The Dielectric Properties And Phase Transitions Of PbTiO₃ And SrTiO₃ Under High Pressure

With the application of thin film electrode integration technology in the diamond anvil cell (DAC), the study of high pressure electrical properties of material has made a skip-type progress. Using the direct-current method, the measurable electrical parameters has not only limited to the sample resistance value. Thanks to he rule electrode geometry and the fixed electrode position, the electrical resistivity value can also be measured accurately in DAC high-pressure experiment at present. In addition, the organic combination between alternating-current impedance spectroscopy measurements and DAC high-pressure technique enables the people to be able to have an understanding to dielectric properties of material under high pressure. In the present paper, we integrate the microcircuit in DAC for dielectric properties measurement using the thin film deposition and photolithograph techniques. With the technique of in situ high-pressure impedance spectroscopy measurement, we are able to measure in situ high-pressure dielectric constant of material simultaneously. In the experiment process, we have a research systematically on the dielectric properties and phase transitions of PbTiO3 and SrTiO3 under high pressure. We obtain the resistance and dielectric constant variation with pressure of these two materials, and analyze their piezoelectric properties and phase transitions qualitatively.Regarding the PbTiO3 sample, the relationship between the resistance and pressure changes in 10.5GPa, 29.8GPa and 45.7GPa, corresponding to different phase transitions, respectively. From the dielectric constant changes with pressure, we find that in the preliminary stage the dielectric constant increases with pressure and achieves the maximum value as the pressure reaches up to 5.6GPa, and then begins to decline. We believe that the maximum value of dielectric constant is in the same range which the MPB exists by theoretical predictions. The increase of dielectric constant usually means an enhancement of the electromechanical coupling effect from the relationship between dielectric and piezoelectric properties. Therefore, we believe that there exists the MPB under high pressure. High-pressure synchrotron X-ray diffraction studies have shown that the first phase transition happens at 10.28GPa, which is very close to our electrical measurement results. From the lattice parameters change with pressure, we find c-axis is more easily compressed than a-axis in tetragonal phase.For SrTiO3 sample, from the relationship between the resistance and pressure, the exceptional change at 10GPa corresponds to the phase transition from cubic to tetragonal. At 21GPa, the resistance increases with pressure and we think the tetragonal phase transforms to orthorhombic phase around this pressure. In the entire pressure range, the dielectric constant deceases linearly with increasing pressure. The dielectric constant peak does not appear shows that SrTiO3 does not exist the MPB as in PbTiO3 sample. From the synchrotron radiation measurements in the range of 0-35GPa, neither diffraction peaks disappear, nor do new peaks appear. Therefore, no phase transitions occur from the diffraction data. As a result, high pressure measurements of some electrical parameters (here resistance) can be used as a local probe to reflect the change of microstructure such as atomic distance.In summary, we integrate a microcircuit on DACs for high pressure dielectric measurement, and have a test on PbTiO3 and SrTiO3, investigate the relationships between the dielectric constant as well as the resistance and the pressure. Meanwhile, we analysis qualitatively the piezoelectric properties and phase transitions. Furthermore, through the ADXRD experiment we investigate the phase transitions and the change of lattice parameters with pressure.