Structure, ferroelasticity and Goldilocks zone phase transitions in C3H5N2Al(SO4)2·6H2O

Abstract

The single crystal growth and sequence of reversible phase transition are described for C3H5N2Al(SO4)2·6H2O. Thermal and structural analyses combined with dielectric studies and optical observations revealed the structural phase transition at T 1 = 339/340 K (I↔II) and T 2 = 347/348 K (II↔III) on heating and cooling, respectively. Both phase transitions are of the first-order type. The symmetry changes from monoclinic to trigonal phase. At 293 K, the large crystals are usually divided into numerous domains of the ferroelastic type that disappear above T 1 on heating and reappear below T 1 on cooling. The domain structure pattern is characteristic for the transition between trigonal and monoclinic phases. The changes of entropy and clear increase of permittivity at T 1 provide evidence for the order–disorder character of this phase transition. The transition at T 2 seems to be displacive.