Rozprawy doktorskie na temat „Electronic Properties - Exotic Transition Metal Oxides”

Transition metal oxides constitute a large class of materials with variety of very interesting properties and important technological utility. A subset with perovskite structure has been the subject matter of the current theoretical investigation with an emphasis on their electronic and structural behavior. An analytical and a computational method are used to calculate physical entities like lattice parameters, bulk moduli, band structures, density of electronic states and charge density distributions for various topologies. Results are discussed and compared with the available experimental findings.

Low-dimensional spin systems with S = 1/2 and a singlet ground state attract intensive research efforts because of the quantum nature of their ground state. Since the discovery of the singlet plaquette CaV4O9 compound more attention has been paid to chemical analogues with S = 1/2, V4+ (d1) ions. For example, the V2O5 network exhibits the characteristics of layered and crystallographic shear structures and there is a large vanadium oxide bronze family, MxV2O5 (M = alkali, alkaline earth metals), which shows several original [V2O5] networks. For this reason, the atomic architectures of V2O5 and its derivatives provide a stage where various lower-dimensional quantum critical phenomena can be realised. Compounds in the series LixV2O5 were synthesised via soft chemistry methods and characterised structurally and magnetically via high-resolution x-ray and neutron diffraction and magnetization measurements using a SQUID magnetometer. Additional weaker peaks observed in certain of the diffraction patterns suggested the occurrence of charge ordering of vanadium ions in an incommensurately modulated superstructure for some of the E-LixV2O5 and E'-LixV2O5 phases. Having discovered a possible magnetic transition (Tf ~ 25 K) in one of these dilute-spin phases, Li0.50V2O5, believed to be associated with the low-dimensional, chain-like arrangement of the V4+ (S = 1/2, d1) cations over the non-magnetic V5+(S = 0, d0) network, a local probe, SR, was applied to investigate its ground state and the ground states of other members of the series LixV2O5 with x = 0.55, 0.64 and 0.72 which also exhibit interesting magnetic behaviour. Lack of oscillation in the SR data revealed the absence of any long-range magnetic order for Li0.50V2O5, Li0.55V2O5, Li0.64V2O5, and Li0.72V2O5, supportive of a true one-dimensional ground state, suggesting the formation of domains of one-dimensional antiferromagnetic chains, consistent with the Bonner-Fisher model for low-dimensional magnetic behaviour which provided a good fit to the SQUID data collected.

Cette thèse porte sur les propriétés de transport anormal des oxydes de métaux de transition, en particulier de la surface de SrTiO₃ ou de l’interface entre SrTiO₃ et LaAlO₃. Dans ces systèmes on observe l’apparition de gaz d’électrons bidimensionnels. Des mesures d’Effet Hall non linéaire indiquent que ces gaz sont constitués de plusieurs sortes de porteurs de charge, et que leurs populations varient de manière non monotone sous l’effet du dopage électrostatique. L’effet des propriétés électrostatiques et des corrélations électroniques sur ces variations sont discutées. Celles-ci sont à l’origine de réponses remarquables en ce qui concerne la conversion du spin en charge dans ces systèmes à l’aide d’un modèle de liaisons fortes et de la théorie de la réponse linéaire. Les effets conjoints du spin-orbite atomique et de la brisure de symétrie d’inversion à l’interface verrouille les nombres quantiques de spin, de caractère orbital et d’impulsion des électrons, et induit des textures de spin complexe dans l’espace réciproque. Ces textures sont responsables de l’apparition des effets Edelstein et Hall de spin dans ces hétérostructures et sont caractéristiques de la nature multi-orbitale de ces systèmes électroniques. Enfin nous conduirons une étude ab initio des hétérostructures STO/LAO/STO pour expliquer les observations expérimentales de nouvelles manières de former un gaz d’électrons à ces interfaces d’oxydes. Nous discuterons des rôles respectifs de la chimie, de l’électrostatique et des défauts dans l’apparition de ce gaz
The anomalous transport properties of transition metal oxides, in particular the surface of SrTiO₃ or at the interface between SrTiO₃ and LaAlO₃ is investigated in this thesis. These systems host two-dimensional electron gases. Nonlinear Hall Effect measurements suggest that several species of carriers are present in these systems, and that their population is varying on a nontrivial manner upon electrostatic doping. The role of the electrostatics properties of the electron gas and of the electronic correlations are discussed in this light. Next we discuss the spin to charge conversion of these systems thanks to tight-binding modeling and linear response theory. The complex interplay between atomic spin-orbit coupling and the inversion symmetry breaking at the interface leads to a complex spin-orbital-momentum locking of the electrons, inducing spin textures. These spin textures are responsible for the appearance of the Edelstein and Spin Hall Effect in these heterostructures and are characteristic of the multi-orbital character of these electronic systems. Finally an ab initio study of STO/LAO/STO heterostructures is performed to explain experimental evidence of new ways to produce an electron gas at this interface. The respective roles of the chemistry, electrostatics and defects are discussed

