Study On Electrical Transport Properties Of Polycrystal BaTiO₃ Under High Pressure

As a typical Perovskite Ferroelectric Material,barium titanate(BaTiO₃)is widely used in capacitors,thermistors,sensors and other fields because of its excellent dielectric,pyroelectric and ferroelectric properties.As another thermodynamic parameter independent of temperature and chemical composition,pressure can reduce the atomic spacing,and then lead to the changes of crystal structure and electronic structure.It is an important source for the generation of new substances,the birth of new theories and the discovery of new phenomena.In this paper,the electrical transport properties,energy band structure and electron local distribution function of polycrystalline BaTiO₃samples under high pressure are studied by using in-situ AC impedance spectroscopy measurement technology and first principle calculation method.The following research results are obtained:(1)The AC impedance spectrum of polycrystalline BaTiO₃ in the pressure range of 0-17.0 GPa was measured in situ under high pressure.It was found that the conductivity type of polycrystalline BaTiO₃ was typical ionic conductivity in the range of 0-3.0 GPa.In the range of 3.0-17.0 GPa,the conductivity type of the sample is electronic conductivity.The change of conduction mechanism is due to the structural phase transition from tetragonal phase to cubic phase of polycrystalline BaTiO₃ at2.0-3.0 GPa.(2)Through the equivalent circuit fitting analysis,we found that the grain boundary resistance has a great influence at low pressure.With the increase of pressure,the effect of grain resistance increases obviously,and finally the grain resistance becomes dominant.The resistance increases abnormally in the pressure range of 15.1-17.0 GPa,which is caused by the increase of energy gap width.From the variation of relaxation frequency peak with pressure,it can be seen that the relaxation frequency of cubic polycrystalline BaTiO₃ is significantly higher than that of tetragonal phase,which is caused by the great difference between electron mobility and ion mobility,which is greater than ion mobility.(3)By calculating the energy band structure of BaTiO₃,it can be seen that both tetragonal and cubic BaTiO₃ are direct band gap semiconductors.In the pressure range of 15.1-17.0 GPa,the band gap width increases with the increase of pressure,which makes it more difficult for electrons to transition from valence band to conduction band,so the resistance increases slightly.(4)By calculating the electronic local function of BaTiO₃ under different pressures,it can be seen that the polarization of Ti-O dipole becomes more difficult with the change of pressure because the electrons aroundTiatom increase with the increase of pressure,which is the reason why the relative dielectric constant of cubic BaTiO₃ sample decreases after pressurization.Through the research of this paper,we hope to provide a valuable reference for the research on the electrical transport properties of other ferroelectric materials under high pressure.