Phase formation studies in the quaternary section Ca_2-<I>y</I>Sr<I>_y</I>Fe_2-<I>x</I>B'<I>_x</I>O<I>_γ</I> : B' = Nb, Ta, 0 ≤ <I>y</I> ≤ 2.0 and 0 ≤ <I>x</I> ≤ 2.0 were carried out. Results are summarised below for B' = Nb; for B' = Ta they were broadly similar. Four solid solution phases were found: i) a cubic solid solution, with a variable compositional extent <I>x</I>, which increased with substitution of Ca by Sr, from 0.45 ≤ <I>x</I> ≤ 0.65 for <I>y</I> = 0, to 0 ≤ <I>x</I> ≤ 1.2 for <I>y</I> = 2.0. ii) A limited orthorhombic solid solution along the Ca₂Fe₂O₅-Ca₂B'₂O₇ join with 0.8 ≤ <I>x</I> ≤ 1.0. iii) A complete range of orthorhombic, brownmillerite solid solution for <I>x</I> = 0 and iv) an orthorhomic, perovskite-related solid solution, isostructural with Ca₂Nb₂O₇, for <I>x</I> = 2.0. A combination of differential thermal analysis, thermogravimetry, Mössbauer spectroscopy, high temperature powder X-ray diffraction, Rietveld refinement, a.c. impedance spectroscopy and magnetic susceptometry has been used to characterise these new phases. Solid solution (i) has variable oxygen content with an oxygen-deficient perovskite structure. As synthesised, for <I>x</I> = 0.6 it contains a mixture of Fe⁴⁺ and Fe³⁺ in the ratio 13/87. The oxygen content could be modified by heat treatment under various atmospheres over the range 5.5. ≤ <I>γ</I> ≤ 5.92 which corresponded to the ratios: 93/7 to Fe³⁺/Fe²⁺ and 42/58 of Fe⁴⁺/Fe³⁺. At higher oxygen contents, <I>γ</I> > 5.7, transformation to an orthorhombic structure was observed. The electrical resistivity was dominated by grain boundary effects. Conduction is electronic and is attributed to hopping between iron in mixed oxidation states. The resistivity increased dramatically on reduction and became <I>n</I>-type, presumably due to a small fraction of Fe²⁺ present.

The physics of doped transition metal perovskite has been an area of intense research in the last few decades due to their interesting magnetic and transport properties. Various exciting phenomena such as, colossal magneto resistance, high Tc superconductivity, multiferroicity, ferroelectricity, high temperature ferromagnetism, etc., have made these systems more fascinating in terms of fundamental study as well as technological applications. There are several intrinsic material characteristics in these perovskite oxides that can impact their magnetic properties. Lattice distortion and chemical in homogeneity are two important ones. Changes in valence and ionic radius in rare earth (A- site) and transition metal (B- site) directly result in structural modification through internal pressure. Consequently, atomic distances and bond angles between the transition metals vary. This, intern, influences the nearest neighbour exchange coupling energy and magnetic interaction. A detailed investigation has been carried out on two A-site doped perovskite namely, La0.85Sr0.15CoO3 & La0.5Sr0.5CoO3 and two B-site doped perovskite, LaMn0.5Co0.5O3 & LuMn0.5Ni0.5O3 with a view to study the impact of chemical in homogeneity and lattice distortion on their respective magnetic ground states. The thesis is organized in seven chapters. A brief summary of each is given below: Chapter 1: Provides a brief introduction about the perovskite structure. Origins of lattice distortions and its effect on the magnetic properties are discussed. It includes a discussion on different types of indirect magnetic interactions involved in perovskite oxide structure. The chapter concludes with a description of spin-glass, phase separation/ cluster-glass, memory effect in glassy magnetism, critical behaviour at phase transition and specific heat in magnetic systems. Chapter 2: This chapter outlines basic principles of the experimental techniques employed for the work presented in this thesis. Chapter 3: Details macroscopic as well as microscopic investigations carried out to understand the glassy magnetic anomalies in La0.85Sr0.15CoO3 samples. The origin of phase separation (PS) has been reinvestigated. Since the magnetic behavior of La0.85Sr0.15CoO3 (LSCO15) lies in the border of spin glass (SG) and ferromagnetic (FM) region in the x-T phase diagram, it is subject to controversial debate for the last several years. While some research groups favour PS, others regard SG behaviour as the dominant phenomenon. In-depth investigation carried out to elucidate these views is outlined in this chapter in two sections. The first section deals with the glassy magnetic anomalies in single crystals of LSCO15 grown by optical floating