Simple view of the Mg2Si1-xSnx phonon spectrum: Sn resonances and mean field

We have studied the phonon spectrum of Mg2Si1-xSnx alloys using both density-functional theory calculations and neutron measurements. In the low tin concentration limit a resonance is obtained in the lower part of the spectrum. At higher concentrations comparison between theory and experiments shows that a simple averaging of the force constants of the perfect crystals Mg2Si and Mg2Sn can explain the main features of the density of states. However we suggest that the tin resonance persists up to large tin concentrations and may therefore be important for transport properties.