On Phase Changes in Crystals Arising from Hindered Molecular Rotation

Abstract

A theory of phase changes due to hindered molecular rotation in crystals is developed on the basis of classical statistical mechanics. It is demonstrated that local hindrance of relative rotation of neighboring molecules in the lattice can produce a non-uniform distribution in orientation below a certain critical temperature. The theory provides a semi-quantitative description of the behavior of the hydrogen halides in certain of their lambda-transitions, with a relative torque potential of the form (ε/2) cos γ for a pair of neighboring molecules, the axes of which are inclined at an angle γ to each other. The barrier height ε is estimated to be between 200 and 300 cal./mole for hydrogen iodide and hydrogen bromide.