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

Room-temperature Brillouin-scattering measurements of the longitudinal and transverse sound velocities for the hexahalometallate K₂SnCl₆ are presented. The elastic constants, refractive indices, volume compressibility are determined from the Brillouin line shifts. Furthermore, the sound velocity in the111-direction is investigated as a function of temperature in the range 252 K-270 K. An acoustic abrupt change of the C₁₁+2C₁₂ mode is observed as the temperature approaches 256 K from above. This has been analysed using the Pippard relations. The conclusion is that this acoustic anomaly may be interpreted by nonlinear coupling of the elastic strain field to the fluctuation of the soft-mode coordinate.