Abstract
The orientational ordering in a system of dipolar molecules located on a trigonal lattice is studied in the eight-sublattices approximation. Our concrete example is SO2 molecules enclosed in β-hydroquinone clathrate. It is assumed that the molecules interact with each other via effective dipole-dipole interactions, and that the crystal field acting on those molecules is of the form WA=Ccos[ ν ( φ - δA ) ], where φ is a rotational angle and A is a sublattice number. If ν is an odd integer, we assume that δA may be different for different A. The mean electric field which depends on the depolarization field is obtained. Various cases of C, ν, δA are investigated. Results are as follows: the possible dipole arrangement is antiferroelectric in all cases; the phase transition for ν=2, 4, 5, 6 is of second order; if ν = 1, no disordered phase occurs; if ν = 3, the phase transition is weakly of first order except for special cases; the C dependence of the transition temperature is weak unless ν = 2; δA chiefly determines the equilibrium direction. A comparison with experiment suggests that the low-temperature phase of SO2-β-hydroquinone has ν = 2 or 3, and that interactions between enclosed SO2 molecules other than dipole-dipole interactions may be also effective.