zone method. Since the sample crystallizes from melt, it possesses good compositional homogeneity and the phase purity is confirmed by XRD pattern. Many characteristics of canonical SG systems are discernible in the magnetic study, such as, kink in field-cooling curve below Tf, frequency-dependent peak shift and the time dependent memory effect. The relaxation time in sub-pico second range (~10-13 s) is very similar to that of the typical SG systems. Time dependent transport relaxation study exhibits memory effect and the time evolution of resistance scales with magnetization and strictly adheres to the stretched exponential behaviour as commonly expected for a SG-like disordered system. However, a detailed study on transport mechanism and temperature-dependent inverse susceptibility reveals the existence of nanoscopic PS in the sample. In the second section, the origin of PS has been examined through a comprehensive study on two sets of LSCO15 polycrystalline samples prepared from the same initial mixture but subjected to different heat treatment processes. This study depicts the dependence of PS on the preparation conditions. The contrasting magnetic behaviour of PS and SG was resolved by experiments of dc magnetization, linear & non-linear ac susceptibility, neutron depolarization and field-cooled magnetic relaxation. Both samples conform to the general characteristics of a glassy behaviour: a kink in FC magnetization, frequency-dependent peak shift (Vogel–Fulcher law), dc bias-dependent peak shift in accordance with de Almeida–Thouless relation, and characteristic relaxation time in the range of 10-13/10-14 s. This is despite their internal spin structure and interaction being much different at a microscopic level. It is found that the sample processed through a proper homogenization process mimics the SG behaviour, whereas the sample prepared by the conventional method behaves like the PS phase. It is confirmed from neutron depolarization experiments that no ferromagnetic correlation exists in the SG phase of La0.85Sr0.15CoO3, a result in contrast to that of PS phase. Higher harmonic ac susceptibility measurement complements the above observation by the evidence that of 2nd order harmonics are not present in the SG phase of La0.85Sr0.15CoO3. The field-cooled magnetic relaxation study makes a distinct reference to the relaxation process and the strength of interaction between PS and SG like phases. In essence, a concerted effect is made to identify and resolve the spin-glass phase from phase-separated/ cluster-glass. This work shows that chemical in homogeneity is a key factor responsible for phase separation in La0.85Sr0.15CoO3; also intrinsic differences between PS and SG are identified that can serve as guiding tools for research in other similar magnetic oxide systems. It is concluded that the true ground state magnetic property of La0.85Sr0.15CoO3 is spin-glass in nature. Chapter 4: This chapter contains two sections. In the first part, the origin of the re-entrant spin-glass (RSG) behaviour in La0.5Sr0.5CoO3 has been investigated using the conventional magnetometer measurements. Polycrystalline samples prepared by the conventional solid-state synthesis exhibit RSG characteristics with a glassy transition at 190 K. The nature of frequency dependence of χ″(T), a pronounced memory effect and the sluggish response in dc magnetization measurement, all of which clearly indicate the re-entrant behaviour. But, once the sample is taken through a rigorous homogenization procedure of repeated grinding and annealing, its phase turns into pure ferromagnetic one. During the course of this homogenization process, the sample loses oxygen with concurrent degeneration of TC to a lower level. In order to regain the oxygen stoichiometry, it is necessary to anneal the sample in oxygen environment at 900 oC, which triggers deleterious ageing effect by which TC falls progressively with time. In the second part, the effect of oxygen stoichiometry on La0.5Sr0.5CoO3 (LSCO50) thin-films has been investigated. The highest TC reported so far for LSCO50 thin film is 250 K, which is significantly less compared to the bulk TC (262 K) of an oxygen stoichiometric compound. This work focuses on achieving the highest ferromagnetic transition temperature (TC) for LSCO50 films under optimized growth conditions. The analysis of experimental data suggests that the Curie temperature can be enhanced to 262 K, irrespective of whether or not, (a) the film on LAO or STO or (b) any induced strain occurs in the LSCO50 