Regular models for solid hydrogen: III

Abstract

A regular model for a hexagonal close-packed lattice of ortho-hydrogen molecules is treated by a first-order statistical approximation. With nearest-neighbour pair energies due mainly to the quadrupole-quadrupole interaction the model is unsymmetric and anisotropic but spatial ordering based on alternate layer sub-lattices occurs at low temperatures. To obtain correct ratios for the numbers of nearest-neighbour site links at different orientations to the crystal axis an octahedron of six sites is used as the basic group in the first-order approximation. It is found that a second-order transition to the layer sub-lattice distribution occurs at T = 3.99°K.