Lattice Dynamics ofMg2Pb at Room Temperature

The phonon-dispersion curves along the [001], [110], and [111] directions in ${\mathrm{Mg}}_2$Pb at room temperature have been measured by neutron inelastic scattering. Although these curves are qualitatively similar to those for ${\mathrm{Mg}}_2$Sn, marked differences exist in some of the longitudinal branches which show large dips at small $q$ for ${\mathrm{Mg}}_2$Pb, suggesting large screening effects of free carriers. A reasonable fit to the measured dispersion curves has been obtained with a nine-parameter shell model. The model was used to calculate the phonon density of states and the temperature dependence of the Debye temperature. The calculated Debye temperature agreed well with the Debye temperature obtained from heat-capacity measurements.