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

CO2 global emissions exceed 30 Giga tonnes (Gt) per year, and the high atmospheric concentrations are detrimental to the environment. In spite of efforts to decrease emissions by sequestration (carbon capture and storage) and repurposing (use in fine chemicals synthesis and oil extraction), more than 98% of CO2 generated is released to the atmosphere. With emissions expected to increase, transforming CO2 to chemicals of high demand could be an alternative to decrease its atmospheric concentration. Transportation fuels represent 26% of the global energy consumption, making it an ideal end product that could match the scale of CO2 generation. The long-term goal of the study is to transform CO2 to liquid fuels closing a synthetic carbon cycle. Synthetic fuels, such as diesel and gasoline, can be produced from syngas (a combination of CO and H2) by Fischer Tropsch synthesis or methanol synthesis, respectively. Methanol can be turned into gasoline by MTO technologies. Technologies to make renewable hydrogen are already in existence, but CO is almost exclusively generated from methane. Due to the high stability of the CO2 molecule, its transformation is very energy intensive. Therefore, the current challenge is developing technologies for the conversion of CO2 to CO with a low energy requirement. The work in this dissertation describes the development of a recyclable, isothermal, low-temperature process for the conversion of CO2 to CO with high selectivity, called Reverse Water Gas Shift Chemical Looping (RWGS-CL). In this process, H2 is used to generate oxygen vacancies in a metal oxide bed. These vacancies then can be re-filled by one O atom from CO2, producing CO. Perovskites (ABO3) were used as the oxide material due to their high oxygen mobility and stability. They were synthesized by the Pechini sol-gel synthesis, and characterized with X-ray diffraction and surface area measurements. Mass spectrometry was used to evaluate the reducibility and re-oxidation abilities of the materials with temperature-programmed reduction and oxidation experiments. Cycles of RWGS-CL were performed in a packed bed reactor to study CO production rates. Different metal compositions on the A and B site of the oxide were tested. In all the studies, La and Sr were used on the A site because their combination is known to enhance oxygen vacancies formation and CO2 adsorption on the perovskites. The RWGS-CL was first demonstrated in a non-isothermal process at 500 °C for the H2-reduction and 850 °C for the CO2 conversion on a Co-based perovskite. This perovskite was too unstable for the H2 treatment. Addition of Fe to the perovskite enhanced its stability, and allowed for an isothermal and recyclable process at 550 °C with high selectivity towards CO. In an effort to decrease the operating temperature, Cu was incorporated to the structure. It was found that Cu addition inhibited CO formation and formed very unstable oxide materials. Preliminary studies show that application of this technology has the potential to significantly reduce CO2 emissions from captured flue gases (i.e. from power plants) or from concentrated CO2 (adsorbed from the atmosphere), while generating a high value chemical. This technology also has possible applications in space explorations, especially in environments like Mars atmosphere, which has high concentrations of atmospheric carbon dioxide.

Perovskite oxides have provided a wide variety of exotic functionalities based on their unique physical and chemical properties. By combining different perovskite oxides, interesting physical phenomena have been observed at the interfaces of perovskite heterostructures. The most interesting among these phenomena is the formation of two dimensional electron gas at the interface of two perovskite materials SrTiO3 and LaAlO3 which led to a number of fascinating physical properties such as metal-insulator transition, super-conductivity, large negative magnetoresistance and so on. This has raised the interest in exploiting the interface of various hybrids structures built on the perovskite oxide backbone. On the other hand, the two dimensional (2D) van der Waals materials such as graphene, MoS2, boron nitride etc. represent a new paradigm in the 2D electron-ics. The functionalities of these individual materials have been combined to obtain new enriched functionalities by stacking different materials together forming van der Waals heterostructures. In this work, we present a detailed study of the interface in hybrid structures made of vander Waals materials (graphene and MoS2) and their hybrids with a perovskite material namely, SrTiO3 which is known as the building block of complex oxide heterostructures. In graphene-MoS2 vertical heterostructure, we have carried out a detailed set of investigations on the modulation of the Schottky barrier at the graphene-MoS2 interface with varying external electric field. By using different stacking sequences and device structures, we obtained high mobility at large current on-off ratio at room temperature along with a tunable Schottky barrier which can be varied as high as ∼ 0.4 eV by applying electric field. We also explored the interface of graphene and SrTiO3 as well as MoS2 and SrTiO3 by electrical transport and low frequency 1/f noise measurements. We observed a hysteretic feature in the transfer characteristics of dual gated graphene and MoS2 field effect transistors on SrTiO3. The dual gated geometry enabled us to measure the effective capacitance of SrTiO3 interface which showed an enhancement indicating the possible existence of negative capacitance developed by the surface dipoles at the interface of SrTiO3 and the graphene or MoS2 channel. Our 1/f noise study and the analysis of higher order statistics of noise also support the possibility of electric field-driven reorient able surface dipoles at the interface.

