Lattice dynamics of $\beta$-Pd${\mathrm{D}}_{0.78}$ at 85 K and ordering effects at 75 K

The phonon dispersion relations of Pd${\mathrm{D}}_{0.78}$ have been measured by inelastic neutron scattering at temperatures slightly above and below the order-disorder transition. The acoustic modes are not sensitive to the ordering process and are somewhat higher in frequency than the corresponding modes in Pd${\mathrm{D}}_{0.63}$. The optic frequencies in the disordered phase of Pd${\mathrm{D}}_{0.78}$ are equal to those of Pd${\mathrm{D}}_{0.63}$. After ordering, a slight increase was observed in some transverse branches.