Quantum Effects on the Orientational Ordering of H2S and D2S Molecules Enclosed in .BETA.-Quinol Clathrate.

A system of planar rotators representing H<SUB>2</SUB>S and D<SUB>2</SUB>S molecules is investigated quantum-statistically, using the mean-field approximation based on an eight-sublattice model. The equilibrium molecular orientation is determined by competition among the dipole-dipole interaction between guest molecules, the host-guest interaction and the quantum kinetic energy. The system has a complex order parameter describing an average direction of dipoles. Both R1 and R3 structures and the effect of the depolarization field are studied. A comparison with experiment by use of theoretical phase diagrams gives: the low-temperature phase has the R1 structure with an antiferroelectric order of dipoles; the direction and arrangement of dipoles in equilibrium for H<SUB>2</SUB>S significantly differ from those for D<SUB>2</SUB>S because of the quantum nature of the rotational motion.