The Dynamics of Molecular Crystals under Pressure

Abstract

AbstractA general procedure for the calculation of the lattice modes in molecular crystals under pressure is given. Only a few modifications are necessary to the general lattice dynamical computer program. Calculations have been performed and the results are presented for naphthalene, anthracene, and orthorhombic sulphur. Although the basic program uses a sum over atom‐atom interactions to derive intermolecular interactions, this is not a necessary assumption in the procedure for the application of pressure. For naphthalene and anthracene the calculated increase in phonon frequency per kilobar of pressure is roughly 0.045 and 0.12 THz for modes with frequency 2 and 6 THz, respectively. Not all modes behave in this manner, and so the calculated pressure variation could prove very useful in mode assignment. The effect of uniaxial pressure on orthorhombic sulphur was also investigated, showing a markedly different frequency variation for some modes but not for others. Thus uniaxial pressure, if experimentally possible, could prove further the success of mode assignment. At 23 kbar uniaxial pressure the orthorhombic structure of sulphur became unstable, resulting in a monoclinic form in the space group C2/c. It is of interest that the simple model for intermolecular interaction can predict a soft mode phase transition, though whether this occurs in practice remains to be seen.