Grenzflächen von Übergangsmetalloxiden in Halbleiterbauelementen bestimmen die Funktionalität auf vielfältige Art und Weise. In dieser Arbeit werden die Nb2O5|Metall- (Metall = Al, Ti, Pt), die TiN|TiO2- und die Si|SrTiO3-Grenzfläche mittels röntgenographischer Methoden sowie der Transmissionselektronenmikroskopie hinsichtlich ihrer Defektchemie, kristallographischen Anpassung und thermischen Stabilität untersucht. Die lokale elektronische Analyse der Nb2O5|Ti- sowie Nb2O5|Al-Grenzfläche zeigt die Ausbildung eines Sauerstoffleerstellengradienten im Nb2O5 durch die Oxidation der unedlen Elektrode. Der elektrische Widerstand dieser beiden Metall-Isolator-Metall-(MIM)-Stapel mit Pt-Bodenelektrode kann reversibel geschalten werden. Diese experimentellen Befunde lassen sich direkt miteinander verknüpfen, da an der Nb2O5|Pt-Grenzfläche weder eine Redoxreaktion stattfindet, noch im Pt|Nb2O5|Pt-Stapel der Widerstand geschaltet werden kann. MIM-Stapel bestehend aus TiN, TiO2 und Au weisen zwar Schaltverhalten des elektrischen Widerstandes auf, lassen aber keine Abweichung der Stöchiometrie im Transmissionselektronenmikroskop erkennen. Die strukturellen Betrachtungen der TiN|TiO2-Grenzfläche verdeutlichen, dass bei der Heteroepitaxie das Aufwachsen der thermodynamisch stabileren TiO2-Modifikation unterdrückt wird, insofern das Substrat eine geeignete kristallographische Orientierung aufweist. So kristallisiert Anatas, eher als Rutil, auf der (001)-Oberfläche des TiN mit einer festen Orientierungsbeziehung. Die thermische Stabilität der Si|SrTiO3-Grenzfläche hängt hingegen stark von der Kationenstöchiometrie des ternären Perowskites ab. Für die Kristallisation der amorphen SrTiO3-Dünnschichten ergibt sich eine Korrelation zwischen der Einsatztemperatur und der Schichtabscheidemethode. / Interfaces of transition metal oxides in semiconductor devices determine their functionalities in a variety of ways. In this work Nb2O5|metal- (metal = Al, Ti, Pt), TiN|TiO2- and Si|SrTiO3-interfaces are investigated by means of X-ray-based methods and transmission electron microscopy with respect to their defect chemistry, crystallographic orientation and thermal stability. From local electronic analysis of the Nb2O5|Ti- as well as Nb2O5|Al-interface the formation of an oxygen vacancy gradient in the Nb2O5 caused by an oxidation of the ignoble electrode can be inferred. The electrical resistance of both types of metal-insulator-metal-(MIM)-stacks, containing a Pt bottom electrode, can be switched reversibly. These experimental findings are directly linked to each other, since at the Nb2O5|Pt-interface no redox reaction based oxygen redistribution takes place and the Pt|Nb2O5|Pt-stack reveals no switching behavior. Using MIM-stacks consisting of TiN, TiO2 und Au switching the electrical resistance is possible, but no stoichiometric deviation was observed in the transmission electron microscope. Structural considerations at the TiN|TiO2-interface clarify that hetero-epitaxy can suppress the growth of the thermodynamically stable TiO2-modification due to suitable crystallographic orientation of the TiN-substrate. Thus, anatase, rather than rutile, crystallizes on the (001)-TiN-surface with a fixed structural coherency. The thermal stability of the Si|SrTiO3-interface strongly depends on the cation stoichiometry of the ternary perovskite. Therefore, crystallization onset temperature correlates to the specific technique of thin film deposition.