Ces dernières années, les oxydes de métaux de transition ont suscité de grands intérêts du point de vue fondamental et technologique. A cet égard, nous nous concentrons sur deux types d'oxydes : le première, le Perovskite double Pb2FeMoO6, avec un potentiel d'application sur des appareils magnétorésistances et spintroniques ; le deuxième, la zircone ZrO2 avec de excellentes propriétés mécaniques et diélectriques pour être utilisée dans les domaines de matériaux structuraux et fonctionnels. Dans la présente étude, nous utilisons la méthode ab-initio (first-principles calculation) pour étudier les détails des orbites décomposés des structures électroniques et des propriétés magnétiques du Pb2FeMoO6 massif de structure parfaite, massif avec des défauts et en structure de plaque. En même temps, les détails des orbites décomposés des structures électroniques, les propriétés mécaniques, dynamiques et diélectriques de six phases de la ZrO2 (cubique, tétragonale, monoclinique, orthorhombique I (Pbca), orthorhombique II (Pnma) et (Pca21)) ont également été étudiés. D'abord nous allons faire les calculs ab-initio sur les propriétés structurales, électroniques et magnétiques de double pérovskite Pb2FeMoO6 massif avec structure parfaite, massif avec défauts et en structure de plaque. La densité des états orbitaux décomposés montre le champ cristallin octaédrique des six atomes d'oxygène autour de métal de transition (des Fe ou des Mo) et divise les cinq états dégénérés des atomes libres de Fe ou Mo dans un états triplement dégénéré t2g (dxy, dyz et dzx) avec une énergie plus faible et dans un états doublement dégénéré eg (dz2 et dx2-y2) avec une énergie plus élevée. La nature semi-métalliques et les propriétés de transport complètes (100%) de spin de polarisation de Pb2FeMoO6 massif et en structures de plaque reflètent un grand potentiel d’application dans les dispositifs magnéto-résistifs et spintroniques. Le caractère semi-métallique est maintenu pour le composé Pb2FeMoO6 désordonné contenant d’antisites Fe(Mo), de lacunes de VFe, VO ou VPb, alors qu'il disparaît quand les antisites Mo(Fe), les échanges entre Fe-Mo ou les lacunes de VMo sont présents même la concentration de défauts est réduite jusqu'à C = 6,25%. Ainsi, les antisites Mo(Fe), les échanges entre Fe-Mo ou les lacunes de VMo doivent être évités afin de préserver le caractère semi-métallique du composé Pb2FeMoO6 et donc être utilisables dans des dispositifs magnéto-résistifs et spintroniques.Ensuite, basé sur la rigidité élastique constantes individuelle calculée Cij de six phases de ZrO2, les propriétés élastiques et mécaniques des agrégats polycristallins ont été prédits. Nous avons donc examiné le caractère isolant de la phase cubique/tétragonale de ZrO2 sous forme film avec différentes combinaisons et différentes épaisseurs possibles dans des plans avec des faibles indices de Miller [(001), (110) et (111)] (pour la phase cubique) et [(001), (100), (110), (101) et (111)] pour la phase tétragonale. Il se trouve que pour les différentes combinaisons et épaisseurs possibles dans ces trois / cinq plans avec faibles indices de Miller, seulement ZrO2-terminé sous forme d’un film orienté dans le plan (110)/(100) et O-terminé sous forme d’un film orienté (111)/(101) des phases cubique/tétragonale de ZrO2 maintiennent le caractère isolant même les épaisseurs d’empilement est réduit jusqu'à deux et trois couches atomiques. Puisque cubique et tétragonale ZrO2 ont grande anisotropie élastique, comme un exemple, le stress et l'énergie de déformation densité ont été calculées pour tous {hkl} -oriented grains d'un film ZrO2 cubique polycristallin
Transition-metal oxides have attracted exceptional research interest in recent years from both fundamental and technological perspectives. In this respect, we focus on two types of oxides, first, the double perovskite, Pb2FeMoO6 for a potential magnetoresistive and spintronics device application, second, zirconia ZrO2 with great mechanical and dielectric properties can be widely used in both structural and functional material fields. In this thesis we use first-principles calculations (ab-initio) to study systematically the detailed orbital-decomposed electronic structures and magnetic properties of Pb2FeMoO6 in the perfected bulk, defected bulk and slab structures. The detailed orbital-decomposed electronic structures, the mechanical, dynamical and dielectric properties of the ZrO2 in six phases (cubic, tetragonal, monoclinic, orthoI (Pbca), orthoII (Pnma) and (Pca21)) have also been studied.Firstly, considering the comparable ionic radius of Pb2+ (1.49Å) with that of Sr2+ (1.44Å), we propose for the first time to substitute Sr2+ ion with Pb2+ ion in Sr2FeMoO6 and a detailed study has been performed on the Pb2FeMoO6 in the perfected bulk, defected bulk and slab structures. The half-metallic nature and a complete (100%) spin-polarized transport properties reflect the bulk and especially slab Pb2FeMoO6 a potential application in magnetoresistive and spintronics devices; The detailed orbital-decomposed density of states show the octahedral crystal-field of the six oxygen atoms around transition-metal Fe or Mo atoms splits the five-fold degenerate states of the free Fe or Mo atoms into triply degenerate t2g (dxy, dyz and dzx) states with lower energy and doubly degenerate eg (dz2 and dx2-y2) states with higher energy, which cannot be observed in previous partial density of states ( ); The Fe3+ and Mo5+ ions are in the (3d5, s=5/2) and (4d1, s=1/2) states with positive and negative magnetic moments respectively and thus antiferromagnetic coupling via oxygen between them; The half-metallic character is maintained for the disordered Pb2FeMoO6 compounds containing FeMo antisite, VFe, VO, or VPb vacancy, while it vanishes when MoFe antisite, Fe-Mo interchange or VMo vacancy are presented even the defect concentration reduce down to C=6.25%. So the MoFe antisite, Fe-Mo interchange or VMo vacancy defects have to be avoided in order to preserve the half-metallic character of the Pb2FeMoO6 compounds and thus usable in magnetoresistive and spintronics devices.Secondly, based on the calculated individual elastic stiffness constants Cij of six ZrO2 phases, the elastic and mechanical properties of the polycrystalline aggregates have been predicted. We further exam the insulating characters of the cubic/tetragonal ZrO2 slabs with various possible terminations and thicknesses within three [(001), (110) and (111)]/five [(001), (100), (110), (101) and (111)] lower index Miller planes. It is found for the first time that among various possible terminations and thicknesses within these three/five lower index Miller planes, only ZrO2-terminated slabs of the (110)/(100) Miller plane and O-terminated slabs of the (111)/(101) Miller plane of cubic/tetragonal ZrO2 maintain the insulating character and thus usable as a gate dielectric oxide in IC industry even the slab thicknesses reduce down to 2 and 3 atomic layers, respectively; Since cubic and tetragonal ZrO2 have larger elastic anisotropy, both stress and strain energy density have been calculated for all {hkl}-oriented grains of a cubic ZrO2 polycrystalline film as one example

In the 4d transition metal oxides, the extension of the 4d orbitals leads to comparable and thus competitive kinetic and coulomb energies. As a result, small perturbations can induce significant changes in their physical properties, giving rise to a class of exotic phenomena that are rarely found in other materials. The ruthenates materials with readily tunable parameters open an avenue to study the strong electronic correlation in the rarely explored territory: the 4d transition metal oxides. The bilayered system, Ca3Ru2O7, belongs to the Ruddlesden-Popper series in which the physical properties are intimately linked to the lattice degrees of freedom. Ca3Ru2O7, with its quasi-2D and severe structure distortion, is believed to be placed in a unique position at which the role of orbital degrees of freedom is highlighted. The system displays strikingly different behaviors when the field is applied along different crystalline axes. A ferromagnetic (FM) state with full spin polarization is achieved for B||a-axis, but colossal magnetoresistance is realized only for B||b-axis by avoiding the ferromagnetic state. In addition, for B rotating within the ac-plane, slow and strong SdH oscillations periodic in 1/B are observed for T.1.5 K in the presence of metamagnetism. For B|| [110], oscillations are also observed but periodic in B (rather than 1/B) and persist up to 15 K. These properties together with highly unusual spin-charge-lattice coupling near the Mott transition (48 K) are driven by the orbital degrees of freedom. Complex thermodynamic properties are also observed in the other ruthenates system such as Sr4Ru3O10 and Pr3RuO7. The Sr4Ru3O10 is a triple-layered system that shows a dedicate balance between fluctuations and order. Besides the anomaly at TC=102K, anomalous behavior at low temperatures are also observed in the thermal study, indicative of an unusual magnetic order in this material. The Pr3RuO7 shows one-dimensional structure with zig-zag chain of corner sharing RuO6 octahedra running in parallel with the rows of edge-shared PrO8 pseudo-cubes. Magnetic and thermal properties studies on its single crystals indicate that the exchange interaction is strongly anisotropic. A Schottky-type anomaly at low temperature suggests that the gorderedh chain Pr ions are still sensitive to a crystal field.

