Brillouin scattering studies of the ferroelastic phase transition in LiCsSO4

Brillouin spectroscopy was used to study the elastic properties of ferroelastic ${\mathrm{LiCsSO}}_{4}$. The sound velocities, linewidths, and intensities of twelve acoustic modes were measured in the temperature range from 150 to 293 K. The mmm (${D}_{2h}$)\ensuremath{\rightarrow}2/m (${C}_{2h}$) phase-transition temperature was found to be 202 K. All nonzero components of the elastic stiffness tensor of the high-temperature phase were determined. The frequency, linewidth, and integrated intensity of the Brillouin line associated with the soft acoustic mode (characterized by ${c}_{66}$) were found to be strongly temperature dependent in the vicinity of ${T}_{c}$. In order to explain all experimental features, the fourth-order Landau free-energy expansion in all strain components was carried out. Using a non-Gaussian approach to critical fluctuations, the effective interaction range was calculated to be \ensuremath{\sim}10 nm.