Structural changes across the glass-transition in a glassy-crystal

The changes in molecular geometry, short-range order and crystal lattice parameter across the calorimetric glass-transition from the orientational-glass state into a plastic crystal, as well as structural changes within the molten state, have been determined from neutron diffraction for the most fragile orientational-glass former known so far, i.e., 1,2-difluoro-1,1,2,2-tetrachloroethane. In addition to its high fragility, the material is of interest due to the existence of two conformers having nonpolar and polar characters, which gives rise to different molecular potential energy surfaces generating an intrinsic source of competing interactions. The radial distributions as well as changes of the intermolecular structure were monitored by the temperature dependence of the lattice parameter. They evidence a remarkably strong interplay between internal and external molecular modes.