Two important mechanisms in preparation of ultrathin films of magnetic oxides were systematically investigated in this work. First, influence of epitaxial strain on resulting magneto-optical properties of La2/3Sr1/3MnO3 (LSMO) ultrathin films was studied. The investigated films were grown by pulsed laser deposition on four different substrates, providing a broad range of induced epitaxial strains. Magnetic properties were found to deteriorate with increasing value of the epitaxial strain, as expected due to the unit cell distortion increasingly deviating from the bulk and effect of the magnetically inert layer. A combination of spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy was used to determine spectra of the diagonal and off-diagonal elements of permittivity tensor. The off-diagonal elements confirmed presence of two previously reported electronic transitions in spectra of all films. Moreover, they revealed another electronic transition around 4.3 eV only in spectra of films grown under compressive strain. We proposed classification of this transition as crystal field paramagnetic Mn t2g → eg transition, which was further supported by ab initio calculations. A key role of strain in controlling electronic structure of ultrathin perovskite films was demonstrated. Dynamic application of...

Deux mécanismes importants reliant la préparation des couches ultraminces d’oxydes magnétiques à leurs propriétés physiques ont été étudiés dans ce travail. En premier lieu, l’influence de la contrainte épitaxiale sur les propriétés magnéto-optiques de la manganite La₂/₃Sr₁/₃MnO₃ (LSMO) a été étudiée. Les couches ultraminces ont été déposées par ablation laser pulsé sur quatre substrats différents, ce qui a fourni différentes valeurs statiques de la contrainte épitaxiale. Les propriétés magnétiques ont été révélées comme se détériorant avec l’augmentation de la contrainte, ce qui était prévisible à cause de la distorsion grandissante de la maille unitaire ainsi qu’à cause de l’effet de la couche magnétiquement inerte. La combinaison de l’ellipsométrie spectroscopique et de la spectroscopie Kerr magnéto-optique a été utilisée afin de déterminer les spectres des éléments diagonaux et non diagonaux du tenseur de permittivité. L’étude des éléments non-diagonaux a confirmé la présence déjà rapportée de deux transitions électroniques dans les spectres de toutes les couches. De plus, elle a révélé une autre transition électronique autour de l’énergie de 4.3 eV, mais seulement dans les spectres des couches déposées avec une contrainte compressive. Nous avons proposé la classification de cette transition comme une transition paramagnétique du champ cristallin Mn t2g → eg. Cette classification a été confortée par des calculs ab initio. Nous avons ainsi montré le rôle clé de la contrainte dans le contrôle des propriétés magnéto-optiques des couches pérovskites ultraminces. En revanche, l’application dynamique de la contrainte par l’utilisation d’une sous-couche piézoélectrique est restée peu concluante. Le transfert de la contrainte entre la sous-couche piézoélectrique et la couche LSMO nécessite des améliorations ultérieures. En second lieu, l’influence de la désorientation du substrat a été étudiée par rapport à la dynamique de l’aimantation dans l’oxyde SrRuO₃ (SRO). Comme attendu, nous avons trouvé qu’un grand angle de désorientation mène à la suppression de la croissance de plusieurs variants cristallographiques du SRO. Au moyen de la microscopie à force magnétique, nous avons montré que la présence de plusieurs variants de SRO mène à l’augmentation de la densité de défauts agissant comme points d’ancrage ou de nucléation pour les domaines magnétiques. Nous avons donc montré que l’emploi d’un substrat vicinal est important pour la fabrication des couches ultraminces de SRO de haute qualité, avec une faible densité de défauts cristallographiques et d’excellentes propriétés magnétiques. / Two important mechanisms in preparation of ultrathin films of magnetic oxides were systematically investigated in this work. First, influence of epitaxial strain on resulting magneto-optical properties of La₂/₃Sr₁/₃MnO₃ (LSMO) ultrathin films was studied. The investigated films were grown by pulsed laser deposition on four different substrates, providing a broad range of induced epitaxial strains. Magnetic properties were found to deteriorate with increasing value of the epitaxial strain, as expected due to the unit cell distortion increasingly deviating from the bulk and effect of the magnetically inert layer. A combination of spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy was used to determine spectra of the diagonal and off-diagonal elements of permittivity tensor. The off-diagonal elements confirmed presence of two previously reported electronic transitions in spectra of all films. Moreover, they revealed another electronic transition around 4.3 eV only in spectra of films grown under compressive strain. We proposed classification of this transition as crystal field paramagnetic Mn t2g → eg transition, which was further supported by ab initio calculations. A key role of strain in controlling electronic structure of ultrathin perovskite films was demonstrated. Dynamic application of strain via use of piezoelectric underlayer remained inconclusive, requiring further improvement of the strain transfer from the piezoelectric layer into the LSMO. Second, influence of substrate miscut on magnetization dynamics in SrRuO₃ (SRO) was studied. As expected we found that high miscut angle leads to suppression of multi-variant growth. By means of magnetic force microscopy we showed that presence of multiple SRO variants leads to higher density of defects acting as pinning or nucleation sites for the magnetic domains, which consequently results in deterioration of magnetic properties. We demonstrated that use of vicinal substrate with high miscut angle is important for fabrication of high quality SRO ultrathin films with low density of crystallographic defects and excellent magnetic properties.

