High Pressure Synthesis And Physical Properties Of New 3d And 5d Oxides

Transition metal oxides have been playing a crucial role in the exploration of novel quantum effects.This dissertation focuses on the high-pressure synthesis and physical properties of several new 3d and 5d transition metal oxides with extraordinary valence states and/or coordination environments.In addition,the related microscopic physical mechanisms are also discussed in detail.The main conclusions are shown as follows.?1?A new 3d transition metal oxide CaCr_0.5Fe_0.5O₃?CCFO?was prepared under high pressure.The charge combination is determined to be Cr⁵⁺/Fe³⁺,indicating the formation of an unusual Cr⁵⁺charge state in the octahedral coordination environment,although Cr⁴⁺and Fe⁴⁺charge states are found to occur in the related perovskites CaCrO₃,CaFeO₃ and Sr₂Fe⁴⁺Cr⁴⁺O₆.Although the CCFO was predicted as a B-site ordered perovskite with antiferromagnetic half metallicity,our experiment reveals that it crystallizes to a B-site disordred one.Moreover,the randomly distributed Cr⁵⁺and Fe³⁺spins lead to short-range ferromagnetic couplings around 120 K,whereas a long-range antiferromagnetic phase transition takes place at 50 K due to the Fe³⁺–O–Fe³⁺superexchange interaction.The first-principles theoretical calculations further demonstrate the B-site Cr⁵⁺/Fe³⁺disordered configuration as well as the insulating state mainly due to the strong correlated effect of Fe³⁺ions.This work opens up a new avenue to explore new transition-metal oxides with exotic charge states and novel physical properties.?2?We for the first time prepare the high-pressure phase for the 3d oxide TeZn O₃?HP-TZO?at high temperature and high pressure conditions.Different from the ambient-pressure orthorhombic phase?AP-TZO?composed of ZnO₅ units,the current high-pressure one crystallizes to a monoclinic structure with space group P2₁/n.Moreover,both ZnO₄ tetrahedral and ZnO₆ octahedral polyhedra are found to occur in this new phase,providing a unique Zn-based material system that simultaneously possesses two distinct coordinated units.Since the outmost orbitals are fully occupied for both Zn²⁺and Te⁴⁺,the compound exhibits diamagnetism and strong insulating behavior with a wide bandgap as large as 6.0 eV.Dielectric constant and specific heat measurements show an obviously anomaly around 240 K.Low-temperature synchrotron X-ray diffraction reveals an isostructural phase transition at this temperature.?3?A novel 5d pyrochlore oxide Cd₂Ir₂O₇ with Ir⁵⁺charge state was prepared by using high-pressure experimental condition.Strong spin-orbit coupling?SOC?plays an important role on the electronic states in most iridates so that an interesting SOC-Mott state is proposed in Sr₂IrO₄ in the assumption of a strong SOC interaction.Thereby the current Cd₂Ir₂O₇ was expected to be a nonmagnetic insulator.In sharp contrast,however,our experimental results show that Cd₂Ir₂O₇ displays metallic electrical transport property as well as short-range ferromagnetic behavior,obviously deviating from the nonmagnetic and insulating J=0 ground state with a strong SOC limit.First-principles calculations well reproduce the experimental results.Due to the significant distortion of IrO₆ octahedra,there exist large mixture between the j_eff=1/2 and j_eff=3/2 bands near the Fermi surface,which leads to the metallic property.This work sheds light on the critical role of a noncubic crystalline field for the electronic properties,which has been ignored in past studies of 5d-electron systems.