Pressure dependence of phonon energies in d₈-naphthalene

The pressure-induced changes of the phonon dispersion curves of the external modes in d 8 -naphthalene (C 10 D 8 ) have been determined at 100K for the ( xi 00) and (00 xi ) directions by coherent inelastic neutron scattering. Typical changes in phonon frequencies of 10 per cent were observed for a pressure difference of 3 kbar. Calculations performed with a quasi-harmonic lattice dynamical model using rigid molecules and based on the atom-atom '6-exp' potential with sets of parameters proposed by Kitaigorodsky (1966) or Williams (1967) and with the set for the best fit to the frequencies at 77K from Mackenzie et al. (1977) are compared with the experimental results. The overall agreement of Delta nu / nu is good in all cases, whereas the frequencies of the calculated phonons are better reproduced by the Williams and Mackenzie parameters.