No description available.

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 group of materials with ABO3 type perovskite structure are very important due to their attractive electrical and magnetic properties for technological applications and have been studied in the form of single crystals, bulk polycrystalline materials and thin films. Recently, efforts have been made to synthesize and understand the growth of ABO3 type perovskite nanostructures because of their distinctive physical properties and potential applications in the nanodevices. The primary aim of the present thesis is to synthesize the perovskites at nano-scale, with zero-dimension (0D), and one-dimension (1D) configurations. Basic work was carried in terms of synthesis – structure – composition correlation. Due to the small nature of the synthesized materials, few attempts were done to examine the physical properties, but to a limited extant. Efforts were also done to emphasize the structural behavior of nano perovskite in comparison with their bulk counterparts. Chapter 1 provides a brief introduction to perovskite materials and nanostructures, their technological applications and the fundamental physics involved. A brief review of the perovskite nanostructures both from fundamental science and technological point of view is provided. Finally the specific objectives of the current research are outlined. Chapter 2 deals with the experimental studies carried out in this thesis. It describes the methods used for the synthesis, experimental set up and the basic operation principles of various structural and physical characterizations such as X-ray diffraction (XRD), thermal analysis, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), compositional analysis (EDX), focused ion beam (FIB), electrical and magnetic studies of the materials prepared. Chapter 3 describes the fabrication of porous anodic aluminum oxide (AAO) templates with different pore size, basic steps for synthesis of nanotubes and the possible growth mechanism of nanotubes in the AAO template. In chapter 4, we report the synthesis of ferroelectric Ba1-xSrxTiO3 (x = 0.0, 0.3) nanoparticles (diameter range: 20-40nm) and Ba1-xSrxTiO3 (x = 0.0, 0.4) nanotubes with diameter about 200nm by the sol-gel method. The Ba1-xSrxTiO3 nanostructures so obtained were characterized by number of techniques, including FE-SEM, XRD, DTA/TGA, FTIR spectroscopy, TEM, HRTEM as well as EDX and SAED. Formation of Y-junctions and multi-branches in Ba1-xSrxTiO3 nanotubes were also observed. The wall of the nanotubes were found to be made of randomly oriented nanoparticles which were confirmed from the HRTEM image. The average thickness of the wall of the nanotubes was found around 15(±5) nm and nanoparticles consisting the wall were found to be in the range of 5-10nm. Diffused phase transition (cubic to tetragonal), shifted to lower temperature side and leaky ferroelectric P–E loops were observed in Ba1-xSrxTiO3 (x = 0.0) ceramic prepared from nanoparticles. Curie temperature was observed at 120oC in the BT nanotube array as confirmed by the dielectric study. The P–E loops of as-prepared Ba1-xSrxTiO3 (x = 0.0) nanotube array were also measured and the hysteresis clearly demonstrates the room temperature ferroelectricity in the as prepared nanotubes, indicating these nanotube array is potential media as ferroelectric information storage. In chapter 5, we report the synthesis of single crystalline nanoparticles and polycrystalline nanotubes of Pb0.76Ca0.24TiO3 (PCT24) by sol-gel processing and characterized by various techniques. The crystallinity and phase purity of the PCT24 nanoparticles and nanotubes were confirmed by the XRD and SAED pattern. Compositional homogeneity and their crystalline structure confirms the formation of the tetragonal perovskite phase. The wall of the nanotubes was found to be made of nanoparticles which were confirmed from the HRTEM analysis. The average thickness of the wall of the nanotubes was found around 20nm and nanoparticles consisting the wall were found to be in the range of 5-8nm. Formation of some single crystalline PCT24 nanorods was also observed as confirmed by SAED and HRTEM analysis. Formations of Y-junctions and multi-branches in this complex functional oxide were observed. Dielectric measurements shows the diffuse phase transition and frequency dependence of Tm (temperature at which real part of dielectric constant shows maxima) suggesting the relaxor type behavior in the PCT24 ceramic prepared from nanoparticles. Polarization study was carried out on PCT24 nanotube array, which shows the ferroelectric nature at room temperature. Chapter 6 reports the synthesis and studies of PbZrO3 (PZ) nanoparticles and PbZr1-xTixO3 for x = 0.0, 0.48 and 1.0 nanotubes. PZ nanoparticles were prepared by a novel sol-gel method based on diol-based solution. Initially, PZ was crystallized with some intermediate m-Z and t-Z phases at 400-550oC and start transforming to orthorhombic at around 600oC and then finally transformed into pure orthorhombic PZ phase at about 700oC. XRD and TEM confirmed the nanocrystalline nature of PZ particles. Curie temperature in the PZ ceramic prepared from PZ nanoparticles was observed around at 205oC, which is lower as compared to the bulk (233oC). P–E hysteresis loops of PZ ceramic prepared from nanoparticles were measured at different applied voltages and single ferroelectric loops of leaky nature were observed rather than antiferroelectrics. The lead zirconate nanoparticles produced may have potential applications as materials used in microelectronics and microelectromechanical systems. PbZr1-xTixO3 for x = 0.0 (PZ), 0.48 (PZT48) and 1 (PT) nanotubes were fabricated by sol-gel method within the closely packed porous alumina templates and characterized by various techniques. The crystallinity of the PZ, PZT48 and PT nanotubes were confirmed via XRD and SAED studies. EDX analysis demonstrated that stoichiometry was formed. Formation of Y-junctions in this complex functional oxide was also observed. The wall of the nanotubes was found to be made up of randomly oriented nanoparticles, which were confirmed by the HRTEM studies and also by a typical SEM image. The average thickness of the wall of the nanotubes was found to be around 10-20nm and nanoparticles consisting the wall was found to be in the range of 3 – 8nm. The Curie temperature was observed at 220oC in the PZ nanotube array. For the first time, PLD has been employed for the synthesis of lead zirconate nanotubes using AAO template. Well-registered arrays of these nanotubes could function as three dimensional (3D) device elements in miniaturized ferroelectric random access memory (FRAM). In chapter 7, we report the synthesis of single crystalline 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (PMN-PT) nanoparticles. PMN-PT nanoparticles were developed by a novel sol-gel method based on diol route. After partial calcination at 450oC/1h, PMN-PT powder morphology started transforming from pyrochlore to perovskite phase. It is interesting to note that this partially crystallized PMN-PT powder was unstable under electron beam and generated freestanding lead nanoparticles after absorbing energy from a focused electron beam. PMN-PT powder annealed at 700°C was fully transformed to perovskite phase and was stable under electron beam. XRD calculations and TEM imaging confirmed the nanocrystalline nature of PMN-PT particles. Magnetic measurements on PMN-PT nanoparticles prepared at 650 and 750oC show room temperature ferromagnetic hysteresis, whereas the bulk or the agglomerated particles show diamagnetic behavior. With an increase of annealing temperature or the particle size the magnetic moment decreases. PMN-PT nanotubes with diameter about 200nm were fabricated successfully by the sol-gel method based on diol route within the closely packed porous nanochannel alumina templates. Phase purity and crystalline perovskite phase formation of PMN-PT nanotubes were confirmed by the XRD and SAED pattern. EDX analysis demonstrated that stoichiometry was formed within accepted limit. The wall of the nanotubes was found to be made of nanoparticles which were confirmed from the HRTEM analysis. The average thickness of the wall of the nanotube was found around 20 nm and nanoparticles consisting the wall were found to be in the range of 10-20 nm. Since electroceramic materials are following a similar trend to miniaturization as conventional semiconductors, the synthesis of nanosized oxidic building blocks is moving into the focus of scientific and technological interest. Ferroelectrics are promising class of materials for the fabrication of electronic devices, as they are already an integral part of modern nanotechnological operations. Chapter 8 deals with the synthesis and properties of BiFeO3 (BFO) nanoparticles and nanotubes. Single crystalline BFO nanoparticles of different size and polycrystalline BFO nanotubes were prepared by sol-gel method. As prepared nanostructures were characterized by various techniques such as XRD, TGA-DTA, FTIR, scanning electron microscope (SEM), transmission electron microscope (TEM), selected-area electron diffraction (SAED), high resolution TEM and energy-dispersive X-ray spectroscopy (EDX). The crystallinity and phase purity of the BFO nanoparticles and nanotubes were confirmed by the XRD, SAED pattern and HRTEM analysis. Compositional homogeneity and their crystalline structure confirms the formation of the rhombohedrally distorted perovskite phase. EDX analysis demonstrated