Mean-field theory of the orientational ordering of dipolar guest molecules in β-hydroquinone clathrate of SO₂ and D₂S

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 cursive Greek chi. 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 cursive Greek chi. 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.