Structure and Energetics of Clusters Relevant to Thorium Tetrachloride Melts

We study within an ionic model the structure and energetics of neutral and charged molecular clusters which may be relevant to molten ThCl4 and to its liquid mixtures with alkali chlorides, with reference to Raman scattering experiments by Photiadis and Papatheodorou. As stressed by these authors, the most striking facts for ThCl4 in comparison to other tetrachloride compounds (and in particular to ZrCl4) are the appreciable ionic conductivity of the pure melt and the continuous structural changes which occur in the melt mixtures with varying composition. After adjusting our model to data on the isolated ThCl4 tetrahedral molecule, we evaluate (i) the Th2Cl8 dimer and the singly charged species obtained from it by chlorine-ion transfer between two such neutral dimers; (ii) the ThCl6 and ThCl7 clusters both as charged anions and as alkali-compensated species; and (iii) various oligomers carrying positive or negative double charges. Our study shows that the characteristic structural properties of the ThCl4 compound and of the alkali-Th chloride systems are the consequence of the relatively high ionic character of the binding, which is already evident in the isolated ThCl4 monomer.

The Three Phases of Solid Deuterated Methane

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.

Molecular Dynamics of the Phase Transition in Solid Deuterated Methane

The rotational dynamics of a single deuterated methane molecule in the presence of its twelve nearest neighbours has been studied by using a computer simulation technique. The three-dimensional motion of the tetrahedral molecule is described by the appropriate algorithm equations, as well as by the well known Lennard-Jones potential. The importance of the inverse-twelfth-power repulsive potential for the dynamics of the deuterated methane molecule is also highlighted. The second-nearest neighbour interactions contribute only 7% to the potential energy of the whole system and this hardly affects the dynamics of the central molecule. A detailed analysis of the direction cosine data reveals a change in the dynamical behaviour of the molecule around the transition temperature, which could be attributed to the singularity observed in the specific heat data. Corresponding to the Lennard-Jones potential, the period of oscillation of the central molecule comes out to be 0�38xlO-12 s. Making use of the (dimensionless) average rotational kinetic energy at the transition, (ERK)critical = 6�3, and the period, the transition temperature is found to be 27�7 K, which is in quite good agreement with one of the ..\-type transition temperatures reported by Clusius et al.