film. Apart from different thin-film growth parameters such as oxygen pressure and substrate temperature during the growth, and post-growth annealing temperature and oxygen pressure, the profile of the laser beam used for ablation of bulk material profile also plays an important role. The elevation of Curie temperature observed in thin-films to that close to the bulk value is believed to be a result of improved stoichiometric composition of oxygen facilitated during thin film growth. However, the strong ageing effect seen is quite close to that is observed in oxygen-annealed polycrystalline sample. Chapter 5: Of the three segments constituting this chapter, the first outlines different magnetic anomalies induced by lattice distortion in LaMn0.5Co0.5O3 (LMCO) single crystals. Single crystals of LMCO compound [(100) orientation] have been successfully grown using the optical floating zone method. Powder as well as single crystal x-ray diffraction analyses provides evidence of large strain dependent structural distortion in as-grown crystals. Spatially resolved 2-D Raman scan reveals that the strain generates a distribution of octahedral distortion in the lattice. While some are compressive in nature, others in the nearby territory relate to tensile distortion. The ac susceptibility measurement elucidates distinct changes in the ferromagnetic transition temperature (TC) in the as grown (strained) crystal. It is possible to release strain by rigorous annealing process. Which also results in a uniform TM-O octahedral deformation. Room temperature 2-D Raman spectra bears testimony to this. Upon annealing, the single crystalline order is diminuend by the atomic rearrangement. This causes tilting of the oxygen octahedra, by decreasing intra-octahedral angle θTM-O-TM, and lowering of exchange energy Jex between the magnetic ions. The transition temperature falls and the magnetic phase merges with that in the strain-free polycrystalline material. A detailed critical analysis performed in the vicinity of paramagnetic to ferromagnetic phase transition in both the samples establishes that the ground state magnetic behaviour, assigned to the strain-free LMCO crystal is of 3D Heisenberg type. But the local octahedral distortion present in the as-grown crystal causes mean field like magnetic interaction at few local sites. This serves as a key drive for the critical exponents to distance from the 3D Heisenberg model towards the mean-field type. The second part of this chapter concerns the anomalous re-entrant glassy magnetic behaviour observed in LMCO single crystals. The ac susceptibility study illustrates the low temperature anomalous glassy magnetic ordering in these crystals. The material behaves like a normal magnetic glass, (frequency-dependent peak-shift in ac susceptibility) in conformance with the phenomenological Vogel-Fulcher law, of spin flips time: ~10-4 s. However, the crystal does not respond to the external dc bias and just as well remains free from memory effect. Anomalous behaviour of this kind is rare in magnetic oxides. The magneto-dielectric effect in LMCO is discussed in the third section of this chapter. The real part of dielectric permittivity (ε′) has a colossal value of 1800 at 220 K and 10 kHz. However as the sample is cooled further, ε′ decreases slowly; followed by dielectric relaxation in the region, 120 - 150 K. Detailed analysis of the temperature dependence of the imaginary part of the dielectric permittivity (ε″) show that there is no relaxor-like phenomena in this compound. The frequency dependence of ε″ reveals that the low frequency region is dominated by Maxwell-Wagner relaxation, whereas, at high frequency, a Debye type relaxation persists. The temperature dependent full-width at half-maximum for this Debye relaxation, peaks at the corresponding TC. The temperature variation of the relaxation time has two domains of different slopes. At zero external field, ε″(ω) has a low activation energy (U = 46.4 meV) in the ferromagnetic region, compared to that in the paramagnetic (60.1 meV) phase. The boundary lies near the corresponding TC. In the presence of external applied field 5 T, U remains unchanged in the ferromagnetic region, but decreases ( U ~ 5 meV) in the paramagnetic phase. These results signify the existence of strong magneto-dielectric coupling in LMCO crystals. The field variation of ε′(ω) at fixed temperature and specific frequency highlights the rise in magnetodielectricity (MD) as well as magneto-loss (ML) with increasing magnetic field. It is perceived that