Abstract
The (NH4)2CuCl4·2H2O crystal exhibits anomalous thermal, piezoelectric, dielectric and elastic properties in the vicinity of the ferroelastoelectric phase transition at TC = 200 K. Macroscopic order parameter is the h36 component of the spontaneous piezoelectric tensor connecting polarisation P3 and strain η6. The temperature change in h36 component was proportional to (TC − T)α, where α = 0.58 ± 0.05. Close to TC changes in dielectric permittivity were negligible. High value of ac conductivity in the high-temperature phase results from protons jumping between disordered ions NH4+ and between molecules of crystallisation water. Only longitudinal ultrasonic waves exhibited an abrupt change upon the phase transition. The relaxation time of the order parameter was determined from anomalous changes in attenuation of the longitudinal waves.
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