De nombreux oxydes de métaux de transition (TMOs) possèdent des propriétés physiques complexes (ferroélectricité, magnétisme, supraconductivité à haute Tc ou magnétorésistance colossale). Les différents degrés de liberté (le réseau, la charge, le spin ou l'ordre orbitalaire) interagissent pour donner des phases différentes, très proches en énergie, qui vont former une grande variété d'états fondamentaux accessibles. La possibilité de fabriquer des hétérostructures de TMOs a encore accru la complexité de ces systèmes, de nouveaux phénomènes apparaissant aux interfaces. Un exemple typique est le gaz d'électrons bidimensionnel (2DEG) créé à l'interface entre deux oxydes isolants, LaAlO3 et SrTiO3, qui montre une transition métal-isolant, du magnétisme ou de la supraconductivité (contrôlée par une tension de grille). Le point de départ de cette thèse a été la découverte d'un 2DEG similaire à la surface nue de SrTiO3 fracturée sous vide, rendant possible l'étude de sa structure électronique par photoémission angulaire.Dans cette thèse, l'étude de surfaces préparées, plutôt que de petites facettes fracturées, a permis l'obtention de données spectroscopiques possédant des largeurs de raie proches des valeurs intrinsèques. Il est alors possible d'étudier les effets à N corps comme la renormalisation de la self-énergie due à l'interaction électron-phonon.Ces recherches sur la structure électronique du 2DEG à la surface de SrTiO3 ont pris un tour nouveau lorsqu'une texture de spin complexe y a été mesurée par photoémission résolue en spin. Nous présentons des résultats qui contredisent ces conclusions et nous discutons des raisons pouvant expliquer ce désaccord.Une des motivations de cette thèse était de savoir si la structure électronique et les propriétés du 2DEG pouvaient être contrôlées. L'étude du 2DEG sur des surfaces (110) et (111) de SrTiO3 révèle que sa structure de bandes (ordre orbitalaire, symétrie de la surface de Fermi, masses effectives) peut être ajustée en confinant les électrons sur des surfaces de différentes orientations du même matériau.Un succès majeure est la mise en évidence de 2DEGs à la surface de nombreux autres TMOs (TiO2-anatase, CaTiO3, BaTiO3) ou d'oxydes plus simples utilisés dans les applications (ZnO). Dans tous ces oxydes, nous avons identifié les lacunes en oxygène comme étant à l'origine de la création des 2DEGs.Dans l'anatase, ou d'autres TMOs en configuration électronique initiale d0, les lacunes en oxygène produisent à la fois des électrons localisés ou itinérants (le 2DEG). Il peut être subtile de prévoir quel est le cas est le plus favorable énergétiquement comme le démontre l'étude de deux polymorphes de TiO2, anatase et rutile. Dans CaTiO3, l’octaèdre formé par les atomes d'oxygène autour du Ti est incliné. Cette rupture de symétrie provoque un mélange des orbitales d et modifie le 2DEG. Dans BaTiO3, la création d'un 2DEG entraîne la coexistence de deux phénomènes normalement incompatibles, la ferroélectricité et la métallicité, dans deux zones spatialement distinctes du même matériau. Ce travail démontre qu'un 2DEG existe aussi à la surface de ZnO qui est, contrairement aux oxydes à base de Ti, plutôt un semiconducteur conventionnel, le caractère des orbitales pour les électrons itinérants étant alors de type s et non de type d.Le principal résultat est la mise au point d'une méthode simple et versatile pour la création de 2DEGs en évaporant de l'aluminium sur des surfaces d'oxydes. Une réaction d'oxydo-réduction entre le métal et l'oxyde permet de créer un 2DEG à l'interface entre le métal oxydé et l'oxyde réduit. Dans cette thèse, les 2DEGs ont été étudiés uniquement par photoémission sous ultra-vide. Cette méthode ouvre la possibilité d'étudier ces 2DEGs dans des conditions de pression ambiante en utilisant, par exemple, des techniques de transport, un pas important vers la production de masse et à bas coûts de 2DEGs dans les oxydes pour de futures