this variation is not due to the magneto resistance of LMCO or caused by LMCO - electrode interfaces. The influence of extrinsic parasitic contributions cannot be ruled out entirely, but the presence of positive MD as well as ML at frequencies above the time constant suggests that the relaxation process and the magneto-dielectric coupling are intrinsic to the LaMn0.5Co0.5O3 system. Chapter 6: This chapter describes the successful synthesis of a new perovskite oxide compound, LuMn0.5Ni0.5O3. The structural characterization employs the Rietveld refinement of powder X-ray diffraction pattern. The compound crystallizes in orthorhombic Pbnm crystal structure. dc magnetization reveals ferromagnetic ordering in the sample. However the low temperature glassy phase spotted in the ac susceptibility measurement might classify it as a re-entrant spin-glass compound. But the display of memory effect until the ferromagnetic transition indicates that intrinsic ant ferromagnetic interaction prevails over the dominant ferromagnetic interaction. A critical behaviour study was carried out in the vicinity of the ferromagnetic to paramagnetic phase transition, which provided the critical exponents: α = 0.37, β = 0.241 ± 0.003, γ = 1.142 ± 0.003 and δ = 5.77 ± 0.03. Interestingly, this set of critical exponents does not match with any of the conventional theories of mean field, 3D Heisenberg, and 3D Ising. Rather it fits quite well with data calculated for the stacked triangular 3D version of the (Z2 × S1) model [α = 0.34 ± 0.06, β = 0.25 ± 0.01, γ = 1.13 ± 0.05 and δ = 5.47 ± 0.27]. This study indicates that the magnetic ground state of LuMn0.5Ni0.5O3 is canted ferromagnetic. Chapter 7: Various important results are summarized in this chapter. It also provides a broad outlook in this area of research.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

A Walker-type multianvil high-pressure facility is capable of high-pressure syntheses and measurements beyond 10 GPa and has been utilized in my research to synthesize the 4d Ruthenium and Rhodium and the 5d Iridium oxides with the perovskite-related structures. Under high-pressure and high-temperature conditions, these families of oxides can be enlarged to a great extent so that enables us not only to address the long-standing problem about ferromagnetism in the perovskite ruthenates but also explore new phenomena associated with the structural and electronic properties in the iridates and rhodates. In the perovskite ruthenates ARuO₃ (A= Ca, Sr, and Ba), a systematic study of the variations of the ferromagnetic transition temperature T[subscript c] and the critical isothermal magnetization as a function of the average A-site cation size and the size variance as well as external high pressures reveals explicitly the crucial role of the local lattice strain and disorder on T[subscript c] and the nature of the localized-electron ferromagnetism. However, such a steric effect is dominated by the electronic effect in another perovskite ruthenate PbRuO₃, which is a paramagnetic metal down to 1.8 K and undergoes a first-order structural transition to a low-temperature Imma phase at Tt [almost equal to] 90 K. Bandwidth broadening due to orbital hybridization between Pb-6s and Ru-4d plays an important role in suppressing the ferromagnetism in the Sr1-zPbzRuO₃ system. The high-pressure sequence of the 9R-BaIrO₃ was explored and three more polytypes, i.e. 5H, 6H and 3C, were identified under 10 GPa. With increasing fraction of the corner- to face-sharing IrO₆/₂ octahedra, the ground states of BaIrO₃ evolve from a ferromagnetic insulator with T[subscript c] [almost equal to] 180 K in the 9R phase to a ferromagnetic metal with T[subscript c] [almost equal to] 50 K in the 5H phase, and finally to an exchange-enhanced paramagnetic metal near a quantum critical point in the 6H phase. In addition to the perovskite SrRhO₃, a new 6H polytype was synthesized for the first time under high pressure and a pressure-temperature phase diagram was given for the 6H-perovskite transformation. Restoration of the Curie-Weiss behavior in the high-temperature magnetic susceptibility [chi](T) of the perovskite SrRhO₃ resolves the puzzle about unusual dependence of [chi]⁻¹ [symbol] T² reported earlier and highlights the importance of spin-orbit coupling in the 4d and 5d transition-metal oxides. / text

