Abstract
Precision measurements of the dispersion relations for phonons along the high-symmetry directions in gamma -Sn (Sn0.8In0.2) have been performed at room temperature 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 wavevector have been identified as Kohn anomalies. The positions and strengths of the anomalies are compared with calculations based on a model electron-ion potential and a spherical Fermi surface. It turns out that the surface of electron distribution exhibits flat parts along Brillouin zone boundaries leading there to closed bands with an energy gap. The Fermi wavevector for the spherical parts is about 4% smaller than predicted for 3.8 conduction electrons per atom.