applications. / Many transition metal oxides (TMOs) show complex physics, ranging from ferroelectricity to magnetism, high-Tc superconductivity and colossal magnetoresistance. The existence of a variety of ground states often occurs as different degrees of freedom (e.g. lattice, charge, spin, orbital) interact to form different competing phases which are quite similar in energy. The capability to epitaxially grow heterostructures of TMOs increased the complexity even more as new phenomena can emerge at the interface. One typical example is the two-dimensional electron system (2DES) at the interface of two insulating oxides, namely LaAlO3/SrTiO3, which shows metal-to-insulator transitions, magnetism or gate-tunable superconductivity. The origin of this thesis was the discovery of a similar 2DES at the bare surface of SrTiO3 fractured in vacuum, making it possible to study its electronic structure by angle-resolved photoemission spectroscopy (ARPES).In this thesis, the study of well-prepared surfaces, instead of small fractured facets, results in spectroscopic data showing line widths approaching the intrinsic value. This approach allows a detailed analysis of many-body phenomena like the renormalization of the self-energy due to electron-phonon interaction.Additionally, the understanding of the electronic structure of the 2DES at the surface of SrTiO3(001) was given an additional turn by the surprising discovery of a complex spin texture measured by spin-ARPES. In this thesis data is presented which contradicts these conclusions and discusses possible reasons for the discrepancy.One major motivation of this thesis was the question if and how the electronic structure and the properties of the 2DES can be changed or controlled. In this context, the study of 2DESs at (110) and (111) surface revealed that the electronic band structure of the 2DES (orbital ordering, symmetry of the Fermi surface, effective masses) can be tuned by confining the electrons at different surface orientations of the same material, namely SrTiO3.A major achievement of this thesis is the generalization of the existence of a 2DES in SrTiO3 to many other surfaces and interfaces of TMOs (TiO2 anatase, CaTiO3, BaTiO3) and even simpler oxides already used in modern applications (ZnO). In all these oxides, we identify oxygen vacancies as the origin for the creation of the 2DESs.In anatase and other doped d0 TMOs, both localized and itinerant electrons (2DES) can exist due to oxygen vacancies. Which of the two cases is energetically favorable depends on subtle differences as demonstrated by studying two polymorphs of the same material (anatase and rutile).In CaTiO3, the oxygen octahedron around the Ti ion is slightly tilted. This symmetry breaking results in the mixing of different d-orbitals demonstrating again why and how the electronic structure of the 2DES can be altered.In BaTiO3, the creation of a 2DES results in the coexistence of the two, usually mutual exclusive, phenomena of ferroelectricity and metallicity in the same material by spatially separating the two.Moreover, this work demonstrates that the 2DES also exists in ZnO which is - compared to the Ti-based oxides - rather a conventional semiconductor as the orbital character of the itinerant electrons is of s and not d-type.The main result of this thesis is the demonstration of a simple and versatile technique for the creation of 2DESs by evaporating Al on oxide surfaces. A redox reaction between metal and oxide results in a 2DES at the interface of the oxidized metal and the reduced oxide. In this thesis the study of such interfacial 2DESs was limited to photoemission studies in ultra high vacuum. However, this technique opens up the possibility to study 2DESs in functional oxides in ambient conditions by e.g. transport techniques, and might be an important step towards cost-efficient mass production of 2DESs in oxides for future applications.

No description available.

