High Pressure Synthesis And Physical Properties Of A-Cr-O Related Compounds

Oxides with Cr⁴⁺ ion and CrO6 octahedral coordination locate close to the crossover of metal and insulator, leading to anomalous electronic state and physical properties. In this thesis, several compounds related to A-Cr-O system were synthesized under high pressure and high temperature conditions. The structures and physical properties of the compounds were investigated. The main points are as follows:?1? Series of single crystals of Sr_xCa_1-xCrO₃ ?0?x?1? were synthesized. The structures were investigated by single-crystal X-ray diffractometer. With the increasing of x, the structure of the samples transforms from orthorhombic to tetragonal to cubic. The samples show antiferromagnetic transition except for SrCrO₃. The antiferromagnetic transition temperatures decreased with increasing x. Magnetic anisotropy was measured for samples x=0,1/6 and 1/3. Semiconductive behavior was observed for all the samples via electrical resistivity measurements. Accurate TN-composition phase diagram was obtained and the local electronic state was evidenced by the preparation and measurement results of Sr_xCa_1-xCrO₃ series which would lay a foundation for the further research.?2? Polycrystalline samples of La_xCa_1-xCrO₃ ?0?x?1? were prepared. The results of X-ray diffraction data were refined with GSAS software package. Orthorhombic structure with space group Pbnm is suitable for all these samples. The lattice parameters increased with x. Transition from C-type antiferromagetism to G-type one occurred when x increases from 0 to 1. The antiferromagnetic transition temperatures show a tendency of dTN/dx-210K/x. The electrical resistivity increased gradually from semiconductor to insulator with the increasing of x. The magnetic and electronic properties exhibited systematic changes from Cr⁴⁺ to Cr³⁺ in La_xCa_1-xCrO₃ system.?3? Single-layered perovskites Ca₂CrO₄ and Sr₂CrO₄ were synthesized, and Sr₂CrO₄ single crystal was obtained. Ca₂CrO₄ shows tetragonal structure with space group I41/acd and spin-glass magnetism proved by the DC and AC magnetic susceptibility measurement. As a new member of A-Cr-O system, Ca₂CrO₄ revealed the suppression of long range magnetic ordering resulted from the lattice distortion. K.2NiF4-type Sr₂CrO₄ with space group I4/mmm shows a weak antiferromagnetic transition with the spin axis along c direction at 95K. The magnetic anisotropy was observed for the single crystal Sr₂CrO₄ sample. Both of Ca₂CrO₄ and Sr₂CrO₄ samples exhibit insulating behavior at ambient pressure as well as under high pressure.