Study On The Structure And Stability Of BaTiO₃-BaGeO₃ System Under High Temperature And High Pressure

Perovskite structure is one of the most thoroughly studied structural types in physics,chemistry,material science and earth science,and it is an important treasure house for understanding the microscopic interactions between constituent elements and discovering new materials,new properties and new phenomena.BaTiO₃ is a typical perovskite structure compound,its excellent electrical properties make it become the raw material for the preparation of many important electronic devices in modern high-tech products,and often through various doping to improve its properties to meet the preparation technology and different electrical properties requirements.Ge and Ti is often the case with the same chemical valence(+4),the improvement of preparation technology and electrical properties of Ge doped BaTiO₃has also been studied by scientic community.However,Ge often forms Ge O₄tetrahedral coordination at ambient pressure instead of octahedral coordination required by perovskite structure,therefore,its doping amount is limited and the improvement of electrical properties is not obvious.Ge and Si are elements of the same main group,and they have similar physical and chemical properties.In earth science research,especially in the research of the lower mantle in which more than 80%of the minerals are two kinds of silicate perovskite,germanate perovskites are often used as analogues of silicate perovskites,and a lot of high temperature and high pressure experimental studies have been carried out.The experimental results show that AGe O₃(A=Mg,Ca,Sr,Mn,Cd,etc.)compounds with pyroxene structure are transformed into perovskite structures under high pressure,and Ge O₄ coordination tetrahedron is transformed into Ge O₆coordination octahedron under high pressure.Therefore,it is speculated that BaTiO₃-BaGeO₃ mixture may form a solid solution under high pressure,greatly expanding the doping amount of Ge in BaTiO₃.In order to confirm this guess,on the one hand,we studied the phase transition behavior of BaGeO₃ and BaTiO₃ at high temperature and high pressure.On the other hand,solid solution synthesis of BaGeO₃-BaTiO₃ mixture was studied.And through high pressure in situ Raman spectroscopy,synchrotron radiation X-ray diffraction to characterize the results,combined with first-principle theoretical calculation,some innovative research results and understandings have been obtained as follows:(1)High temperature and high pressure experimental study was carried out on the pseudowollastonite phase BaGeO₃.The 6H-type perovskite phase BaGeO₃ was synthesized for the first time in the pressure range of 17.4-32.1 GPa,and it was found that the cubic perovskite phase was formed by further phase transformation in the pressure range of 32.1-43.2 GPa.After the formation of perovskite phase,there were some unique phenomena on decompression.The 6H-type perovskite phase BaGeO₃has a higher spreading speed along the c-axis than along the a-axis on decompression.When decompressing to ambient pressure,Raman spectrum results show that the perovskite phase is amorphous,and synchrotron X-ray diffraction results show that the perovskite phase can be maintained.The Birch-Murnaghan equation of state was used to fit the P-V data of 6H-type perovskite phase BaGeO₃,and the zero pressure cell volume V₀ and the volume elastic modulus K₀ were V₀=373.0(3)A³,K₀=150(2)GPa,K₀?=4(fixed),respectively.The first-principles calculation results of 6H-type perovskite phase BaGeO₃ are in good agreement with the experimental results.The cubic perovskite phase BaGeO₃ transformed into a new cubic perovskite phase when decompressed to?8.6 GPa.The amorphous process is accompanied with the transformation of Ge O₆ octahedron into Ge O₄ tetrahedron.(2)The high temperature and high pressure experimental study of tetragonal perovskite phase BaTiO₃ was conducted.Under the temperature and pressure conditions of 29.9 GPa and 1600 K,the structure of cubic perovskite phase BaTiO₃remains unchanged,which extends the stable temperature and pressure range of cubic perovskite phase BaTiO₃.The cubic perovskite phase BaTiO₃ is transformed into tetragonal perovskite phase at ambient pressure on decompression which means the transformation of tetragonal perovskite phase BaTiO₃ to cubic perovskite phase is reversible.(3)The BaTiO₃-BaGeO₃ mixtures of 1:1,1:2 and 1:4 were experimentally studied at high temperature and high pressure.The solid solutions were synthesized at the temperature and pressure of?30 GPa and 1600 K.After the solid solutions are formed,almost all of them undergo phase transition on decompression,and all the solid solutions remain as the crystal phase when decompressing to ambient pressure.The results provide a new idea for the synthesis of Ba(Ti_x,Ge_1-x)O₃ solid solution,and also supplement the structural phase transition behavior of Ba(Ti_x,Ge_1-x)O₃ solid solution at high pressure.The minimum pressure of Ba(Ti_0.5,Ge_0.5)O₃ solid solution synthesis is 9.9 GPa,large size solid solution samples can be synthesized by multiple anvil high-pressure and high-temperature device or other device.Combined with high temperature and high pressure experimental results of BaTiO₃-BaGeO₃ system above,it can be seen that the synthesis of 6H-type perovskite phase and cubic perovskite phase BaGeO₃ expands the structure and phase transition sequence of pseudowollastonite phase BaGeO₃ under high pressure.The study on the structure and stability of BaGeO₃ at high temperature and high pressure is of great significance for understanding the phase transition law of silicate perovskite and its stability,and the physical and chemical properties and changes of the earth's lower mantle.The cubic perovskite phase BaTiO₃ can be maintained under higher temperature and pressure conditions,which expands its stable temperature and pressure range.The synthesis of Ba(Ti_x,Ge_1-x)O₃ solid solution at high temperature and high pressure indicates that the amount of Ge entering BaTiO₃ under high pressure is greatly increased,and the minimum synthesis pressure of the solid solution is not high,which is convenient for the synthesis of large-size samples to carry out electrical and other properties measurement,and provides the possibility for the synthesis of high-performance solid solution materials.