Orientational disorder and melting transition: Phenomenology and modelling with relevance to solid halogens and H-2

Experiment and computer simulation indicate that zig-zag I-2 (H-2) chains play a role in the behaviour of I-2 (H-2) solids under high pressure. Therefore, the phenomenological treatment of Pople and Karasz, which includes an energy barrier between different molecular orientations, is utilized to correlate a reduced melting temperature T-m with the inverse of T-c, the orientational disorder temperature. Contact is also made with a possibly relevant model, due to Longuet-Higgins, in which flexible molecules perform torsional oscillations under the influence of an intermolecular interaction energy. Further pressure experiments could provide an experimental test of the phenomenology and/or modelling.