Temperature Dependence of the Brillouin Spectra in Sn_2p_2s(se)_6 Ferroelectric Crystals

Abstract

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 LP = 0.28.

The second order phase transition in Sn_2P_2S_6 crystals: anharmonic oscillator model

“…However, it was impossible to obtain correct symmetry assignment of soft optic mode in this model at lattice instability modelling. Then the polarizable ion models in different variations were applied [7,8]. The results of the latter calculations were found to be in good agreement with experimental and theoretical description of PT in Sn 2 P 2 S 6 crystals.…”

“…In the same manner, the line which corresponds to the Curie temperature was obtained using (8), as well as it was drawn in figure 3. …”

Lattice dynamics and phase transitions in Sn_{2}P_{2}S(Se)_{6} ferroelectric crystals

“…The calculations given for the acoustic surfaces enable one to make a comparison of the sound velocity orientation dependencies obtained theoretically with those received experimentally on BS [43] (figure 7). As it is seen, the polarizable ion model qualitatively reflects anisotropy of Sn 2 P 2 S 6 crystal elastic properties.…”

Acoustic and elastic properties of Sn2P2S6crystals

“…In order to analyse thoroughly the AO interaction in Sn 2 P 2 S 6 crystals, one needs to acquire comprehensive data on the acoustic wave velocities enough to construct the so-called acoustic slowness surfaces, determine obliquity parameter defining deviation of the acoustic energy flow from the acoustic wavevector and find deviation of the polarization vector from its purely transverse or longitudinal orientation. Some preliminary results concerned with the elastic properties of these crystals have already been reported in our recent works [9,12]. However, the corresponding data are not sufficient for the complete analysis of AO interactions in Sn 2 P 2 S 6 crystals.…”