Ising theories of the plastic phase-transition in adamantane [tricyclo[3.3.1.13,7] decane

Abstract

We have calculated some of the properties of adamantane crystals using three approximations to the solution of the compressible Ising model. We used an ‘atom-atom’ intermolecular potential and a single variable parameter, the low-temperature thermal expansion, which was taken from experiment. The mean-field Ising approximation gave good agreement with a wide range of crystal properties; but it failed to describe the detailed behaviour near the molecular orientational order-disorder phase transition. A series expansion solution to the Ising model, allowing clustering in both orientation and centres-of-mass, gave excellent agreement with specific heat data, even near T c. Retention of a uniform lattice, allowing only clustering in orientation, and not in centres-of-mass, was only a slight improvement on the mean-field solution. This contrasts with magnetic and alloy order-disorder systems.