HIGH-PRESSURE BRILLOUIN-SCATTERING STUDY OF AMMONIUM FLUORBERYLLATE

We have studied the temperature and pressure dependence of the elastic constants of ammonium fluorberyllate {(NH4)(2)BeF4 or AFB} using Bril louin scattering. We report here results of the temperature and pressu re dependence of the elastic constant C-55, which shows anomalies both at the normal to incommensurate and incommensurate to commensurate fe rroelectric phase transition at ambient pressure. Application of high hydrostatic pressure at constant temperature results in an increase of the elastic constant when approaching the phase transition to the inc ommensurate phase from the commensurate phase. This is contrary to the behaviour of C-55 when this transition is approached at constant pres sure with increasing temperature. We interpret the fact that (partial derivative In C-55/partial derivative T)(p) has the same sign and it m uch larger than -(beta/kappa(T))(partial derivative In C-55/partial de rivative p)(T), where beta and kappa(T) are the volume thermal expansi on and isothermal compressibility, respectively, as an indication that anharmonicities (cubic or quartic, or both) play an important role in producing the observed anomaly of C-55 at the commensurate to incomme nsurate phase transition. Copyright (C) 1996 Elsevier Science Ltd