Amorphous transition metal oxides [aTMOs], have emerged as innovative functional materials for wide-ranging electronic, optical and energy-related applications. However, no systematic and broadly applicable method exists to assess their atomic-scale correlations, and since the optical and electronic processes are local structure-dependent, still there are not well-stablished mechanisms that suitably explain the physical properties of aTMOs. This thesis presents experimental and theoretical studies of the atomic short-range order, optical and electronic properties, and state-defects induced by Li+-ion-intercalation and oxygen-vacancies in amorphous titanium aTiO2 and tungsten aWO3 thin-film oxides. Those properties play a key role for application in high energy-density Li+-ion batteries and in switchable dynamical modulation of solar-irradiation transmittance for energy efficient "smart windows", where the disorder-dependent Li+-ion-intercalation and oxygen-vacancy-induced defect-states influence charge-carrier transfer mechanisms. After introducing the scope of this thesis, the fundamental theoretical concepts describing the experimental findings on amorphous solids are reviewed. Thereafter, a comprehensive analysis on the optical absorption phenomena experimentally observed in oxygen-deficient and Li+-ion-intercalated aLixTiO2−y and aLixWO3−y thin-films and a discussion on the electrochromic properties are presented. The optical absorption is described in the framework of the small polaron absorption model. Finally, a state-of-the-art systematic procedure involving theory and experiment in a self-consistent computational framework is implemented to unveil the atomic-scale structure of aTiO2 and aWO3, and its role for the electronic properties. The procedure is based in Reverse Monte Carlo [RMC] and Finite Difference Method [FDM] simulations of X-ray-Absorption spectra to construct a disordered theoretical model having the same bonding and coordination distribution as the experimental system. Ab-initio molecular dynamics simulations and density functional theory are then used to assess defect-states induced by Li+-ion-intercalation and oxygen-vacancies in aTiO2 and aWO3 oxides. The schemes introduced in this study offer a consistent route to experimentally and theoretically assess the role of the atomic-scale structure on the optical and electronic properties of aTiO2 and aWO3 and could be extended to the study of other aTMOs. The final results provide crucial insight towards the understanding of optical and electronic mechanisms where disorder-dependent ion-intercalation and oxygen-vacancy-induced localized defect-states influence charge transfer mechanisms of crucial importance for wide ranging optical and energy-related application of aTiO2 and aWO3 oxides.

The commercialization of lithium-ion batteries has played a pivotal role in the development of consumer electronics and electric vehicles. In recent years, much research has focused on the development and modification of the active materials of electrodes to obtain higher energies for a broader range of applications. High voltage spinel materials including LiNi_0.5Mn_1.5O_4-δ (LNMO) have been considered as promising cathode materials to address the increasing demands for improved battery performance due to their high operating potential, high energy density, and stable cycling lifetimes. In an effort to elucidate fundamental structure-property relationships, this thesis explores the tunable properties of single-crystal LNMO. Utilizing facile molten salt synthesis methods, the structural and electronic properties of LNMO can be well controlled. Chapter 2 of this thesis focuses on uncovering the effect of molten salt synthesis parameters including molten salt composition and synthetic temperature on the materials properties. A range of imaging, microscopic, and spectroscopic techniques are used to characterize structural and electronic properties which are investigated in tandem with electrochemical performance. Results indicate the Mn oxidation state is highly dependent on synthesis temperature and can dictate performance, while the molten salt composition strongly influences the particle morphology. In Chapter 3, we explore the concept of utilizing LNMO as a tunable support for heterogeneous metal nanocatalysts, where alteration of the support structure and electronics can have an influence on catalytic properties due to unique support effects. Ultimately, this work illustrates the tunable nature of single-crystal LNMO and can inform the rational design of LNMO materials for energy applications.
M.S.
The development of lithium-ion batteries has been fundamental to the expansion and prevalence of consumer electronics and electric vehicles in the twenty-first century. Despite their ubiquity, there is an ongoing drive by researchers to address the limitations and improve the quality and performance of lithium ion batteries. Much research has focused on altering the composition, structure, or properties of electrodes at the materials level to design higher achieving batteries. A fundamental understanding of how composition and structure effect battery performance is necessary to progress toward better materials. This thesis focuses on investigating the properties of LiNi_0.5Mn_1.5O_4-δ (LNMO). LNMO material is considered a promising cathode material to meet the increasing consumer demands for improved battery performance. Through the synthesis methods, the shape of individual particles and the global electronic properties of LNMO can be tuned. In this work, specific synthesis parameters are systematically tuned and the properties of the resultant LNMO materials are explored. Electrochemical testing also evaluates the performance of the materials and offers insights into how they may fair in real battery systems. In an effort to potentially recycle spent battery materials, LNMO is also utilized as a catalyst support. Alteration of shape and electronic properties of the LNMO support can influence the catalytic properties, or the ability of the material to enhance the rate of a chemical reaction. Overall, this thesis explores how LNMO can be tuned and utilized for different applications. This work provides insights for understanding LNMO properties and direction for the development of future battery materials.

