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 $\mathrm{Cmcm}-{(T}_1=273\mathrm{}\hspace{0.5em}\mathrm{K})\to \mathrm{Pmcn}-{(T}_2=120\mathrm{}\hspace{0.5em}\mathrm{K})\to {P2\mathrm{}}_1\mathrm{cn}.\mathrm{}$ Pronounced pretransitional effects are observed in macroscopic properties in a large temperature region $\left(\approx 200\hspace{0.5em}\mathrm{K}\right)$ above $T_1.$ The low-temperature transition at $T_2=120\mathrm{}\hspace{0.5em}\mathrm{K}$ is found to be proper ferroelectric. A deviation from the Curie-Weiss law appears in the temperature dependence of the dielectric susceptibility in the region of the ferroelectric transition below $T_2.$ Frequency dependent measurements of the dielectric permittivity yield evidence that ferroelectric domain-wall motion is responsible for this deviation from the standard behavior.

• Received 20 July 1999

DOI:https://doi.org/10.1103/PhysRevB.62.6143