Piezoelectric and Ultrasonic Studies of Mixed Ag<sub>x</sub>Cu<sub>1-x</sub>InP<sub>2</sub>(S,Se)<sub>6</sub> Layered Crystals

Abstract

The paper reviews recent results of ultrasonic and piezoelectric investigation in CuInP2S6 family ferroelectric layered crystals and their solid solutions in the temperature range 100-360 K. It was shown that, Cu substitution by Ag lowers the phase transition temperature. In investigated AgxCu1-xInP2(S,Se)6 crystals above the phase transition (PT) temperature the piezoelectric response was absent and appeared only below the transition. At low temperatures T < 220 K the layered AgxCu1-xInP2Se6 crystals are ferroelectric. Piezoelectric sensitivity in the ferroelectric phase increases with DC field applied along the c-axis, then saturates, and after reversion of voltage the piezoelectric signal decreases, at field near coercive changes sign, and saturates again at high voltage of opposite polarity In the paraelectric phase under external DC electric field, applied along c-axis normal to layers, thin AgxCu1-xInP2Se6 plates can effectively excite and detect ultrasonic waves, due to electrostriction. The same behaviour was observed and in AgxCu1-xInP2S6 crystals. The critical ultrasonic velocity anomalies were observed in the vicinity of PT. In CuInP2S6 crystals with addition of In i.e. indium rich materials the phase transition temperature could be raised to T > 330 K what is important for applications in medical diagnostics ultrasonic transducers. In all these materials the poling conditions were investigated and it was shown that after long time exposing in DC field the piezoelectric sensitivity sufficiently increases and electromechanical coupling constants as high as > 50 % could be obtained.