Orientational ordering in solid parahydrogen and orthodeuterium

Abstract

The low-temperature pressure-induced orientational ordering transition in solid parahydrogen (p-H2) and orthodeuterium (o-D2) is investigated using a model consisting of quantum rotors on a rigid fcc lattice. A correlated variational wave function of the Jastrow type is used to describe the zero-point orientational fluctuations which inhibit ordering into a Pa3 structure. Using the bare electrostatic quadrupole–quadrupole interaction, the predicted ordering pressure for o-D2 is about half the experimental value; realistic intermolecular potentials, with anisotropic exchange repulsion, yield even lower estimates. Hence, at high pressure the hydrogen molecule appears more isotropic (spherical) than indicated by the model potentials used here. Possible explanations for this are discussed, a likely candidate being the assumption of a rigid lattice and hence the neglect of translation-rotation coupling.