Optical and Ultrasonic Investigations of Phase Transitions in Ferroelectric RbHSeO$\text{}_{4}$ Single Crystal

Birefringence and ultrasonic measurements were done for ferroelectric RbHSeO4 crystal. Influences of both stresses (σ$\text{}_{i}$) and pressure on the properties and phase transition were studied as well. The observed dependence of δ(Δn$\text{}_{b}$) = f(σ$\text{}_{5}$) shows hysteresis loop characteristic of ferroelectric phase whereas the dependence of δ(Δn$\text{}_{c}$) = f(σ$\text{}_{6}$) shows hysteresis loop characteristic of ferroelastic phase. Birefringence measurements prove the phase transition to ferroelectric phase at 373 K. Additionally, the phase transition was found at 398 K. The new phase may be regarded as an incommensurate one. On the grounds of the stresses influence the phase diagram is constructed in coordinates T, σ$\text{}_{5}$. The linear decrease in T$\text{}_{c}$ with increase in pressure is observed in ultrasonic studies and it is similar to the results obtained in DSC measurements