Abstract
Precision measurements of the dispersion relations for some low-energy phonons in Cd have been performed at 77K by inelastic neutron scattering. Phonon group velocities have been extracted from the data with an accuracy of 5-10%. Several abrupt changes of group velocity as a function of wave vector have been identified as Kohn anomalies. The wave vectors and strengths of the anomalies are compared with calculations based on a model electron-ion potential and a spherical Fermi surface. Discrepancies between experiment and theory are qualitatively explained in terms of deformations of the Fermi surface from sphericity. These deformations are consistent with those determined by Stark and Falicov (1967). Extension of the calculations to include third-order terms in the model potential leads to the prediction of further phonon anomalies, none of which is observed.