that stoichiometric BiFeO3 was formed within accepted limit. The HRTEM analysis confirmed that wall of the BFO nanotubes was made of nanoparticles, which were randomly oriented in the wall. The average thickness of the wall of the nanotubes was found to be around 15 nm and nanoparticles consisting the wall were found to be in the range of 3-6nm. Formation of Y-junctions in this complex functional oxide was observed. Magnetic measurements show clearly the enhancement of ferromagnetism in BFO nanotubes and ferroelectric loops were also observed in these nanotubes, that indicates the multiferroic nature of these nanotubes. BFO nanostructures at a large scale might be important for many applications such as memory elements in nanoscale devices in future. Chapter 9 reports the synthesis of a series of crystalline La1-xCaxMnO3 (x = 0, 0.3, 0.5, 0.7) nanoparticles with average diameter about 20 nm by an improved sol-gel method. The crystallinity and phase formation of as prepared nanoparticles was confirmed via XRD, SAED and HRTEM studies. EDX analysis demonstrated that desired stoichiometric was formed. Magnetic characterization reveals that the PM-FM transitions (Tc) occurs around at 205, 235, 235 and 230 K for x = 0, 0.3, 0.5, 0.7, respectively. The strong irreversibility between zero field cooling (ZFC) and field cooling (FC) magnetization curves, a cusplike peak in ZFC curve and unusual shape of M versus H loop at T = 5 K gives strong support for surface spin glass behavior. The highly stable charge ordering state in bulk manganites is suppressed, while the ferromagnetism is enhanced in these nanoparticles (x = 0.5 and 0.7). La0.7Ca0.3MnO3 were fabricated by sol-gel method within the closely packed porous alumina templates. The wall of the nanotubes was found to be made up of randomly oriented nanoparticles (8-12nm) as confirmed by HRTEM studies. The strong irreversibility between ZFC and FC magnetization curves as well as a cusplike peak in ZFC curve gives strong support for surface spin glass behavior. Magnetization value as obtained from M-H loop was about 28.5% of expected value, suggesting the existence of a magnetic dead layer, which avoids the propagation of exchange interaction between magnetic grains. The PM-FM transition was observed at 235 K. Chapter 10 gives the summary and conclusions of the present study and also discusses the possible future work that could after more insights into the understanding of the perovskite nanostructures. Highlight of the present work (i) Successful growth of nanostructures in both particles and tube forms, and study of their structure – composition correlations. (ii) Present work could optimize the necessary chemistry to successfully grow nanoparticles and nanotubes of various perovskite compositions. (iii) Successful studies of physical properties of nanoparticles and nanotubes, ofcourse, to a limited extent. However the properties observed in the present nanostructures have a strong indication of nonlinear phenomena similar to their bulk counterparts. (iv) It was reported in the literature, the observation of ferromagnetic behavior in several nonmagnetic compositions at nano-scale. Surprisingly, similar ferroelectric behavior was noticed even in our perovskite complex oxides such as relaxors (PMN-PT). A clear interaction of magnetic spin and an electric dipole was evident in these oxides such as relaxors and also multiferroics at nano-scale (~10-20 nm). (v) In ferromagnetic compositions such as LCMO, a very interesting spin-glass type behavior was observed.

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

Transition metals (TMs) are ‘elements whose atoms have partially filled d-shell, or which can give rise to cations with an incomplete d-shell’. In TMs, the d-shell overlaps with next higher s-shell. Most of the TMs exhibit more than one (multiple) oxidation states. Some TMs, such as silver and gold, occur naturally in their metallic state but, most of the TM minerals are generally oxides. Most of the minerals on the planet earth are metal oxides, because of large free energies of formation for the oxides. The thermodynamic stability of the oxides is determined from the Ellingham diagram. Ellingham diagram shows the temperature dependence of the stability (free energy) for binaries such as metal oxides. Ellingham diagram also shows the ease of reducibility of metal oxides. TM oxides of general formulas MO, M2O3, MO2, M2O5, MO3 are known to exist, many of them being the ultimate products of oxidation in air in their highest oxidation states. In addition, TM oxides also exist in lower oxidation states which are prepared under controlled conditions. The nature of bonding in these oxides varies from mainly ionic (e.g. NiO, CoO) to mainly covalent (e.g. OsO4). Simple