A one-parameter model of a rigid-unit structure

Abstract

The Rigid-Unit model has been developed to describe phase transitions in which polyhedral units of several atoms maintain their integrity on collective motion. Previously, lattice dynamics calculations have been performed on such structures at zero temperature and with infinitesimal rotations of the rigid units. Molecular Dynamics calculations were carried out on the β-Cristobalite structure to observe its behaviour under the excitation of several Rigid-Unit Modes (RUMs) at finite temperatures. Rigid tetrahedra interacted via harmonic forces at the vertices - the ‘one-parameter’ model. RUMs are seen, in this model, to freely coexist with no particular preference for one set of RUMs over another.

Key Words:

• High symmetry phase
• Dynamic disorder
• Computer simulation