Abstract
Room-temperature Brillouin-scattering measurements of the longitudinal and transverse sound velocities for the cubic crystals K2SnCl6, K2ReCl6, (NH4)2SnCl6 and (NH4)2SiF6 are presented. The elastic constantsc11,c12 andc44 are determined from the angular variation of the Brillouin line shifts. Furthermore, for K2SnCl6 the sound velocities in the [100]- and [111]-direction are investigated as a function of temperature, 256K≦T≦330K. A strong softening of thec11–c12 mode is observed in the para-distortive phase as the temperature approaches the phase transition temperatureTc1=262K from above, whilec44 shows no anomaly within the experimental error. These acoustic anomalies are interpreted theoretically by a mechanism based on the coupling of the elastic strain field to the fluctuations of the soft-mode coordinates.
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