Zone-Centre Lattice Vibrations in α-Sulphur Including Effects of Hydrostatic Pressure

The zone-centre phonon frequencies in orthorhombic sulphur have been calculated at hydrostatic pressures of 0 kbar, 5 kbar and 10 kbar by applying a dynamical model which includes both external and internal vibrational degrees of freedom of the molecular motion. The calculations are carried out within the harmonic approximation with the intermolecular force constants calculated using a 6-exp atom-atom potential function and with the intramolecular force constants found from spectroscopic studies. Results are compared with those from a rigid molecule model calculation and from experiment. An analysis of the calculated eigenvectors predicts that not all the modes can be classified in the conventional way, as either internal or external, but for some modes both types of motion contribute substantially. Some suggestions are made for improving the dynamical model in order to remove the discrepancies between theory and experiment found in the frequency splittings of the high-frequency intramolecular modes.