Study On Structural Phase Transition Properties Of Niobate Materials

Niobate are environmentally friendly piezoelectric materials with great application prospects.For this reason,researchers have carried out a lot of scientific research on them.Pressure,as a physical parameter independent of temperature and chemical parameters,results in the reduction of atomic bond length and volume,the overlap of electronic orbits and the change of bonding modes between atoms,which results in the change of crystal microstructures and the formation of structural phase transitions,such as structural phase transitions and electronic phase transitions.In this paper,the structure and thermal properties of typical antiferroelectric NaNbO₃ under high pressure were systematically studied by using diamond anvil pressing technology,Raman scattering technology,X-ray Diffraction and differential scanning calorimetry.1.The high pressure Raman spectra of NaNbO₃ in the pressure range of 0-32GPa and the wavenumber range of 40-1000 cm^-1 are measured and analyzed.Firstly,the structural phase transition at 2 GPa is confirmed by 0.5 GPa pressure step of NaNbO₃.The phase transition at 7 and 9 GPa are consistent with previous studies.It is observed for the first time that NaNbO₃ exhibited significant difference in Raman spectra above 18 GPa.Therefore,the sequence of phase transition was Pbma→HP-I（2 GPa）,HP-I→HP-II（7 GPa）,HP-II→Pbnm（9 GPa）and Pbnm→HP-IV（18 GPa）.The structure of HP-IV phase remained stable to 32 GPa,and there is no evidence indicate NaNbO₃ transform to a cubic phase.Therefore,the temperature of Tc decreased from 614℃to room temperature at least at the rate of dT/dP=19.8℃GPa.During decompression,structural changes near 2,7 and 9 GPa are observed.However,below 7 GPa,the Raman spectra of HP-I phase are significantly different from the spectra observed at increasing pressure cycle.This result shows the irreversibility of the structural disorder caused by Na⁺displacement indicating the crystal structure within this pressure range may be coexistence of HP-I and Pbnm phase.After releasing the pressure,the ambient structure of NaNbO₃ is basically recoverable.2.The high-pressure X-ray diffraction of NaNbO₃ was measured and analyzed.The results show the structural phase transition appear near 6⁷.9 GPa and 9.1 GPa.The compression factors corresponding to the three axes of HP-I phase are calculated to be k_a=0.00108（2）,k_b=0.00108（1）,k_c=0.00110（1）.It is indicate that the lattice compression of HP-I phase is isotropic.At the same time,the decrease rate of cell volume is lower in this pressure range,which indicates that lattice compression contributes little to structural phase transition.Combining with the high pressure Raman experiment results,it can be concluded that the structural phase transition in this pressure range is mainly caused by the interaction between Na⁺and NbO₆.Up to9.1 GPa,NaNbO₃ transforms into Pbnm phase,and the volume of the high-pressure phase V=216.4508?³.The significant decrease in volume indicates that the structure phase changes is first-order phase transformation.At the same time,the b parameters of the Pbnm phase change continuously with the pressure,and a and c parameters are discontinuous.The corresponding compression factors are ka=-0.00309（5）,k_b=-0.00103（2）and k_c=-0.00343（5）,respectively,which indicates that the compressibility of lattice parameters in all directions is anisotropic.The compressibility of axes a and c is three times that of axes b,which indicates that the rapid compression of axes a and C contributes most to the distortion of oxygen octahedron,and the distortion of oxygen octahedron occurs along axes a or c.It is calculated that K₀（HP-I）=304.6 GPa,K₀^’（HP-I）=4（fixed）,K₀（Pbnm）=95.9GPa,K₀^’（Pbnm）=4（fixed）.3.The performance of NaNbO₃ at high temperature was measured and analyzed.With the decrease of temperature,the phase transition temperatures of NaNbO₃ are at630℃（Cubic→T₂）,560℃（T₂→T₁）,510℃（T₁→S）and 290℃（S→P）,respectively,...