The Investigation About Crystal Structure And Lattice Dynamics Of Niobate-based Ferroelectric Materials Under High Pressure

Perovskite type ferroelectric oxide KNbO₃ has attracted much attention due to its excellent physical properties and environmental friendliness.They have been widely used in high-density memory and energy collection devices.In ferroelectric materials,the change of crystal structure is closely related to the size and direction of the electric polarization.It is very important to use external parameters such as temperature or pressure to effectively control the crystal structure,and then adjust the piezoelectric effect and physical and thermal properties of ferroelectric materials.High pressure can effectively reduce the distance between molecules,change the intermolecular force,and then change the structure and properties of materials;while high temperature can not only change the state of matter of materials,but also improve the activation performance of materials,and accelerate the physical and chemical reactions.In this paper,we have systematically investigated the structural and thermal properties of typical niobate materials KNbO₃ by Raman spectroscopy,synchrotron X-ray diffraction method under high pressure using diamond anvil cell high pressure technology,and differential scanning calorimetry(DSC).Some new phenomena were found in this study.In high pressure Raman scattering experiments,the structural phase transition of KNbO₃ was studied up to 53.8 GPa using inert gases Ar,Ne and He as pressure transmiting media,respectively.The experimental results show that KNbO₃ undergoes orthorhombic(O)?tetragonal(T)phase transition at 7?8.2 GPa,tetragonal(T)?cubic(C)phase transition at 10.9 GPa and cubic(C)?orthorhombic Gd FeO₃ phase transition at 47GPa.The effect of pressure on KNbO₃ is reversible during pressure releasing process,and there is no hysteresis in the phase transition.However,the coexistence of high pressure tetragonal phase and room temperature orthorhombic phase may coexist in pressure quenched KNbO₃ at ambient condition,and it does not completely return to initial pure orthorhombic phase.The results of high-pressure synchrotron X-ray study show that the phase transition sequence of KNbO₃ is orthorhombic(O)?tetragonal(T)?cubic(C)up to 16 GPa,which is consistent with the results of high-pressure Raman studies which is studied to higher pressures.Our results obtained at different pressure transmitting media used in this study shows that when the helium(He)is used as the pressure transmitting medium,the hydrostatic condition inside the sample cavity is better than other pressure transmitting media,but is can cause irreparable damage to the diamond anvil.Therefore,it is suggested that helium can be used as the pressure transmitting medium when the target pressure is less than 60GPa.At last,the physical and thermal properties of KNbO₃ were as studied by DSC.The results show that:during the heating process,orthorhombic(O)?tetragonal(T)and tetragonal(T)?cubic(C)phase transitions occur at about 188?and 413?;during the cooling process,cubic(C)?tetragonal(T)and tetragonal(T)?orthorhombic(O)phase transitions occur at about 430?and 220?.