Phase transitions in ferroelectric nonachlorodiantimonate [(CH 3)3NH]3Sb2Cl9 studied by calorimetric and dielectric methods

Phase transitions occurring in a ferroelectric crystal tris(trimethylammonium)nonachlorodiantimonate (III) (TMACA) were studied by calorimetric, dielectric and pyroelectric methods. Anomalies of dielectric and thermal properties were found around 200 K and at ca. 364 K. The anomalies in the low-temperature region exhibit a pronounced relaxational character. A detailed study of the behaviour of TMACA around 364 K clearly indicates that in fact TMACA undergoes two closely lying phase transitions, one of them being probably of the first order and the other of the second order. The temperature of the second order transition was determined to amount to (364.0 ± 0.3) K irrespective of the method employed, whereas the first order transition temperatures are detected by the dielectric measurements ca. 1 K below those found from the calorimetric measurements. An unusual sequence of the phase transitions was observed in DSC measurements : both on cooling and on heating the samples through the transition region, the second order phase transition precedes the first order one. The onset to ferroelectricity was found to be associated with the first order phase transition.