (RP) series, described by the formula A_n+1B_nO_3n+1, are presented. Compounds having n = 2, 3 and ∞ are discussed in Chapters 3, 4 and 5, respectively, their properties being studied by powder X-ray and neutron diffraction, SQUID magnetometry and magnetotransport measurements. In Chapter 3 (A₃B₂O₇) it is shown that Sr_2-xNd_1+xMn₂O₇ (0 ≤ x ≤ 0.5) and Sr₂PrMn₂O₇ exhibit colossal, negative magnetoresistance (CMR) below 150K. The zero field resistivity signal is reduced by 2-3 orders of magnitude in a field of 14T, at ~4.2K. The compounds Sr₂HoMn₂O₇ and Sr₂YMn₂O₇ show no significant magnetoresistance. In contrast to the tetragonal Sr,Nd and Sr,Pr compositions, these compounds show a symmetry lowering to space group P4₂/mnm, and spin glass freezing on the Mn sublattice at ~20K. Chapter 4 (A₄B₃O₁₀) focuses on A- and B-cation substitutions in the parent compound Ca₄Mn₃O_10-δ. Substitution of Ca by Sr yields Sr₄Mn₃O_10-δ, an orthorhombic (Cmca) compound, composed of trimers of face sharing octahedra. The magnetic susceptibility of this sample is interpreted in terms of direct and indirect antiferromagnetic (AFM) Mn-Mn exchange interactions. The orthorhombic (Pbca) RP phase Ca_3.95La_0.05Mn₃O_10-δ has a magnetic transition at ~114K, suggesting that the antiferromagnetic groundstate, with associated weak ferromagnetism arising through the Dzyaloshinsky-Moriya interaction observed in Ca₄Mn₃O_10-δ, is preserved on introduction of a fraction of La³⁺ dopant cations. Ca_3.95La_0.05Mn₃O_10-δ displays CMR at 4K, with the resistivity signal reduced to 18% of the zero field value, in 14T. B-cation substitution yields Ca₄Mn₂TiO_9.93, an orthorhombic (Pbca) RP phase in which the Mn:Ti cation distribution, deduced from the combined results of anomalous dispersion X-ray experiments and neutron diffraction studies, is 59.8%(2.6):40.2%(2.6) and 70.1%(1.3):29.9%(1.3) across the 4b and 8c octahedral sites, respectively. Neutron diffraction studies at 5K show the presence of only short range magnetic interactions in this insulating material, the resistivity of which is reduced by just 10% at 75K in 14T. Chapter 5 (ABO₃ or A₂BB'O₆) describes the mixed B-cation phases, La₂GaMnO₆ and Nd₂GaMnO₆, containing Mn³⁺, 3d⁴ cations. Both are cation disordered, orthorhombic (Pnma) materials, and Nd₂GaMnO₆ exhibits a static, cooperative Jahn-Teller (JT) distortion. La₂GaMnO₆ contains a relatively higher proportion of dynamic, cooperative JT distortions, and as such exhibits isotropic ferromagnetism at 5OK and 5K (2.80(5)μ_B per Mn aligned along y at 5K), predicted by the 'quasistatic hypothesis', which describes the correlation between electron spin configurations of neighbouring JT cations. The magnetic structure of Nd₂GaMnO₆ at 5K and 1.7K is modelled as 'A_X F_Y G_Z' for the Mn sublattice, and 'F_γ' for the Nd sublattice, using Bertaut's notation, resulting in competing ferromagnetic and antiferromagnetic superexchange interactions on the Mn sublattice along the [010] direction of the unit cell. Both materials are highly insulating and neither displays CMR, with resistivity values in each being reduced to just 96% of the zero field value at 200K, in a field of 14T.