Hafnium oxide is widely used for resistive switching devices, and recently it has been discovered that ferroelectricity can be established in (un-)doped hafnium oxide as well. Previous studies showed that both switching mechanisms are influenced by oxygen vacancies. For resistive switching, typically amorphous oxide layers with an asymmetric electrode configuration are used to create a gradient of oxygen vacancies. On the other hand, ferroelectric switching is performed by having symmetric electrodes and requires crystalline structures. The coexistence of both effects has recently been demonstrated. In this work, a detailed analysis of the reversible interplay of both switching mechanisms within a single capacitor cell is investigated. First, ferroelectric switching cycles were applied in order to drive the sample into the fatigued stage characterized by increased concentration of oxygen vacancies in the oxide layer. Afterwards, a forming step that is typical for the resistive switching devices was utilized to achieve a soft breakdown. In the next step, twofold alternation between the high and low resistance state is applied to demonstrate the resistive switching behavior of the device. Having the sample in the high resistance state with a ruptured filament, ferroelectric switching behavior is again shown within the same stack. Interestingly, the same endurance as before was observed without a hard breakdown of the device. Therefore, an effective sequence of ferroelectric—resistive—ferroelectric switching is realized. Additionally, the dependence of the forming, set, and reset voltage on the ferroelectric cycling stage (pristine, woken-up and fatigued) is analyzed giving insight into the physical device operation.

First, the effect of transition metal oxide (e.g., V2O5, Co2O3, etc.) on the physical properties (e.g., density, glass transition temperature (Tg), optical properties and mechanical properties) and chemical durability of a simplified borosilicate nuclear waste glass was investigated. Adding V2O5 in borosilicate nuclear waste glasses decreases the Tg, while increasing the fracture toughness and chemical durability, which benefit the future formulation of nuclear waste glasses. Second, structural study of ZrO2/SiO2 substitution in silicate/borosilicate glasses was systematically conducted by molecular dynamics (MD) simulation and the quantitative structure-property relationships (QSPR) analysis to correlate structural features with measured properties. Third, for bioactive glass formulation, mixed-network former effect of B2O3 and SiO2 on the structure, as well as the physical properties and bioactivity were studied by both experiments and MD simulation. B2O3/SiO2 substitution of 45S5 and 55S5 bioactive glasses increases the glass network connectivity, correlating well with the reduction of bioactivity tested in vitro. Lastly, the effect of optical dopants on the optimum analytical performance on atom probe tomography (APT) analysis of borosilicate glasses was explored. It was found that optical doping could be an effective way to improve data quality for APT analysis with a green laser assisted system, while laser spot size is found to be critical for optimum performance. The combined experimental and simulation approach adopted in this dissertation led to a deeper understanding of complex borosilicate glass structures and structural origins of various properties.

Manifold research has been done to understand the detailed mechanisms behind the performance instabilities of ferroelectric capacitors based on hafnia. The wake-up together with the imprint might be the most controversially discussed phenomena so far. Among crystallographic phase change contributions and oxygen vacancy diffusion, electron trapping as the origin has been discussed recently. In this publication, we provide evidence that the imprint is indeed caused by electron trapping into deep states at oxygen vacancies. This impedes the ferroelectric switching and causes a shift of the hysteresis. Moreover, we show that the wake-up mechanism can be caused by a local imprint of the domains in the pristine state by the very same root cause. The various domain orientations together with an electron trapping can cause a constriction of the hysteresis and an internal bias field in the pristine state. Additionally, we show that this local imprint can even cause almost anti-ferroelectric like behavior in ferroelectric films.

