Pressure effects on relaxor ferroelectricity in disordered Pb (Sc _1/2 Nb_1/2) O₃

High-pressure Brillouin and Raman scattering spectroscopy and x-ray diffraction measurements were carried out on disordered Pb (Sc_1/2 Nb_1/2) O₃, considered to be a model system for phase transitions in relaxor ferroelectrics and related materials. The observed pressure-dependent Raman spectra are unusual, with the relaxor state distinguished by broad Raman bands. Raman spectra as a function of pressure reveal a new peak at 370 cm-1, with two peaks near 550 cm-1 merge above 2-3 GPa, indicating a structural phase transition in this pressure range consistent with earlier dielectric measurements. A significant softening in the longitudinal acoustic mode is observed by Brillouin scattering. Both the temperature and pressure dependencies of the linewidth reveal that the longitudinal acoustic mode softening arises from electrostrictive coupling between polar nanoregions and acoustic modes. X-ray diffraction indicates that the pressure-volume compression curve changes near 2 GPa.