The work presented involved simulation and experimental studies aimed at improving the methodology of positron annihilation lifetime spectroscopy (PALS), and applied PALS to gain a better understanding of doping mechanisms in ABO3 perovskite oxide materials. Reliable decomposition of PALS spectra requires an accurate description of the instrument resolution function (IRF) and the extrinsic, source component, annihilation events. The source terms include annihilations with the crystallites of the radionuclide and in the thin foil normally used to support the source. In principle both the IRF and the source correction terms can accurately be determined if samples exhibiting a true single lifetime component are measured. A series of annealing studies was performed on commercially available high purity polycrystalline metal samples to reduce the defect concentration below the approximate 0.1 ppm detection limit of PALS. The study showed that despite the numerous reports in literature it was not possible to reproduce the results with similar annealing conditions or sample purity. The possibility of utilising two-lifetime materials to enable the extraction of source correction terms is analysed using simulations, and by experiments on commercially available pure polycrystalline metals. The positron source is commonly deposited on, and supported by, a thin Kapton foil. As part of this work variable energy PALS (VE-PALS) performed at the Munich Research Reactor FRMII on Kapton foils were analysed. This enabled one of the source correction terms to be unambiguously determined. In consequence, the source correction terms for a Kapton supported positron source were extracted from measurements using annealed nickel exhibiting two positron lifetime components. PALS was applied to a study on donor doping of PbTiO3 ceramics using a series of lanthanide-ions. It has been proposed that the smaller Ln-ions may act as amphoteric dopants substituting either on the A-site as a donor, or on the Bsite as an acceptor. In this study Ln-ions in size from La down to Er were studied. A systematic variation in the average positron lifetime was observed where the value was constant from La to Gd and then reduced for the smaller ions. The decrease in average lifetime provides evidence for a reduction in the fraction of trapping to A-site related vacancy defects. The onset of a reduction in the average lifetime between Gd and Dy provides evidence for a change in the doping mechanism resulting in a relative reduction in the fraction of A-site vacancy positron trapping. In contrast to PbTiO3, donor doping of SrTiO3 normally results in electron charge compensation. Recently this has been very clearly demonstrated for La3+ doped SrTiO3 thin films grown by molecular beam epitaxy (MBE) which exhibit exceptional electron mobilities. A series of MBE films grown at University of California Santa Barbara were measured by VE-PALS at FRMII and have been analysed here. Strontium vacancies were identified, and a reduced bulk lifetime component was also observed. This enabled bulk lifetime values to be obtained from two of the films which were in good agreement with the previously obtained values from single crystal samples. A PALS study was also performed on a series of B-site donor, Nb, doped SrTiO3 crystals. High intensity reduced bulk components were observed and enabled measurements of the bulk lifetime. The highest Nb doping level samples showed the most intense reduced bulk lifetime but also clearly demonstrated the presence of Sr vacancies. The observation of A-site vacancy defects for both Nb-doped and La-doped SrTiO3 suggest that formation of these defects is preferred and are independent of the site of incorporation of the donor ion. Studies were also performed on acceptor doped SrTiO3. PALS measurements were made on a series of Fe-doped SrTiO3 ceramic samples, and VE-PALS measurements on pulsed laser deposition of Fe-doped SrTiO3 thin film samples were analysed. The positron lifetime measurements on the ceramic samples showed a dominant 166(3) ps component, a value less than the Ti-vacancy lifetime. It is proposed that the component contains a contribution from positrons trapping at oxygen vacancy substitutional Fe impurity complexes with a local charge that is neutral or negative. The measurements on the series of Fe-doped PLD SrTiO3 films suggest a complex relation between the vacancy defect content of a film and both the Fe-doping and PLD growth conditions. Films grown with higher laser fluence values contained Sr vacancy defects, in contrast to previous studies of acceptor doped perovskites. Films grown with low laser fluence or with high Fe-content showed dominant trapping to Ti-vacancy related defects.

