Mean-Field Theory of the Orientational Ordering of Dipolar Guest Molecules in β-Hydroquinone Clathrate of SO₂ and D₂S

Abstract

A clathrate consists of both a `host' system and a `guest' system composed of the enclosed molecules. Treating the guest system as a diluted dipolar one, we have investigated the effects on orientational phase transitions of the cavity occupancy x, a crystal field W due to host molecules, the difference between atom-atom potentials and the ignorance of those contributions by the host system which are not involved in W. It is shown that self-consistency makes the transition temperature T_c a nonlinear function of x. The occurrence of a successive phase transition is shown to depend on the higher order terms of W and on the difference in the potential model. It is also clarified that the host system may affect the estimation of T_c. We have further investigated to what extent classical statistical mechanics is applicable at low temperatures. Results are discussed in relation to the experiments.