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Abstract
A temperature dependence of hypersound velocities at transition from a paraelectric phase to ferroelectric one near a Lifshitz point in Sn2P2(Se0.28S0.72)6 crystals was investigated by Brillouin scattering spectroscopy. Clear anomalies were observed in behavior of sound velocities and attenuation of waves propagating along Z axis of crystals. These anomalies are connected with instability of acoustic phonons that is an origin of the incommensurate phase appearance in mixed crystals with x > 0.28.