The current energy and environmental scenario in the world demands acute attention on sustainable repurposing of waste CO2 to high value hydrocarbons that not only addresses the CO2 mitigation problem, but also provides pathways for a closed loop synthetic carbon cycle. Difference in the scales of global CO2 emissions (about 40 Gtpa, 2017) and the carbon capture and sequestration (CCS) facilities (estimated cumulative 40 Mtpa, 2018) provokes active research on this topic. Solar thermochemical (STC) and visible light photocatalysis are two of the most promising routes that have garnered attention for this purpose. While STC has the advantages of high CO2 conversion rates, it operates at high temperatures (more than 1000 °C) limiting its industrial implementation. Photocatalysis, on the contrary, is plagued by the poor quantum efficiency and conversion rates, although its exhibits the benefits of low temperature operation. Thus, any significant progress towards low temperature STC and visible light photocatalytic CO2 reduction is a giant leap towards a greener and sustainable energy solution. This dissertation is an effort towards improving both the STC and photocatalytic CO2 reduction. Reverse water gas shift - chemical looping (RWGS-CL) is a modified STC approach that has the potential for low temperature CO2 conversion. RWGS-CL process uses mixed metal oxides like perovskite oxides (ABO3) for the conversion to CO, a potential feedstock for subsequent hydrocarbon production. Generation of oxygen vacancy defects on these perovskite oxides is a key step of RWGS-CL and thus, oxygen vacancy formation energy has been found to be a key descriptor for this process. Using density functional theory based calculations, this intrinsic material property has been used towards rational design of better catalysts. Highest rate of CO2 conversion at the low temperatures of 450 °C was demonstrated by earth abundant perovskite oxide via RWGS-CL. This low temperature and stable CO2 conversion process enables thermal integration with subsequent Fischer Tropsch processes for the hydrogenation of CO to hydrocarbons. Parallel to the developments on materials discovery, another crucial parameter that deserves attention is the surface termination effects of the perovskite oxides. Hence, the site specificity of the bulk and surface oxygen vacancies have been probed in detail towards elucidating the CO2 conversion performance over these materials. In the view of recent progress on the growth of selective crystal facets and terminations, this study opens new avenues for enhanced CO2 conversion performance not only through bulk composition variation, but also via exposing desired crystal facets. Type-II semiconductor heterojunctions (staggered type) are promising candidates for efficient photocatalytic reactions, not only because of their capabilities of electronic density of states tuning, but also their ability to segregate the excited electrons and holes into different materials thereby restricting exciton recombination. Metal oxynitride heterojunctions have recently demonstrated promising activity on visible light water splitting. Elucidating the structure-function relationships for these materials can pave the way towards designing better CO2 conversion photocatalysts. This dissertation focuses on unravelling the roles of material composition, anion vacancy defects and lattice strain towards modulating the electronic density of states of lateral and vertical heterojunctions of (ZnO)X(AlN)1-X and (ZnO)X(GaN)1-X. The heterojunctions consist of periodic potential wells that allows for restricting interlayer charge transport. Increased ZnO concentration was explicitly shown to decrease the band gap due to N 2p and Zn-3d repulsion. Biaxial and vertical compressive strain effected increased band gap while tensile strain reduced the same. Oxygen vacancies was found to have different effect on the electronic state of the materials. When present in charged state (+2), it promotes mid gap state formation, while in neutral state it revealed increased electronic densities near the valence band and conduction band edges. These fundamental site specific material property tuning insights are essential for designing better photocatalysts for future.

This dissertation is divided into two parts. In the first, the general paradigm of scanned probe microscopy is outlined with a focus on atomic force microscopy and a few of its variations. Magnetic force microscopy is covered in detail as it forms the basis of the second part of this dissertation. The core elements and extra features of the instrument are described with attention paid to the upgrades made by the author. In the second part of this dissertation, background information on perovskite oxides and the inverse spinel system, magnetite, is given. Magnetic force microscopy studies were done on three thin film systems and are detailed. In the first study, ferromagnetic manganite films were subjected to discontinuous changes in strain due to structural transitions in their barium titanate substrates. The resulting effect on the magnetic domains was observed. In the second study, the ferromagnetism of a tensile-strained LaCoO₃ film was studied across temperatures from 4.3 K to 90 K and applied fields up to [mu]₀H=1.1 T. Finally, the properties of antiphase domains in magnetite films of varying film strain due to transition metal buffer layers was probed by imaging the magnetic domains which are pinned to the antiphase boundaries. / text

