Title: TEMPERATURE DEPENDENCE OF THE BRILLOUIN SPECTRA IN SN$_2$P$_2$S(SE)$_6$ FERROELECTRIC CRYSTALS
Author(s): R.M.Yevych, S.I.Perechinskii, A.A.Grabar, Yu.M.Vysochanskii, V.Yu.Slivka (Institute for Solid State Physics and Chemistry, Uzhgorod University, 46 Pidgirna Str., 88000 Uzhgorod, Ukraine)

A temperature dependence of hypersound velocities at transition from a paraelectric phase to ferroelectric phase in Sn$_2$P$_2$S$_6$ crystals was investigated by Brillouin scattering spectroscopy. Based on these data, full set of elastic constants was determined. Similar measurements were also performed for Sn$_2$P$_2$(Se$_{0.28}$S$_{0.72}$)$_6$ mixed crystals in the paraelectric phase near a Lifshitz point. The sound velocity indicatrices' evolution at transition from Sn$_2$P$_2$S$_6$ to Sn$_2$P$_2$(Se$_{0.28}$S$_{0.72}$)$_6$ was observed and softening of transverse acoustic phonons in the paraelectric phase near the Lifshitz point was found. An instability of the acoustic phonons is induced by an interaction with a soft optic mode which is the origin of an incommensurate phase appearance in mixed crystals Sn$_2$P$_2$(Se$_{x}$S$_{1-x}$)$_6$ with $x>x_{\mathrm{LP}}=0.28$.

Key words: Brillouin scattering, soft mode, ferroelectrics, Lifshitz point
PACS:78.35.+c