P. Kuzel et al., BRILLOUIN AND ULTRASONIC STUDIES OF PHASE-TRANSITIONS IN CS2CDBR4 .2.PHENOMENOLOGICAL INTERPRETATION, Physical review. B, Condensed matter, 49(10), 1994, pp. 6563-6574
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.