Oxide heterostructures are attracting huge interest in recent years due to the special functionalities of quasi two-dimensional quantum gases. In this thesis, the electronic states at the interface between perovskite oxides and pyrite compounds have been studied by first-principles calculations based on density functional theory. Optimization of the atomic positions are taken into account, which is considered very important at interfaces, as observed in the case of LaAlO3/SrTiO3. The creation of metallic states at the interfaces thus is explained in terms of charge transfer between the transition metal and oxygen atoms near the interface. It is observed that with typical thicknesses of at least 10-12 °A the gases still extend considerably in the third dimension, which essentially determines the magnitude of quantum mechanical effects. To overcome this problem, we propose incorporation of highly electronegative cations (such as Ag) in the oxides. A fundamental interest is also the thermodynamic stability of the interfaces due to the possibility of atomic intermixing in the interface region. Therefore, different cation intermixed configurations are taken into account for the interfaces aiming at the energetically stable state. The effect of O vacancies is also discussed for both polar and non-polar heterostructures. The interface metallicity is enhanced for the polar system with the creation of O vacancies, while the clean interface at the non-polar heterostructure exhibits an insulating state and becomes metallic in presence of O vacancy. The O vacancy formation energies are calculated and explained in terms of the increasing electronegativity and effective volume of A the side cation. Along with these, the electronic and magnetic properties of an interface between the ferromagnetic metal CoS2 and the non-magnetic semiconductor FeS2 is investigated. We find that this contact shows a metallic character. The CoS2 stays quasi half metallic at the interface, while the FeS2 becomes metallic. At the interface, ferromagnetic ordering is found to be energetically favorable as compared to antiferromagnetic ordering. Furthermore, tensile strain is shown to strongly enhance the spin polarization so that a virtually half-metallic interface can be achieved, for comparably moderate strain. Our detailed study is aimed at complementing experiments on various oxide interfaces and obtaining a general picture how factors like cations, anions, their atomic weights and elecronegativities, O vacancies, lattice mismatch, lattice relaxation, magnetism etc play a combined role in device design.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18996号 / 工博第4038号 / 新制||工||1622(附属図書館) / 31947 / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 田中 勝久, 教授 平尾 一之, 教授 三浦 清貴 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Solid Oxide Fuel Cells could play a key role in energy systems of the future because they can directly convert the chemical energy of fuels into electrical energy in a reliable and energy efficient way. The choice of materials for the components of fuel cells is crucial for the achievement of the high performance and the low price necessary to establish fuel cell technology in the energy market. Current state of the art anodes consisting of nickel and yttria stabilised zirconia (Ni/YSZ) offer good electrochemical performance but suffer from limitations like carbon deposition, redox instability and sulphur poisoning. This thesis explores the properties of composite fully ceramic anodes consisting of a skeleton of yttria stabilised zirconia (YSZ) or cerium gadolinium oxide (CGO) and a perovskite phase based on B-site doped lanthanum strontium titanate. The perovskite phase was fabricated in situ inside the pores of the skeleton material by the infiltration of an aqueous precursor and subsequent firing (impregnation method). Material characterisation of the composite anodes was carried out by X-ray diffraction and the microstructure investigated by electron microscope techniques. The electrochemical performance was tested by IV-curves and impedance spectroscopy. Particularly the investigation of the connection between the microstructure of the impregnated anodes and their electrochemical performance is a main objective of this work. The electrochemical performance of cells with a CGO skeleton and an impregnated lanthanum strontium titanate phase was found to be inferior compared to cells with a YSZ skeleton, even if the ionic conductivity of CGO is known to be higher than the ionic conductivity of YSZ. The difference was assigned to mass transport problems tightly connected to the different microstructure of the composite anodes. A significant improvement of the performance could be achieved by the utilisation of A-site deficient perovskites as impregnated phase in a YSZ skeleton. Cells with composite anodes of YSZ and La₀.₄Sr₀.₄Ti₀.₉₄Mn₀.₀₆O[sub](3-δ) show power densities of 156.2 mW/cm² at a measuring temperature of 750 °C compared to 58.5 mW/cm² measured in a similar cell with A-site stoichiometric LSTM, both cells having an electrolyte thickness of around 60 μm. The superiority of the performance of anodes with A-site deficient perovskites is mainly due to a lower ohmic resistance of only 0.5 Ω*cm², indicating better conductivity of the composite with A-site deficient perovskites. The investigation of the microstructure of composite anodes with A-site deficient perovskites showed the decoration of the surface with nanoparticles after reduction. These nanoparticles originate from exsolution of ions from the B-site of the perovskite and can't be found in A-site stoichiometric perovskites. The influence of fabrication parameters like firing temperature of the skeleton, firing temperature after impregnation or vacuum impregnation on the microstructure and electrochemical performance of the composite anodes was studied. Particularly the increase of the firing temperature of the skeleton from 1400 °C to 1500 °C resulted in an impressive improvement of total cell resistance and maximal power density.