Successive phase transitions in [C(NH2)3]4Cl2SO4 crystal—dielectric, pyroelectric and optical evidences

Abstract

Studies of the dielectric and pyroelectric properties of a [C(NH2)3]4Cl2SO4 crystal along the ferroelectric c-axis and the optical observation along the b-axis in the temperature range 330–380K covering the phase transitions region were performed with various heating/cooling rates. At faster heating, a diffused dielectric anomaly in the range of phase transitions was observed. Slow heating (0.01K/min) shows two successive phase transitions: III→II at T2h=354.5K and II→I at T1h=357.5K. Pyroelectric studies showed polar properties of phases III and II. The disappearance of polarization is diffused at the first heating run. The sample kept for 400min in phase II undergoes a direct sharp II→I transition during heating. Optical observations on the heating run at a 0.01K/min rate confirmed successive phase transitions with symmetry changes according to a scheme: orthorhombic (III)→orthorhombic (II)→tetragonal (I). The dielectric, pyroelectric and optical studies revealed a slow III→II transition. On the cooling run, we deal with one transition at T1c equal to 355K between the tetragonal phase (I) and the orthorhombic phase (II).