Thermoelectric energy (TE) conversion can be used to create electricity from temperature gradients. Hence power can be generated from waste heat using TE materials, e.g. from the exhaust in automotives. This power in turn may lead to a reduction of gas consumption by reducing the alternator load on the engine. Because of the increasing demand and limited availability of energy sources, there is strong and renewed interest in advancing thermoelectric materials. Past research shows that the best TE materials are narrow band gap semiconductors composed of heavy elements, exhibiting a large Seebeck coefficient, S, combined with high electrical conductivity, σ, and low thermal conductivity, κ. Various research projects have been attempted during the past four years of my Ph.D. studies. These include the synthesis, crystal structure studies, electronic structure calculations and thermoelectric properties of transition metal oxides and thallium main group chalcogenides. Because of the good thermal stability, lack of sensitivity to the air, and non-toxicity, transition metal oxides are potential candidates for commercial thermoelectric applications. During the investigation of oxides for thermoelectric application, several interesting features of different transition metal oxides have been discovered: 1. A new quaternary layered transition-metal oxide, Na2Cu2TeO6, has been synthesized under air using stoichiometric mixtures of Na2CO3, CuO and TeO2. Na2Cu2TeO6 crystallizes in a new structure type, monoclinic space group C2/m with a = 5.7059(6) Å, b = 8.6751(9) Å, c = 5.9380(6) Å,  = 113.740(2)°, V = 269.05(5) Å3 and Z = 2, as determined by single crystal X-ray diffraction. The structure is composed of[Cu2TeO6] layers with the Na atoms located in the octahedral voids between the layers. Na2Cu2TeO6 is a green nonmetallic compound, in agreement with the electronic structure calculation and electrical resistance measurement. 2. An n-type narrow band gap semiconductor, LaMo8O14, exhibiting the high Seebeck coefficient of -94 μVK-1 at room temperature has been investigated. 3. Pb0.69Mo4O6 with a new modulated structure and stoichiometry was determined from single-crystal X-ray diffraction data. The compound crystallizes in the tetragonal super space group, P4/mbm(00g)00ss, with a = 9.6112(3) Å, c = 2.8411(1) Å, q = 0.25c*, which is different from the previously reported structure. As for the research of thermoelectric properties of thallium main group chalcogenides, three new ternary thallium selenides, Tl2.35Sb8.65Se14, Tl1.97Sb8.03Se13 and Tl2.04Bi7.96Se13, have been discovered. All three compounds crystallize in the same space group P21/m with different cell parameters, and in part different Wyckoff sites, hence different structure types. The three selenides with similar structures are composed of distorted edge-sharing (Sb,Bi)Se6 octahedra, while the distorted Tl/(Sb, Bi) sites are coordinated by 8 - 9 Se atoms. Electronic structure calculations and physical property measurements reveal they are semiconductors with high Seebeck coefficient but low electrical conductivity, and therefore not good thermoelectrics. On the other hand, our transport property measurements on the unoptimized Tl2SnTe3 sample show interesting thermoelectric properties of this known compound. Advanced thermoelectrics are dominated by antimonides and tellurides so far. The structures of the tellurides are mostly composed of NaCl-related motifs, hence do not contain any Te–Te bonds. All of the antimonide structures containing Sb–Sb bonds of various lengths are much more complex. The Sb atom substructures are Sb24– pairs in β-Zn4Sb3, linear Sb37– units in Yb14MnSb11, planar Sb44– rectangles in the skutterudites, e.g., LaFe3CoSb12, and Sb8 cubes interconnected via short Sb–Sb bonds to a three-dimensional network in Mo3Sb5Te2. The results of electronic structure calculations suggested that these interactions have a significant impact on the band gap size as well as on the effective mass around the Fermi level, which represent vital criteria for advanced thermoelectrics. The crystal structure and electronic structure investigation for the unique T net planar Sb–Sb interactions in Hf5Sb9 will be also presented, although Hf5Sb9 is metallic compound with poor thermoelectric performances.

