Ultrasonic Study of Ferroelectric (NH₄)₂BeF₄ under Hydrostatic High Pressure

Abstract

Ultrasonic velocites have been determined on the ferroelectric (NH 4 ) 2 BeF 4 as functions of both temperature and pressure. Special attention has been paid to the incommensurate phase which appears in a narrow temperature range. The temperature dependence of sound velocities corresponding to the elastic constants c 11 , c 22 , c 33 , c 44 , c 55 , c 66 has been obtained at several constant pressures. A distinct pressure dependence was observed in the variation of the c 22 component with temperature. The ultrasonic attenuation of the longitudinal sound propagated along the a -axis has been measured under pressure. From the result of the attenuation measurement and the Brillouin scattering data, it was found that the relaxation time in the normal phase is given by τ eff =τ 0 /(( T - T I )/ T I ) -1.3 at atmospheric pressure. The elementary relaxation time τ 0 is determined to be (1.1±0.3)×10 -13 sec.