Dilatometric and optical studies of phase transitions in ferroelectric [(CH3)2NH2]3CuCl5 single crystal

Dilatometric measurement of thermal expansion and birefringence measurement were done for [(CH3)2NH2]3CuCl5 single crystal along three crystallographic axis in the temperature range 305-240 K. Thermal expansion showed clearly two successive phase transitions at T 1 = 278.3 ± 0.3 K and T 2 = 253.4 ± 0.2 K on cooling run with thermal hysteresis of a few degrees. The obtained on cooling changes of |ΔV/V| are equal to (2.37 ± 0.15) x 10-3 at 278.3 K and (2.61 ± 0.15) x 10-3 at T 1 and T 2, respectively. Jump like anomalies of linear birefringence confirm the first-order character of both phase transitions