The two-dimensional electronic systems (2DESs) have kept surprising physicists for the last few decades. Examples include the integer and fractional quantum Hall effects, cuprate superconductivity, and graphene. This thesis is intended to develop suitable theoretical tools which can be generalized to study new types of 2DESs with strong correlation feature. The first part of this thesis describes the investigation of heterostructures made by Mott insulators. This work is mostly motivated by the significant improvement of techniques for layer-by-layer growth of transition metal oxides in the last few years. We construct a toy model based on generalized Hubbard model complemented with long-ranged Coulomb interaction, and we study it by Hartree-Fock theory, dynamical mean-field theory, and Thomas-Fermi theory. We argue that interesting 2D strongly correlated electronic systems can be created in such heterostructures under several conditions. Since these 2D systems are formed entirely due to the gap generated by electron-electron interaction, they are not addiabatically connected to a noninteracting electron states. This feature makes these 2D systems distinguish from the ones created in semiconductor heterostructures, and they may be potential systems having non-Fermi liquid behaviors. The second part of this thesis is devoted to the study of collective excitations in high-temperature superconductors. One important achievement in this work is to develop a time-dependent mean-field theory for t-U-J-V model, an effective low energy model for cuprates. The time-dependent mean-field theory is proven to be identical to the generalized random-phase approximation (GRPA) which includes both the bubble and ladder diagrams. We propose that the famous 41 meV magnetic resonance mode observed in the inelastic neutron scattering measurements is a collective mode arising from a conjugation relation, which has been overlooked in previous work, between the antiferromagnetic fluctuation and the phase fluctuation of the d-wave superconducting order parameter near momentum ([pi, pi]). Furthermore, we find that this collective mode signals the strength of the antiferromagnetic fluctuations which are responsible for the suppression of the superfluid density in the underdoped cuprates even at zero temperature. Finally, we perform a complete analysis on an effective model with parameters fitted by experimental data of Bi2212 within the GRPA scheme and conclude that the short-range antiferromagnetic interactions which are a remnant of the parent Mott-insulator are more likely the pairing mechanism of the High-T[subscript c] cuprates.
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Electronic structure of transition metal oxides has been a subject of intense research since decades due to the wide spectrum of properties that they exhibit, like high temperature superconductivity, metal-insulator transitions (MIT), phase separation etc. Among these, colossal magnetoresistance (CMR), i.e. a sharp drop in the electrical resistance by the application of an external magnetic field, is a property of fundamental and technological importance. In the present study we investigate several of these interesting properties ranging from colossal magnetoresistance, metal-insulator transitions and phase separation phenomena on a wide range of magnetoresistive systems. All these properties originate in transition metal oxides due to a competition between the strong inter-atomic Coulomb interaction strength within the transition metal d electrons and a large hopping interaction strength between the metal d and oxygen 2p states. In this thesis we report the investigation of the electronic and magnetic structures of some magnetoresistive oxides, including various double perovskites and manganites, using various high energy spectroscopies in conjunction with various theoretical approaches. The samples for the present experimental investigation were prepared by different synthetic routes, such as solid state reaction, nitrate method, d.c arc melting and float zone method, and were characterized by x-ray diffraction, four probe resistivity, magnetic susceptibility, optical absorption and energy dispersive analysis of x-rays while some of the samples were supplied by our collaborators. Various spectroscopic techniques like x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) , bremsstrahlung isochromat spectroscopy (BIS), x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism spectroscopy (XMCD) , electron energy loss spectroscopy (EELS), spatially resolved photoelectron spectroscopy and M¨ ossbauer spectroscopy were used to probe the samples. Theoretical methods include configuration interaction cluster approach to fit the XAS and XMCD spectra while ab initio band structure calculations along with the least-square fitting procedure was used to fit some of the valence and conduction bands. Following a general