Some observations on phase transformations in molecular crystals

Abstract

This paper illustrates the use of calorimetric data in interpreting phase behavior in molecular crystals. Values of entropy of transition are used to estimate the number of distinguishable molecular orientations in a disordered phase, and symmetry and steric considerations are used to find a reasonable description of these orientations. As the number of molecular orientations in a plastic crystal is 10 or higher, it is proposed that orientations occur in which less than the full molecular symmetry is displayed at lattice sites. Thus, a molecule of tetrahedral symmetry may assume two orientations in which the common molecular and lattice symmetry elements belong to the T d point group and an additional eight with the elements of C 3v . From this consideration, the entropy of transition for typical tetrahedral molecules is calculated to be R In 10 (4.58 cal deg −1 mole −1, in accord with experimental evidence. Molecules of approximately tetrahedral shape but less than tetrahedral symmetry form plastic crystals with disorder corresponding to the same ten orientations plus additional disorder due to the lower molecular symmetry.