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

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 330–380 K 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 (0.01 K/min) shows two successive phase transitions: III→II at T2 h=354.5 K and II→I at T1 h=357.5 K. 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 400 min in phase II undergoes a direct sharp II→I transition during heating. Optical observations on the heating run at a 0.01 K/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 355 K between the tetragonal phase (I) and the orthorhombic phase (II).

► Dielectric, pyroelectric, optical studies of [C(NH2)3]4Cl2SO4 crystal were performed.
► Diffused dielectric and pyroelectric anomalies are observed at faster heating.
► Slow heating shows two successive phase transitions.
► Optical studies showed symmetry changes: orthorhombic→ orthorhombic→ tetragonal.
► On cooling only one phase transition occurs.