TEMPERATURE DEPENDENCE OF THE BRILLOUIN SPECTRA IN Sn<sub>2</sub>P<sub>2</sub>S(Se)<sub>6</sub> FERROELECTRIC CRYSTALS

oleh: R.M.Yevych, S.I.Perechinskii, A.A.Grabar, Yu.M.Vysochanskii, V.Yu.Slivka

Format: Article
Diterbitkan: Institute for Condensed Matter Physics 2003-01-01

Deskripsi

A temperature dependence of hypersound velocities at transition from a paraelectric phase to ferroelectric phase in Sn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> 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<sub>2</sub>P<sub>2</sub>(Se<sub>0.28</sub>S<sub>0.72</sub>)<sub>6</sub> mixed crystals in the paraelectric phase near a Lifshitz point. The sound velocity indicatrices' evolution at transition from Sn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> to Sn<sub>2</sub>P<sub>2</sub>(Se<sub>0.28</sub>S<sub>0.72</sub>)<sub>6</sub> 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<sub>2</sub>P<sub>2</sub>(Se<sub>x</sub>S<sub>1-x</sub>)<sub>6</sub> with x &gt; x<sub>LP</sub>=0.28.