binary oxides of the compositions, MO, generally possess the rock salt structure (e.g. NiO), while the dioxides, MO2, possess the rutile structure (e.g. TiO2); many sesquioxides, M2O3, possess the corundum structure (e.g. Cr2O3). TMs form important ternary oxides like perovskites (e.g. CaTiO3), spinels (e.g. MgFe2O4) and so on. In TM oxides, the valence (outer) d-shell could be empty, d0 (e. g. TiO2), partially filled, dn (1≤ n≤ 9) (e.g. TiO, VO, NiO etc.) or completely filled, d10 (e.g. ZnO, CdO, Cu2O etc.). The outer d electrons in TM oxides could be localized or delocalized. Localized outer d electrons give insulators/semiconductors, while delocalized/itinerant d electrons make the TM oxide ‘metallic’ (e.g. ReO3, RuO2). Partially filled dn states are normally expected to give rise to itinerant (metallic) electron behaviour. But most of TM oxides with partially filled d shell are insulators because of special electronic energy (correlation energy) involved in d electron transfer to adjacent sites. Such insulating TM oxides are known as Mott insulators (e. g. NiO, CoO etc.). Certain TM oxides are known to exhibit both localized (insulating) and itinerant (metallic) behaviour as a function of temperature or pressure. For example, VO2 shows a insulator–metal transition at ~340K. Similar transitions are also known for V2O3, metal-rich EuO and so on. The chemical composition and bonding of TM oxides, which determine the crystal and electronic structures, give rise to functional properties. Table 1 gives representative examples. Properties like ionic conductivity and diffusion are governed by both the crystal structure and the defect structure (point defects), whereas properties such as magnetism and electron transport mainly arise from the electronic structures of the materials. Accordingly, TM oxides provide a platform for exploring functional materials properties. Among the various functional materials properties exhibited by transition metal oxides, the present thesis is devoted to investigations of lithium ion battery cathodes, inorganic pigments and magnetic perovskites. Over the years, most of the lithium containing first row transition metal oxides of rock salt derived structure have been investigated for possible application as cathode materials in lithium ion batteries (LIBs). First major breakthrough in LIBs research was achieved by electrochemically deinserting and inserting lithium in LiCoO2. A new series of cathode materials for LIBs were prepared by incorporating excess lithium into the transition metal containing layered lithium oxides through solid solution formation between Li2MnO3–LiMO2 (M = Cr, Mn, Fe, Co, Ni), known as lithium-rich layered oxides (LLOs). LLOs exhibit improved electrochemical performance as compared to the corresponding end members and hence received significant attention as a potential next generation cathode materials for LIBs in recent times. LiCoO2 (R-3m) crystallizes in the layered α-NaFeO2 structure with the oxygens in a ccp arrangement. Li+ and Co3+ ions almost perfectly order in the octahedral sites (3a and 3b) to give alternating (111) planes of LiO6 and CoO6 octahedra. Table 1. Materials properties exhibited by representative TM oxides. Property Example(s) Ferroelectricity BaTiO3, PbTiO3, Bi4Ti3O12 Nonlinear Optical Response LiNbO3 Multiferroic response BiFeO3, TbMnO3 Microwave dielectric properties Ba3ZnTa2O9 Relaxor Dielectric Properties Pb3MgNb2O9, Colossal Magnetoresistance Tl2Mn2O7 Metallic ‘Ferroelectricity’ Cd2Re2O7 Superconductivity AOs2O6(A = K, Rb, Cs) Redox deinsertion/insertion of LiCoO2 lithium Photocatalysis/water splitting TiO2 Pigment Ca(1-x)LaxTaO(2-x)N1+x (yellow-red), YIn1-xMnxO3 (blue) Metallic Ferromagnetism CrO2 Antiferromagnetism NiO, LaFeO3 Zero thermal expansion ZrW2O8 The reversible capacity of LiCoO2 in common LIBs is relatively low at around 140 mA h g-1 (half of theoretical capacity), corresponding to: LiCo3+O2 → Li0.5Co3+0.5Co4+0.5O2 + 0.5Li+ + 0.5e– . Substitution of one or more transition metal ions in LiCOO2 has been explored to improve the electrochemical performance. The structure of LLOs is described as a solid solution or nano composite of Li2MnO3 (C2/m) and LiMO2 (R-3m). The electrochemical deinsertion/insertion behaviour of LLOs is complex and also not yet understood completely. The present thesis consists of four parts. After a brief introduction (Part 1), Part 2 is devoted to materials for Li-ion battery cathode, consisting of three Chapters 2.1, 2.2 and 2.3. In Chapter 2.1, we describe the synthesis, crystal structure, magnetic and electrochemical characterization of new LiCoO2 type rock salt oxides of formula, Li3M2RuO6 (M = Co, Ni). The M =Co oxide adopts the LiCoO2 (R-3m) structure, whereas the M = Ni oxide also adopts a