Micro-Brillouin scattering study of ferroelastic phase transitions inKSc(MoO4)2

Abstract

High-resolution micro-Brillouin scattering studies of ferroelastic ${\mathrm{K}\mathrm{S}\mathrm{c}(\mathrm{M}\mathrm{o}\mathrm{O}}_{4}{)}_{2}$ have been performed in the temperature range 293--125 K giving information about the temperature dependence of the acoustic phonons propagating in the [100], [010], and [001] directions. Strong anomalies in sound velocity and attenuation have been observed for the sound waves corresponding to the ${c}_{11},$ ${c}_{44},$ and ${c}_{66}$ elastic constants. These results indicate a $3\ifmmode\bar\else\textasciimacron\fi{}\stackrel{\ensuremath{\rightarrow}}{m}2/m$ improper ferroelastic transition at 260 K followed by two transitions into other ferroelastic and monoclinic phases at 240 and 180 K. The comparison of the observed anomalies with the results of phenomenological theory suggests the following phase transition sequence in ${\mathrm{K}\mathrm{S}\mathrm{c}(\mathrm{M}\mathrm{o}\mathrm{O}}_{4}{)}_{2}:$ $P3\ifmmode\bar\else\textasciimacron\fi{}m\stackrel{\ensuremath{\rightarrow}}{1}C2/\stackrel{\ensuremath{\rightarrow}}{c}C2/\stackrel{\ensuremath{\rightarrow}}{m}C2/c.$ The present results suggest that below 260 K a simultaneously ferroelastic and incommensurate phase may exist, in agreement with the previous reports.