Lattice Dynamics of a Single Crystal of Pd${\mathrm{D}}_{0.63}$

The frequency-wave-vector dispersion relations of a single crystal of Pd${\mathrm{D}}_{0.63}$ have been measured by coherent neutron inelastic scattering at 78, 150, 220, and 295 K. The acoustic modes are considerably lower in frequency than the corresponding modes for pure Pd, in accord with the observed lattice expansion. The optic modes, in which D motions predominate, show considerable dispersion. In particular, the shape of the [100] LO branch shows conclusively that second-like-neighbor D-D interactions are comparable to the first-like-neighbor D-D interactions.