Pressure effects on grain boundary, electrical and vibrational properties of the polycrystalline BaTeO3

Abstract

The evolutions of alternate current (AC) impedance spectra, direct current (DC) resistivity and Raman spectra in polycrystalline BaTeO3 have been investigated at high pressures. The abrupt changes observed in electrical transport measurements at 12.73 GPa indicated a kind of phase transition, which was confirmed to be related to the structural phase transition by high-pressure Raman scattering experiments. From the impedance spectra, both grain resistance and grain boundary resistance decreased with increasing pressure, and two clear discontinuities occurred at 12.73 GPa and 17.47 GPa, respectively. After the transition, the contribution to the total resistance by the grain boundary effect dramatically declined and the grain resistance began to play a dominant role. Besides, the electric polarization process was changed due to the phase transition, and it led to a significant rise of the relaxation frequency. From the decompression impedance spectra, it could be clearly seen that the phase transition was irreversible, and it was probably attributed to the irreversible change of the grain boundary for the two different crystal structures. By varying the temperature, the transport behaviour at high pressure was still of semiconductor type but with different slopes for the ambient and high-pressure phases. High-pressure Raman measurements revealed an irreversible structural phase transition occurring above 15 GPa, and the results were consistent with the observed changes of the electrical properties at high pressures.