similar layered structure related to Li2TiO3. Magnetic susceptibility measurements reveal that in Li3Co2RuO6, the oxidation states of transition metal ions are Co3+, Co2+ and Ru4+, whereas in Li3Ni2RuO6, the oxidation states are Ni2+ and Ru5+. Li3Co2RuO6 orders antiferromagnetically at ~10K. On the other hand, Li3Ni2RuO6 presents a ferrimagnetic behaviour with a Curie temperature of ~100K. Electrochemical Li-deinsertion/insertion studies show that high first charge capacities (between ca.160 and 180 mA h g−1) corresponding to ca.2/3 of theoretical capacity are reached albeit, in both cases, capacity retention and cyclability are not satisfactory. Chapter 2.2 presents a study of new ruthenium containing LLOs, Li3MRuO5 (M = Co and Ni). Both the oxides crystallize in the layered LLO type LiCoO2 (α-NaFeO2) structure consisting of Li[Li0.2M0.4Ru0.4]O2 layers. Magnetic susceptibility data suggest that the oxidation states of transition metals are Li3Co3+Ru4+O5 for the M = Co compound and Li3Ni2+Ru5+O5 for the M = Ni compound. Electrochemical investigations of lithium deintercalation–intercalation behaviour reveal that both Co and Ni phases exhibit attractive specific capacities of ca. 200 mA h g-1 at an average voltage of 4 V, that has been interpreted as due to the oxidation of Co3+ and Ru4+ in Li3CoRuO5 and Ni2+ to Ni4+ in the case of Li3NiRuO5. Thus, we find that ruthenium plays a favourable role in LLOs than in non-LLOs in stabilizing higher reversible electrochemical capacities. In Chapter 2.3, we describe the synthesis, crystal structure and lithium deinsertion–insertion electrochemistry of two new LLOs, Li3MRuO5 (M=Mn, Fe) which are analogs of the oxides described in Chapter 2.2. The Li3MnRuO5 oxide adopts a structure related to Li2MnO3 (C2/m), while the Li3FeRuO5 oxide adopts a near-perfect LiCoO2 (R-3m) structure. Lithium electrochemistry shows typical behaviour of LLOs for both oxides, where participation of oxide ions in the electrochemical processes is observed. A long first charge process with capacities of 240 mA h g-1 (2.3 Li per f.u.) and 144 mA h g-1 (1.38 Li per f.u.) is observed for Li3MnRuO5 and Li3FeRuO5, respectively. Further discharge–charge cycling points to partial reversibility. X-ray photoelectron spectroscopy (XPS) characterisation of both pristine and electrochemically oxidized Li3MRuO5 reveals that in the Li3MnRuO5 oxide, Mn3+ and Ru4+ are partially oxidized to Mn4+ and Ru5+ in the sloping region at low voltage, while in the long plateau, O2- is also oxidized. In the Li3FeRuO5 oxide, the oxidation process appears to affect only Ru (4+ to 5+ in the sloping region) and O2- (plateau), while Fe seems to retain its 3+ state. Another characteristic feature of TMs is formation of several coloured solid materials where d–d transitions, band gap transitions and charge transfer transitions are involved in the colouration mechanism. Coloured TM oxides absorbing visible light find important applications as visible light photocatalyst (for example, yellow BiVO4 for solar water splitting and red Sr1-xNbO3 for oxidation of methylene blue) and inorganic pigments [for example, Egyptian blue (CaCuSi4O10), Malachite green (Cu2CO3(OH)2), Ochre red (Fe2O3)]. Pigments are applied as colouring materials in inks, dyes, paints, plastics, ceramic glazers, enamels and textiles. In this thesis, we have focused on the coloured TM oxides for possible application as inorganic pigments. Generally, colours arise from electronic transitions that absorb visible light. Colours of the inorganic pigments arise mainly from electronic transitions involving TM ions in various ligand fields and charge transfer transitions governed by different selection rules. The ligand field d–d transitions are parity forbidden but are relaxed due to various reasons, such as distortion (absence of center of inversion) and vibronic coupling. The d-electrons can be excited by light absorption in the visible region of the spectrum imparting colour to the material. Charge transfer transitions in the visible region are not restricted by the parity selection rules and therefore give intense colours. Here we have investigated the colours of manganese in unusual oxidation state (Mn5+) as well as the colours of different 3d-TM ions in distorted octahedral and trigonal prismatic sites in appropriate colourless crystalline host oxides. These results are discussed in Part 3 of the thesis. In Chapter 3.1, we describe a blue/green inorganic material, Ba3(P1−xMnxO4)2 (I) based on tetrahedral Mn5+O4 :3d2 chromophore. The solid solutions (I) which are sky-blue and turquoise-blue for x ≤ 0•25 and dark green for x ≥ 0•50, are readily synthesized in air from commonly available starting materials, stabilizing the Mn5+O4 chromophore in an isostructural phosphate