The Three Phases of Solid Deuterated Methane

Abstract

By using a computer simulation technique, the three phases of solid deuterated methane have been interpreted in terms of the changes in dynamical behaviour of the CD4 molecule located at the centre of a tetragonal unit cell having dimensions a = b = 5·872 A and c = 5·95 A. The central molecule is influenced by twelve nearest neighbour molecules situated at the mid-points of the sides of the unit cell. The rotational motion of the tetrahedral molecule is described by the appropriate algorithm equations, as well as by the well-known Lennard-Jones potential. The present calculations reveal two transition regions separating three distinct phases, which can be attributed to the two ,x-type transitions depicted by Clusius et al. (1937). Making use of the (dimensionless) average rotational kinetic energy corresponding to the lower transition region, (ERK)critical = 4·8, and the experimental libration frequency 6700 m-1 reported by Savoie and Fournier (1970), the transition temperature turns out to be 21·2 K, close to the experimentally observed value.