Elastic constants and softening of acoustic mode in K2SnCl6 crystal studied by Brillouin scattering

In this paper we present the Brillouin Scattering measurementsof the longitudinal and transverse sound velocities for the hexahalome-atllate KSnClat room temperature.The elastic constants,refractiveindex,volume compressibility are determined from the Brillouinline shifts.Furthermore,the sound velocity in the [111] direction is inves-tigated as a function of temperature 252K<T<270K.An acousticadrupt change of the c+2cmode is observed as the temperature ap-proaches 256K from above,which has been analysed by means of thePippard relations.The conclusion is that KSnClundergoes first orderphase transition near T=256K.This acoustic anomaly can be interpretedby the nonlinear coupling of the elastic strain field to the fluctuation ofthe soft-mode coordinate.