High-pressure Brillouin scattering of the single-crystal PbSc_{1/2}Ta_{1/2}O_{3} relaxor ferroelectric

Abstract

The effect of pressure on the acoustic modes and adiabatic elastic constants of the perovskite-type ($AB$O${}_{3}$) relaxor compound PbSc${}_{1/2}$Ta${}_{1/2}$O${}_{3}$ have been studied by Brillouin spectroscopy at room temperature up to 9.2 GPa. It is shown that the elastic constants are very sensitive to the pressure-induced structural transformations that were established by diffraction and Raman scattering analysis. Changes in the evolution of the pseudocubic elastic constants ${c}_{11}$ and ${c}_{44}$ occur at the two phase-transition pressures ${p}_{c}$${}_{1}$ $=$ 1.9 GPa and ${p}_{c2}$ \ensuremath{\sim} 5.5 GPa. Changes in evolution of ${c}_{12}$ occur at the two characteristic pressures ${p}_{1}$${}^{*}$ $=$ 1.2 GPa and ${p}_{2}$${}^{*}$ \ensuremath{\sim} 3.0 GPa, which mark changes in the local structure that precede the corresponding phase transitions. The elastic anisotropy and the width of the transverse acoustic mode increase in the pressure range between ${p}_{c}$${}_{1}$ and ${p}_{c}$${}_{2}$ with a maximum near ${p}_{2}$${}^{*}$. A strong decrease in the Cauchy parameter is observed at pressures above ${p}_{2}$${}^{*}$, which indicates an enhancement of the covalent character of chemical bonding. Increasing pressure gradually suppresses the central peak typical of the relaxor ergodic state, and the quasielastic scattering vanishes at a pressure slightly above ${p}_{2}$${}^{*}$.