<p dir="ltr">The field of nanoclusters includes a broad range of sizes and structures that influence both their physical and chemical properties. Scientists use several techniques, such as atom-by-atom substitution, to synthesize atomically precise nanoclusters, and ligand shell mixing to protect nanoclusters from unwanted side reactions, while controlling their reactivity and solubility. These combined techniques can provide stable products, but isomers and structural analogs often remain in the product mixture, complicating the structural characterization of individual nanoclusters. Leading structural characterization techniques in nanocluster research are often limited in their ability to examine both the structure of the metal core and ligand shell in sufficient detail. The primary aim of this research is to systematically characterize the structures and chemical properties of several types of transition metal oxide nanoclusters of interest to applications in energy production, catalysis, and magnetic resonance imaging, without requiring purification. Specifically, this work focuses on 1) Polyoxovanadates (POV) with a mixture of methoxy, ethoxy, and ether ligands, 2) Fe- and W-substituted POV alkoxides, and 3) Octanuclear iron oxide clusters substituted with In atoms. Mass spectrometry techniques enable the structural characterization of individual clusters from multicomponent mixtures without interference. Specifically, we use ion mobility spectrometry to explore how surface ligands affect the metal core in mixed-ligand POV alkoxide species. We examine structure-specific fragments to identify the positions of ligands and heteroatoms within the metal core of mixed-ligand species and W and Fe-substituted POV methoxides. Additionally, we use ion soft-landing to purify W-substituted POV methoxide anions on surfaces for characterization using cyclic voltammetry and infrared spectroscopy. We discovered unique characteristics of each nanocluster including the position of heteroatoms, ligands shell mobilities, structures and collisional cross sections, and provided first insights into the redox properties of W-substituted POV alkoxide. These results highlight the growing influence of mass spectrometry in the field of nanocluster characterization and design.</p>

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.

Rechargeable Li-ion batteries and supercapacitors are the most promising electrochemical energy storage devices in terms of energy density and power density, respectively. Recently, nanostructured materials have gained enormous interest in the field of energy technology as they have special properties compared to the bulk. Commercially available Li-ion batteries, which are the most advanced among the rechargeable batteries, utilize microcrystalline transition metal oxides as cathode materials which act as lithium insertion hosts. To explore better electrochemical performance the use of nanomaterials instead of conventional materials would be an excellent alternative. High Li-ion insertion at high discharge rates causes slow Li+ transport which in turn results in concentration polarization of lithium ions within the electrode material, causing a drop in cell voltage. This eventually, leads in termination of the discharge process before realizing the maximum capacity of the electrode material being used. This problem can be addressed by decreasing the average particle size which leads to an increase in surface area of the electrode material. Nanostructured materials, because of their high surface area and large surface to volume ratio, to some extent can overcome the problem of slow diffusion of ions. Supercapacitors are electrical energy storage devices which can deliver large energy in a short time. A supercapacitor can be used as an auxiliary energy device along with a primary source such as a battery or a fuel cell to achieve power enhancement in short pulse applications. Active materials for supercapacitors are classified into three categories: (i) carbonaceous materials, (ii) conducting polymers and (iii) metal oxides. Among the materials studied over the years, metal oxides have been considered as attractive electrode materials for supercapacitors due to the following merits: variable oxidation state, good chemical and electrochemical stability, ease of preparation and handling. The performance of supercapacitors can be enhanced by moving from bulk to nanostructured materials. The theme of the thesis is to explore novel routes to synthesize nanostructured materials for Li-ion batteries and supercapacitors, and to investigate their physical and electrochemical characteristics. Chapter I is an introduction of various types of electrochemical energy systems such as battery, fuel cell and supercapacitor. A brief review is made on electrode materials for Li-ion batteries and supercapacitors, and nanostructured materials. Chapter II deals with the study of nanostrip orthorhombic V2O5 synthesized