The phonon dispersion of the intermetallic has been studied by inelastic neutron scattering in its high-temperature phase of structure, stable above . The dispersion has been examined at a temperature of ; some selected branches were also studied at (i.e. very close to the phase transition) and at . In general, the phonons are very strongly damped and some branches have not been found at all. For transverse phonons with propagation, the frequency does not change as a function of the temperature, but the widths become larger towards higher temperatures. The migration enthalpy and several other thermodynamic quantities have been calculated from the densities of states. The anomalous behaviour of the alloy is related to high vacancy densities.