Inelastic-neutron-scattering line shapes in PdD0.63

Previous measurements of the phonon-dispersion curves in a single crystal of Pd${\mathrm{D}}_{0.63}$ have been augmented by detailed inelastic-neutron-scattering measurements of the optical-phonon line shapes at 80 K. The line shapes observed at this temperature show substantial broadening and subsidiary structure. The data have been compared to line shapes calculated from a Born-von Kármán type model in which the nonstoichiometric structure of the crystal is explicitly taken into account. Although the model results cannot reproduce all details of the observed line shapes, the observed and calculated linewidths agree reasonably well, which demonstrates that the broadening is primarily an effect of nonstoichiometry and not, for example, due to anharmonicity. The Pd-D force constants derived from the nonstoichiometric model agree closely with those obtained from an earlier analysis of the dispersion curves based on a conventional Born-von Kármán approach in which the crystal was treated as stoichiometric PdD. The near-neighbor D-D force constants obtained in the present analysis, however, are some 50% larger than those deduced with a stoichiometric model.