BRILLOUIN AND ULTRASONIC STUDIES OF PHASE-TRANSITIONS IN CS2CDBR4 .2.PHENOMENOLOGICAL INTERPRETATION

The Brillouin and ultrasonic results are interpreted using a phenomeno logical free-energy development. The ferroelastic phase transition at 156 K is of a displacive type and is well described by a static model: the variation of the elastic constants bilinearly coupled to the orde r parameter is in quantitative agreement with recent Raman data. It is shown that the order parameter Q1 of the sequence Pnma(I) <-> 252 K i ncommensurate (IC) <-> 235 K P2(1)/n11 (C) is a relaxator which signif icantly contributes to the scattering in the Brillouin frequency range . The differences between the ultrasonic and the Brillouin results are well understood. (i) A qualitative interpretation of the behavior of C22 is given: it is strongly influenced by the fluctuations of the ord er parameter and, concerning the Brillouin scattering, by a significan t Landau-Khalatnikov contribution below T(i). (ii) In the vicinity of the Pnma <-> IC transition, the ultrasonic data concerning C44 are wel l interpreted in terms of a static model with a bilinear Q1 epsilon 4 coupling, while the Brillouin scattering, in the geometry which usuall y gives access to C44 through the dynamics of the strain epsilon 4, is interpreted as resulting from the cumulative contribution of three te rms involving three pair correlation functions of the order parameter Q1 and the strain epsilon4 which allow to fit the characteristic stron gly asymmetric shape of the spectra and which explain the considerable shift of the Brillouin maxima below the ''static'' ultrasonic curve. Finally, we show that an improved interpretation of the Brillouin and ultrasonic data is found assuming that the virtual Pnma <-> P2(1)/n11 transition is of the first order.