Mode softening, precursor phenomena, and intermediate phases in PbZrO3

Single crystals of PbZrO${}_{3}$ have been studied by Brillouin light scattering, birefringence, and piezoelectric coefficient measurements. The structural phase transition at ${T}_{c}$ is shown to be related to the softening of a longitudinal acoustic (LA) mode, whereas precursor effects are deduced from a central peak in the light scattering spectrum, finite birefringence, and piezoelectric coefficient above the structural instability. An intermediate phase in the vicinity of ${T}_{c}$ is apparent where a coexistence of the LA mode with a high frequency mode appears, and in addition the transverse acoustic (TA) mode becomes visible.