A mechanism of a phase transition based on a new symmetry and structure of bis(thiourea) thiazolium bromide

A redetermination of the crystal structure of bis(thiourea) thiazolium bromide (hereafter T2ThBr) at 250 K (HT) and 100 K (LT) points to different space group symmetries than the ones reported in [A. Pajzderska, J. Wąsicki, H. Małuszyńska, P. Czarnecki, L. Toupet, E. Collet, J. Chem. Phys. 129 (2008) 104501]. Our structure determination of T2ThBr shows that the two phases are centrosymmetric, Cmcm and Pmcn, and that the guest is disordered over at least four symmetry related positions in the HT form and over at least two positions in the LT form. The thiourea molecule is disordered over two positions independent from the temperature. A new model of the disordered structure is presented together with a new classification and a mechanism of the phase transition.