Multiferroic materials exhibit simultaneously, magnetic and electric order. In a magnetoelectric composite structure, a coupling is induced via an interfacial elastic interaction between magnetostrictive and piezoelectric materials enabling the control of the magnetisation by applying an electric field and vice versa. However, despite the potential of such coupling, experimental limits of theoretical models were observed. This work sheds some light on these limits by focusing the research on the chemistry of nanocomposite CoFe2O4 and BaTiO3, particularly at the interfaces where the coupling predominates. A comparison of the most common conductive oxides, Nb doped SrTiO3 and SrRuO3, was made for the bottom electrode application. The variation of conductive properties in Nb-SrTiO3 thin films at high temperature has been quantified when artificially strained and 60 nm SrRuO3 film was found to be the best bottom electrode choice for room temperature use. Epitaxial growth of magnetic CoFe2O4 was achieved on various metal oxide substrates despite large lattice mismatches. Crystallographic properties and strain evaluation were investigated and a Stranski-Krastanov growth mechanism, arising from the PLD deposition, was predominant. A notable drop of magnetisation was observed depending on the growth template, particularly on BaTiO3 substrates, the piezoelectric counterpart of the magnetoelectric structures. However, an encouraging magnetoelectric coupling induced by thermal phase transition of BaTiO3 was revealed. For BaTiO3, a control of the growth direction was realised by varying the deposition pressure, and the existence of both 180° and 90° ferroelectric domains was observed for films up to 300 nm in thickness. However, both the ferroelectric and piezoelectric properties were reduced in the thin films due to the clamping effect of the substrate. Finally, highly crystalline multilayers of CoFe2O4 and BaTiO3 were prepared on SrRuO3 buffered SrTiO3 substrates. It was found that the degradation of both magnetic and ferroelectric properties was proportional to the increase in the number of interfaces. A thorough microscopic study revealed interdiffusion and chemical instability occurring between CoFe2O4 and BaTiO3 at the interface. This undesired effect was partially recovered by the insertion of an ultra thin layer of SrTiO3, acting as a barrier layer at every interface. This research shows how interfacial chemistry need to be understood to achieve high magnetoelectric coupling in these types of epitaxial engineered structures.

[Truncated abstract] In the past decade, the synthesis of superparamagnetic iron oxide nanoparticles (SPIONs) has received considerable attention due to their potential applications in biomedical fields. However, success in size and shape control of the SPIONs has been mostly achieved through organic routes using large quantities of toxic or/and expensive precursors in organic reaction medium at high reaction temperature. This has limited the biomedical applications of SPIONs and therefore, development of a synthetic method under aqueous condition that is reproducible, scalable, environmentally benign, and economically feasible for industrial production is of paramount importance in order to fully realise their practical applications. Spinning Disc Processing (SDP) has been used to synthesise superparamagnetic magnetite (Fe3O4) nanoparticles at room temperature via a modified chemical precipitation method under continuous flow condition and offer a potential alternative to be applied to SPIONs production. SDP has extremely rapid mixing under plug flow conditions, effective heat and mass transfer, allowing high throughput with low wastage solvent efficiency. The synthesis process involves passing ammonia gas over a thin aqueous film of Fe2+/3+ which is introduced through a jet feed close to the centre of a rapidly rotating disc (500-2500 rpm). Synthetic parameters such as precursor concentrations, temperature, flow rate, disc speed, and surface texture influence the particle sizes. ... Magnetic silica microspheres are receiving great attention for possible applications in magnetic targeting drug delivery, bioseparation and enzyme isolation. However, the current available methods for preparation suffer from the setback of low loading of Fe3O4 nanoparticles in the silica microsphere, which result in low magnetic moment, thereby limiting their practical applications. Therefore it is of considerable importance to develop new alternative synthetic methods for fabricating magnetic silica microspheres with high magnetic nanoparticles loading. Superparamagentic Fe3O4 nanoparticles (8-10 nm diameter) and curcumin have been encapsulated in mesoporous silica in a simple multiplestep self assembly approach process with high Fe3O4 nanoparticles loading (37%). The synthesis involves loading of curcumin in the Cetyltrimethylammonium bromide (CTAB) micellar rod in the presence of superparamagnetic Fe3O4 nanoparticles via a parallel synergistic approach. The synthesised magnetic mesoporous silica composite material is stable, superparamagnetic with high saturation magnetisation before and after curcumin leaching experiment. Under physiological pH in phosphate buffer, the curcumin is slowly released over several days. These magnetic mesoporous silica are expected to have great potential as targeted drug delivery systems.