host. We suggest that the covalency/ionicity of P–O/Mn–O bonds in the solid solutions tunes the crystal field strength around Mn(V) such that a blue colour results for materials with small values of x. The material could serve as a nontoxic blue/green inorganic pigment. In Chapter 3.2, an experimental investigation of the stabilization of the turquoise-coloured Mn5+O4 chromophore in various oxide hosts, viz., A3(VO4)2 (A = Ba, Sr, Ca), YVO4, and Ba2MO4 (M = Ti, Si), has been carried out. The results reveal that substitution of Mn5+O4 occurs in Ba3(VO4)2 forming the entire solid solution series Ba3(V1−xMnxO4)2 (0 < x ≤ 1.0), while, with the corresponding strontium derivative, only up to about 10% of Mn5+O4 substitution is possible. Ca3(VO4)2 and YVO4 do not stabilize Mn5+O4 at all. With Ba2MO4 (M = Ti, Si), we could prepare only partially substituted materials, Ba2M1−xMn5+xO4+x/2 for x up to 0.15, that are turquoise-coloured. We rationalize the results that a large stabilization of the O 2p-valence band states occurs in the presence of the electropositive barium that renders the Mn5+ oxidation state accessible in oxoanion compounds containing PO43−, VO43−, etc. By way of proof-of-concept, we synthesized new turquoise-coloured Mn5+O4 materials, Ba5(BO3)(MnO4)2Cl and Ba5(BO3)(PO4)(MnO4)Cl, based on the apatite – Ba5(PO4)3Cl – structure. Chapter 3.3 discusses crystal structures, and optical absorption spectra/colours of 3d-transition metal substituted lyonsite type oxides, Li3Al1-xMIIIx(MoO4)3 (0< x ≤1.0) (MIII = Cr, Fe) and Li3-xAl1-xMII2x(MoO4)3 (0< x ≤1.0) (MII = Co, Ni, Cu). Crystal structures determined from Rietveld refinement of PXRD data reveal that in the smaller trivalent metal substituted lyonsite oxides, MIII ions occupy the octahedral (8d, 4c) sites and the lithium ions exclusively occur at the trigonal prismatic (4c) site in the orthorhombic (Pnma) structure; on the other hand, larger divalent cations (CoII/CuII) substituted derivatives show occupancy of CoII/CuII ions at both the octahedral and trigonal prismatic sites. We have investigated the colours and optical absorption spectra of Li3Al1-xMIIIx(MoO4)3 (MIII = Cr, Fe) and Li3-xAl1-xMII2x(MoO4)3 (MII = Co, Ni, Cu) and interpreted the results in terms of average crystal field strengths experienced by MIII/MII ions at multiple coordination geometries. We have also identified the role of metal-to-metal charge transfer (MMCT) from the partially filled transition metal 3d orbitals to the empty Mo – 4d orbitals in the resulting colours of these oxides. B The ABO3 perovskite structure consists of a three dimensional framework of corner shared BO6 octahedra in which large A cation occupies dodecahedral site, surrounded by twelve oxide ions. The ideal cubic structure occurs when the Goldschmidt’s tolerance factor, t = (rA + rO)/{√2(rB + rO)}, adopts a value of unity and the A–O and B–O bond distances are perfectly matched. The BO6 octahedra tilt and bend the B – O – B bridges co-operatively to adjust for the non-ideal size of A cations, resulting deviation from ideal cubic structure to lower symmetries. Ordering of cations at the A and B sites of perovskite structure is an important phenomenon. Ordering of site cations in double (A2BB'O6) and multiple (A3BB'2O9) perovskites give rise to newer and interesting materials properties. Depending upon the constituent transition metals and ordering, double perovskite oxides exhibit a variety of magnetic behaviour such as ferromagnetism, ferrimagnetism, antiferromagnetism, spin-glass magnetism and so on. We also have coupled magnetic properties such as magnetoresistance (Sr2FeMoO6), magnetodielectric (La2NiMnO6) and magnetooptic (Sr2CrWO6) behaviour. Here we have investigated new magnetically frustrated double perovskite oxides of the formula Ln3B2RuO9(B = Co, Ni and Ln = La, Nd). The Chapter 4.1 describes Ln3B2RuO9 (B = Co, Ni and Ln = La, Nd) oxides (prepared by a solid state metathesis route) which adopt a monoclinic (P21/n) A2BB'O6 double perovskite structure, wherein the two independent octahedral 2c and 2d sites are occupied by B2+ and (B2+1/3Ru5+2/3) atoms, respectively. Temperature dependence of the molar magnetic susceptibility plots obtained under zero field cooled (ZFC) condition exhibit maxima in the temperature range 25–35K, suggesting an antiferromagnetic interaction in all these oxides. Ln3B2RuO9 oxides show spin-glass behavior and no long-range magnetic order is found down to 2 K. The results reveal the importance of competing nearest neighbour (NN), next nearest neighbor (NNN) and third nearest neighbour (third NN) interactions between the magnetic Ni2+/Co2+ and Ru5+ atoms in the partially ordered double perovskite structure that conspire to thwart the expected ferromagnetic order in these materials.