Optical, elastic, and dielectric studies of the phase transitions in lawsonite

Results of optical birefringence, ultralow-frequency elastic, dilatometric, and dielectric measurements of lawsonite crystals are presented in a wide temperature range including two successive phase transitions Cmcm-(T-1=273 K)-->Pmcn-(T-2=120 K)-->P2(1)cn. Pronounced pretransitional effects are obs erved in macroscopic properties in a large temperature region (approximate to 200 K) above T-1. The low-temperature transition at T-2=120 K is found t o be proper ferroelectric. A deviation from the Curie-Weiss law appears in the temperature dependence of the dielectric susceptibility in the region o f the ferroelectric transition below T-2 Frequency dependent measurements o f the dielectric permittivity yield evidence that ferroelectric domain-wall motion is responsible for this deviation from the standard behavior.