Molecular mobility, phase transitions and phase transformations in crystalline ortho- and meta-carboranes Original Research Article

The thermally stimulated depolarisation currents (TSDC) technique was used to study the slow molecular mobility in the crystalline forms of highly symmetric polar isomers of carborane. As a consequence of the high molecular symmetry, that originates a molecular shape with a high degree of globularity, plastic crystalline phases with relatively fast reorientational mobility are expected. The TSDC results showed that there is an orientational glass transformation in phase III of ortho-carborane but not in meta-carborane, reinforcing the conclusions of the calorimetric study that suggests that phase III of m-carborane is an orientationally ordered phase. Furthermore, a slow molecular mobility is detected by TSDC in phase II of o-carborane, some degrees below the II→I transition temperature. The mobility in phase II of m-carborane is too fast to be probed by TSDC. It is suggested that the difference in the dynamic behaviour of the two isomers probably arises from their different polarity.