introduction in Chapter 1, the details of various experimental and theoretical techniques are discussed in Chapter 2 of this thesis. Recently, a double perovskite, Sr2FeMoO6, belonging to a general family of halfmetallic ferromagnetic oxides, has shown a spectacularly large magnetoresistance even at the room temperature and at relatively small applied magnetic fields compared to the extensively investigated class of magnetoresistive manganites. Physical properties of this compound is strongly influenced by the Fe -Mo ordering. We hence synthesized Sr2FeMoO6 sample, both with high and low degree of Fe/Mo ordering. Spectroscopic investigations of these samples suggest the presence of Fe rich and Mo rich domains of the type Sr2Fe1+xMo1−xO6 in disordered Sr2FeMoO6 at times. This prompted us to prepare bulk samples of Sr2Fe1+xMo1−xO6. In Chapter 3 we address various issues related to Fe/Mo ordering like saturation magnetization, variation of TC, and CMR as well as oxidation state of Fe and Mo in Sr2FeMoO6using this new series, ”Sr2Fe1+xMo1−xO6” as it offers a better control on the Fe/Mo bonds by controlling x. On the basis of the electron spectroscopic studies in conjunction with a configuration interaction cluster calculation model coupled with the conduction band, we claim that Fe remains in 3+oxidation state throughout the series, where as Mo changes its valency to maintain the charge neutrality. An analysis of the magnetic momentas a function of x suggests that Fe at the ”wrong” crystallographic site is coupled anti-parallel to the Fe moments at the ”correct” site. Additionally, Mo depolarizes to the extend proportional to the number of Mo sites in the near-neighbor co-ordination shell. Continuing with the double perovskites in Chapter 4 we investigate the electronic and magnetic structure of Sr2FeMoO6, Ca2FeMoO6 and Ba2FeMoO6using XAS and XMCD studies. We find that the conventional XAS and XMCD calculations based on configuration interaction of a typical fragment, FeO6in this case, is insufficient to reproduce the experimental spectrum as the compounds considered here are metallic. In order to include the non local charge transfer, we coupled FeO6 octahedra to a conduction band which mimics the Mo band. Within this model we obtained a good fit to the experimental spectrum. Chapter 5 deals with another series of double perovskite (Sr1−yCay)2FeReO6which exhibits a rich phase diagram since it undergoes a metal insulator transition (MIT) with composition at low temperatures. This system becomes more interesting due to the presence of a temperature driven MIT for higher y compositions. We find that the MIT is not related to the change in valency of Fe and Re. Analysis of the near Fermi edge valence band spectra suggests opening up of a soft gap. The main reason for MIT in this system is most likely the presence of strong electron-electron correlation between multiple electrons at the Re site, which is caused by the mismatch of the Re ionic radius and change in the crystal structure across MIT. Another issue which has been extensively investigated in this thesis is phase separation in manganites presented in Chapter 6. We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional unique sensitivity to chemical compositions, to investigate high quality single crystal samples of La1/4Pr3/8Ca3/8MnO3 in the first section. This unique probe establishes the formation of distinct insulating domains embedded in the metallic host at low temperatures, significantly in the absence of any perceptible chemical inhomogeneity, with the domain-size at least an order of magnitude larger than the previous largest estimate. We also provide compelling evidence of memory effects in such domain formation and morphology, suggesting an intimate connection between these electronic domains and long-range strains, often thought to be an important ingredient in the physics of doped manganites. In second part of this chapter we discuss another system namely Eu0.5Y0.5MnO3 which undergoes a chemical phase separation forming alternate stripes of Eu rich (Y deficient) orthorhombic phase and Y rich (Eu deficient) hexagonal phases. These stripes are amazingly straight and run parallel over millimeters. One more system that we investigated is a mixture of ferromagnetic La5/8Sr3/8MnO3and insulating ferroelectric LuMnO3 taken in ratio 3:7, here too the attempt to make a single crystal resulted into a chemical phase separation forming strips of metallic La5/8Sr3/8MnO3and insulating LuMnO3 throughout the sample surface. Preliminary studies suggests that strain between the chemically and crystallographically different species may result into such interesting morphology. In Chapter 7 we study pseudo-one dimensional compounds Sr3CuIrO6 and Sr3ZnIrO6 using photo electron spectroscopy. The experimental results were fitted using band structure calculations with Full Potential Linearized Augmented Plane Wave (FP-LAPW) method.