by a two-step procedure, with the formation of a vanadyl ethylene glycolate precursor and post-calcination treatment. The precursor and the final product are characterized for phase and composition by powder X-ray diffraction (XRD), infrared (IR) spectroscopy, thermal analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The morphological changes are investigated using field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HRTEM). It is found that the individual strips have the following dimensions, length: 1.3 μm, width: 332 nm and thickness: 45 nm. The electrochemical lithium intercalation and de-intercalation of nanostrip V2O5 is investigated by cyclic voltammetry (CV), galvanostatic charge-discharge cycling, galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy. Chapter III describes the synthesis of nanoparticels of LiMn2O4 by microwave assisted hydrothermal method. The phase and purity of spinel LiMn2O4 are confirmed by powder XRD analysis. The morphological studies are carried out using FE-SEM and HRTEM. The electrochemical performance of spinel LiMn2O4 is studied by using CV and galvanostatic charge-discharge cycling. The initial discharge capacity is found to be about 89 mAh g-1 at a current density of 21 mA g-1 with reasonably good cyclability. Chapter IV deals with synthesis of MoO2 nanoparticles through ethylene glycol medium and its electrochemical characterization. XRD data confirms the formation MoO2 on monoclinic phase, space group P21/c. Polygon shape of MoO2 is observed in HRTEM. MoO2 facilitates reversible insertion-extraction of Li+ ions between 0.25 to 3.0 V vs. Li/Li+. CV and galvanostatic charge-discharge cycling are conducted on this anode material to complement the electrochemical data. Chapter V reports the synthesis of nanostructured MnO2 at ambient conditions by reduction of potassium permanganate with aniline. Physical characterization is carried out to identify the phase and morphology. The as prepared MnO2 is amorphous and it contains particles of 5 to 10 nm in diameter. On annealing at a temperature > 400 °C, the amorphous MnO2 attains crystalline α-phase with a concomitant change in morphology. A gradual conversion of nanoparticles to nanorods (length 500-750 nm and diameter 50-100 nm) is evident from SEM and TEM studies. High resolution TEM images suggest that nanoparticles and nanorods grow in different crystallographic planes. The electrochemical lithium intercalation and de-intercalation of nanorods was performed by (CV) and galvanostatic charge-discharge cycling. The initial discharge capacity of nanorod α-MnO2 is found to be about 197 mAh g-1 at a current density of 13.0 mA g-1. Capacitance behavior of amorphous MnO2 is studied by CV and galvanostatic charge-discharge cycling in a potential range from -0.2 to 1.0 V vs. SCE in 0.1 M sodium sulphate solution. The effect of annealing on specific capacitance is also investigated. Specific capacitance of about 250 F g-1 is obtained for as prepared MnO2 at a current density of 0.5 mA cm-2 (0.8 A g-1). Chapter VI pertains to electrochemical supercapacitor studies on nanostructured MnO2 synthesized by polyol method. Although X-ray diffraction (XRD) pattern of the as synthesized nano-MnO2 shows poor crystallinity, it is found that it is locally arranged in δ-MnO2 type layered structure composed of edge-shared network of MnO6 octahedra by Mn K-edge X-ray Absorption Near Edge Structure (XANES) measurement. Annealed MnO2 shows high crystalline tunneled based α-MnO2 as confirmed by powder XRD pattern and XANES. As synthesized MnO2 exhibits good cyclability as an electrode material for supercapacitor. In Chapter VII, capacitance behavior of nanostrip V2O5, TiO2 coated V2O5 and nanocomposites of PEDOT/V2O5 are presented. Structural and morphological studies are carried out by powder XRD, IR, TGA, SEM and TEM. Cyclic voltammogram of pristine V2O5 shows the regular rectangular shape indicating the ideal capacitance behavior in aqueous 0.1 M K2SO4. The SC value of pristine V2O5 is found to be about 100 F g-1. Nanostrip V2O5 is modified with TiO2 using titanium isobutoxide to enhance the capacitance retention upon cycling. Only 48 % of the initial capacitance remains in the case of pristine V2O5 after 100 cycles, while TiO2 coated V2O5 exhibits better cyclability with capacitance of 70 % of the initial capacitance. The capacitance retention is attributed to the presence of TiO2 on the surface of V2O5 which prevents the vanadium dissolution into the electrolyte. Microwave assisted hydrothermally synthesized PEDOT/V2O5 nanocomposites are utilized as capacitor materials. The initial SC of PEDOT/V2O5 (237 F g-1) is higher than that of either pristine V2O5 or PEDOT. The enhanced electrochemical performance is attributed to synergic effect and an enhanced bi-dimensionality. Details of the above studies are described in the thesis with a conclusion at the end of each Chapter.

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.