Field-induced effects in the relaxor ferroelectric Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 studied by micro-Brillouin scattering

Field-induced changes of Brillouin scattering spectra in a relaxor-based ferroelectric Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystal have been examined under an electric field oriented along the pseudocubic [001] direction. A complete cycle of the Brillouin shift-electric field loop shows that sound velocities of one longitudinal and two transverse modes are softened during the polarization reversal. In particular, one transverse mode, which corresponds to the (c11-c12)/2 elastic stiffness coefficient, becomes markedly soft from 18.6 to 14 GHz and then restores its initial value at two distinct electric fields. In addition, two peaks have also been observed in the field dependence of the full width at half maximum of this mode. It implies that, at least two kinds of polarization rotation process are involved in the switching process, which seems to be related with the existence of intermediate monoclinic symmetry during the field-induced phase transition from rhombohedral to tetragonal phase