Electronic structure of transition metal oxides has been a subject of intense research since decades due to the wide spectrum of properties that they exhibit, like high temperature superconductivity, metal-insulator transitions (MIT), phase separation etc. Among these, colossal magnetoresistance (CMR), i.e. a sharp drop in the electrical resistance by the application of an external magnetic field, is a property of fundamental and technological importance. In the present study we investigate several of these interesting properties ranging from colossal magnetoresistance, metal-insulator transitions and phase separation phenomena on a wide range of magnetoresistive systems. All these properties originate in transition metal oxides due to a competition between the strong inter-atomic Coulomb interaction strength within the transition metal d electrons and a large hopping interaction strength between the metal d and oxygen 2p states. In this thesis we report the investigation of the electronic and magnetic structures of some magnetoresistive oxides, including various double perovskites and manganites, using various high energy spectroscopies in conjunction with various theoretical approaches. The samples for the present experimental investigation were prepared by different synthetic routes, such as solid state reaction, nitrate method, d.c arc melting and float zone method, and were characterized by x-ray diffraction, four probe resistivity, magnetic susceptibility, optical absorption and energy dispersive analysis of x-rays while some of the samples were supplied by our collaborators. Various spectroscopic techniques like x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) , bremsstrahlung isochromat spectroscopy (BIS), x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism spectroscopy (XMCD) , electron energy loss spectroscopy (EELS), spatially resolved photoelectron spectroscopy and M¨ ossbauer spectroscopy were used to probe the samples. Theoretical methods include configuration interaction cluster approach to fit the XAS and XMCD spectra while ab initio band structure calculations along with the least-square fitting procedure was used to fit some of the valence and conduction bands. Following a general introduction in Chapter 1, the details of various experimental and theoretical techniques are discussed in Chapter 2 of this thesis. Recently, a double perovskite, Sr2FeMoO6, belonging to a general family of halfmetallic ferromagnetic oxides, has shown a spectacularly large magnetoresistance even at the room temperature and at relatively small applied magnetic fields compared to the extensively investigated class of magnetoresistive manganites. Physical properties of this compound is strongly influenced by the Fe -Mo ordering. We hence synthesized Sr2FeMoO6 sample, both with high and low degree of Fe/Mo ordering. Spectroscopic investigations of these samples suggest the presence of Fe rich and Mo rich domains of the type Sr2Fe1+xMo1−xO6 in disordered Sr2FeMoO6 at times. This prompted us to prepare bulk samples of Sr2Fe1+xMo1−xO6. In Chapter 3 we address various issues related to Fe/Mo ordering like saturation magnetization, variation of TC, and CMR as well as oxidation state of Fe and Mo in Sr2FeMoO6using this new series, ”Sr2Fe1+xMo1−xO6” as it offers a better control on the Fe/Mo bonds by controlling x. On the basis of the electron spectroscopic studies in conjunction with a configuration interaction cluster calculation model coupled with the conduction band, we claim that Fe remains in 3+oxidation state throughout the series, where as Mo changes its valency to maintain the charge neutrality. An analysis of the magnetic momentas a function of x suggests that Fe at the ”wrong” crystallographic site is coupled anti-parallel to the Fe moments at the ”correct” site. Additionally, Mo depolarizes to the extend proportional to the number of Mo sites in the near-neighbor co-ordination shell. Continuing with the double perovskites in Chapter 4 we investigate the electronic and magnetic structure of Sr2FeMoO6, Ca2FeMoO6 and Ba2FeMoO6using XAS and XMCD studies. We find that the conventional XAS and XMCD calculations based on configuration interaction of a typical fragment, FeO6in this case, is insufficient to reproduce the experimental spectrum as the compounds considered here are metallic. In order to include the non local charge transfer, we coupled FeO6 octahedra to a conduction band which mimics the Mo band. Within this model we obtained a good fit to the experimental spectrum. Chapter 5 deals with another series of double perovskite (Sr1−yCay)2FeReO6which exhibits a rich phase diagram since it undergoes a metal insulator transition (MIT) with composition at low temperatures. This system becomes more interesting due to the presence of a temperature driven MIT for higher y compositions. We find that the MIT is not related to the change in valency of Fe and Re. Analysis of the near Fermi edge valence band spectra suggests opening up of a soft gap. The main reason for MIT in this system is most likely the presence of strong electron-electron correlation between multiple electrons at the Re site, which is caused by the mismatch of the Re ionic radius and change in the crystal structure across MIT. Another issue which has been extensively investigated in this thesis is phase separation in manganites presented in Chapter 6. We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional unique sensitivity to chemical compositions, to investigate high quality single crystal samples of La1/4Pr3/8Ca3/8MnO3 in the first section. This unique probe establishes the formation of distinct insulating domains embedded in the metallic host at low temperatures, significantly in the absence of any perceptible chemical inhomogeneity, with the domain-size at least an order of magnitude larger than the previous largest estimate. We also provide compelling evidence of memory effects in such domain formation and morphology, suggesting an intimate connection between these electronic domains and long-range strains, often thought to be an important ingredient in the physics of doped manganites. In second part of this chapter we discuss another system namely Eu0.5Y0.5MnO3 which undergoes a chemical phase separation forming alternate stripes of Eu rich (Y deficient) orthorhombic phase and Y rich (Eu deficient) hexagonal phases. These stripes are amazingly straight and run parallel over millimeters. One more system that we investigated is a mixture of ferromagnetic La5/8Sr3/8MnO3and insulating ferroelectric LuMnO3 taken in ratio 3:7, here too the attempt to make a single crystal resulted into a chemical phase separation forming strips of metallic La5/8Sr3/8MnO3and insulating LuMnO3 throughout the sample surface. Preliminary studies suggests that strain between the chemically and crystallographically different species may result into such interesting morphology. In Chapter 7 we study pseudo-one dimensional compounds Sr3CuIrO6 and Sr3ZnIrO6 using photo electron spectroscopy. The experimental results were fitted using band structure calculations with Full Potential Linearized Augmented Plane Wave (FP-LAPW) method.