Synthesis,Structure And Electrical Properties Of Hexagonal Perovskite ⅤB/ⅥB Oxides Under High Pressure

High-temperature and high-pressure synthesis is an emerging material preparation method that can be used to stabilize unconventional perovskite materials.In recent years,three main types of unconventional perovskite materials have been used for high-pressure studies,including Se MO₃,double perovskite A₂BB’O₆and AA’₃B₄O₁₂（A and A’are Pb,Bi or alkaline earth,rare earth metals,B and B’are transition metal ions）,where the latter two classes of materials are ferromagnetic above room temperature.However,there are relatively few high pressure studies on hexagonal perovskites.In this paper,two eight-layer shift hexagon perovskites and one six-layer shift hexagon perovskite were synthesized by means of high temperature and high pressure.The phase and structure of the material were analyzed by combining X-ray diffraction（XRD）,powder neutron diffraction（PND）,scanning electron microscopy（SEM）and selected area electron diffraction（SAED）.The chemical properties were analyzed to determine the valence and magnetic properties of the elements in the material.At the same time,the microwave dielectric properties,electrical conductivity and UV-Vis absorption properties of the materials were studied.The main results are as follows:（1）Two new compounds at high pressure were obtained by high-temperature and high-pressure synthesis,using normal pressure twinned octagonal hexagonal perovskite tantalate Ba₈MTa₆O₂₄（M=Zn and Mn）as the initial raw materials.Using XRD data,NPD data and selected area electron diffraction data to analyze and refine their structure,it is confirmed that the two new high-pressure compounds are shifted octagonal hexagonal perovskite tantalate with space group P-3m1.The unit cell parameters of Ba₈Zn Ta₆O₂₄ are a=5.7922（7）（?）,c=18.9317（9）（?）,where the Ba O₃ layers are arranged in a（hhcc）₂ manner along the c-axis,Zn and Ta mix occupy the B-site perovskite sites.The variable temperature XRD data show that the two displaced structural materials have good thermal stability in the temperature range of 25°C-1000°C.The electrical properties show that the materials are dominated by grain response(7.9×10^-12 F/cm).Microwave dielectric data show that Ba₈Zn Ta₆O₂₄ has a high microwave dielectric constant（32）and a temperature coefficient of resonant frequency close to zero.The magnetic test shows that the Ba₈Mn Ta₆O₂₄ material is paramagnetic,and the experimental effective magnetic moment is 6.2μ_B/f.u,indicating that the Mn ion is in the 2+oxidation state.（2）The strontium tungstate compound Sr₃W₂O₈ was prepared by high-pressure synthesis technology.The structure of shift hexagon perovskite was determined by XRD and SAED data.the space group is C2/c.The WO₆ octahedron showed tilt and deformation,which was due to the AO₃ layer under high pressure.The TG data,SAED data and XPS data prove the existence of oxygen vacancies in the compound and the W ion is in the 5+oxidation state.The magnetic properties show that the sample exhibits unique diamagnetism,which is different from the magnetic characteristics exhibited by other materials of the same type.（3）The hexagonal perovskite oxide Ba₇Nb₄Mo O₂₀ was prepared by the high temperature solid-phase method.After applying different pressures of 5-10 GPa,the XRD diffraction data show a single phase,and the SEM data show that the element distribution was uniform.The element ratio is the same as that under normal pressure.The SAED data also show a typical hexagonal perovskite structure consistent with normal pressure,which confirm that its crystal structure did not change under high pressure,which may be related to the compressibility of ions in the compound and the strength of chemical bonds.