The ultrasonic and piezoelectric investigations of Sn_2P_2(S,Se)_6 and CuInP2Ss ferroelectric - semiconductors single crystals have been performed. In these materials the ultrasonic anomalies at ferroelectric phase transitions (PT) were observed. Below phase transition point the comparatively strong piezoeffect was observed in these compounds. The variety of photosensitive ultrasonic phenomena such as acoustoelectric interaction, acoustoelectric voltage, relaxing ultrasonic attenuation oscillations and the photostimulated shift of phase transitions to lower temperature were observed and investigated in all these polar crystals of Sn_2P_2S_6 family.
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