Abstract
Anisotropy of hypersound velocities at room temperature in Sn 2 P 2 S 6 and Sn 2 P 2 (Se 0.28 S 0.72 ) 6 crystals is investigated by Brillouin scattering spectroscopy using different scattering geometries. The dispersion of velocity of the longitudinal sound waves propagating along [001] crystallographic direction is measured. The significant changes of velocity of the transversal sound waves propagating and polarized in (010) symmetry plane were observed in accordance with previously reported experimental data. The existence of Lifshitz point in Sn 2 P 2 (Se 0.28 S 0.72 ) 6 crystals and the contribution of acoustic phonons in soft mixed optic-acoustic modes at the ferroelectric phase transitions may be the reason of such a behaviour of elastic properties of crystals under study.
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