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

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.