Physical properties of transition metal oxides synthesized by floating zone method and spark plasma sintering
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Transition metal oxides have attracted growing attention over the last few decades because of rich physical properties they exhibit. Perovskite structure transition metal oxides AMO₃ are of particular interest to the design of functional materials in modern techniques, since a variety of ways can be used to tune the physical properties of AMO₃. Single crystals of Y₁₋ₓLaₓTiO₃ are grown by floating zone method to study the magnetic transition from ferromagnetic in YTiO₃ to G-type antiferromagnetic in LaTiO₃. Y₁₋ₓ LaₓTiO₃ shows similar magnetic phase diagram with RTiO₃ family, and the magnetism and the transition temperature can be finely tuned by varying the La doping x. By measuring the change of magnetic transition temperatures on single crystal samples under uniaxial stress, the correlation between the lattice distortions and the cooperative orbital ordering can be distinguished. Double perovskite CaMnTi₂O₆ is the first columnar A-site ordered perovskite exhibiting ferroelectric property. Spark plasma sintering (SPS) is used to successfully synthesize gram-level Ca₂₋ₓMnₓTi₂O₆, which has the same crystal structure and similar high-T [subscript c] ferroelectric property. Through neutron diffraction, the detailed information of the structure is obtained, and the driving force for ferroelectricity is identified. Inspired by the successful synthesis of double perovskite Ca₂₋ₓMnₓTi₂O₆, perovskites La₁₋ₓPrₓRuO₃ are obtained by SPS as well. The substitution of La by smaller rare earth ion Pr gives rise to the crossover from itinerant to localized electronic behavior. A systematical study of physical properties is made and an unusual second-order metal insulator transition is found in La₁₋ₓPrₓRuO₃. The A²⁺V₂O₄ spinels have the smallest gap caused by electron-electron correlations in the single-valent spinels, and the V-V bond length in these spinels decreases as the A-site cation is replaced by cations in the order of A = Cd, Mn, Fe, Mg, Zn, Co. The density functional theory (DFT) calculation and transport properties of CoV₂O₄ under pressure indicate that CoV₂O₄ might be at the crossover between localized electron and itinerant electronic behavior. In order to clarify this, the series of AV₂O₄ spinels (A = Cd, Mn, Fe, Mg, Zn, Co) are studied with in situ high-pressure x-ray and neutron diffraction at different temperatures.