Orientationally disordered hydrogen. Isotropic model of quadrupolar glass
Abstract
A model is constructed to describe the thermodynamic properties of an ortho–para mixture of solid molecular hydrogen in the region without any long-range orientational order. After the unitary transformation of the original Hamiltonian of a real hydrogen system to the local order axes system of coordinates, a version of the thermodynamic perturbation theory is developed. The model Hamiltonian of hydrogen is presented as a system of two-dimensional pseudospins interacting through isotropic exchange that changes sign on the average and existing in random local-anisotropy type fields that are conjugate to the order parameter of quadrupolar glass and that appear in the fourth order perturbation theory. The effect of these fields on the possible types of ordering of quadrupolar glass is discussed. A method is proposed to reveal random anisotropy-type fields from the experimental data on the thermodynamic properties of a mixture of ortho- and parahydrogen.