Orientational Order of Guest H₂S Molecules in β-Quinol Clathrate

The orientational ordering of guest H₂S molecules is studied using a mean-field theory on the basis of quantum statistical mechanics; it is assumed that the guest system is a quenched one consisting of a normal mixture of ortho and para H₂S, and that the guest molecules which are influenced by a crystalline field, of which strength is estimated from far-infrared spectra, couple with one another via dipolar interactions. A phase diagram based on an effective dipole moment p is obtained. It shows: If p = 0.800D, a second-order transition between a partially disordered and a disordered phase occurs at T = 15.2 K, and the temperature dependence of the heat capacity well coincides with experiment; if p < 0.778D, no cooperative order is expected.