Analysis of translation-rotation coupling in an orientationally disordered ionic crystal

Abstract

Molecular dynamics calculations are used to study the coupling between translations and rotations in a model appropriate to solid sodium cyanide in its orientationally disordered high temperature cubic phase. Static properties are analysed via the formalism of Michel and Naudts through the introduction of a wavevector dependent parameter λ which measures the extent of translation-rotation coupling. The dynamics of the crystal are shown to be described accurately by a Mori-type theory, similar to that adopted by de Raedt and Michel, in which collective time correlation functions are expressed in terms of λ and certain other static averages. The same approach is used to discuss the crystal dynamics in the region of long wave-length, where the molecular dynamics calculations yield no direct information.