Brillouin light scattering in niobium doped lead zirconate single crystal

Brillouin light scattering experiments were performed for lead zirconate single crystals doped with niobium. Special attention was paid to the elastic mode softening near phase transition temperatures. The results are compared with data obtained by Raman light scattering experiments. We observed that the interaction between acoustic and optic modes is responsible for symmetry breaking far above T_C, leading to polar regions' appearance. No changes in the acoustic mode frequency and its damping are observed at T_C, where ε(T) exhibits a maximum value. The absence of these changes and the central peak observed in Raman experiments suggest that the phase transition at T_C is mainly of the order-disorder type. The origin of other phase transitions is discussed as well.