Glassy crystals: Correlation between molecular symmetry and glass transition

Using the symmetry departure concept, a tentative classification of molecular glassy crystals is presented. In particular, it is shown that plastic crystals of symmetrical molecules which do not lead to glassy crystals have their counterparts in plastic crystals of homologous substituted molecules which do. Other classes take into account heterocyclic molecules, incompatible symmetries and molecular interconversion motions. An extension of this classification is suggested to handle the cases of fullerene C-60 and of some molecular mixtures.