Raman Scattering in ${\mathrm{Mg}}_2$Si, ${\mathrm{Mg}}_2$Ge, and ${\mathrm{Mg}}_2$Sn

The one-phonon Raman scattering of ${\mathrm{Mg}}_2$Si, ${\mathrm{Mg}}_2$Ge, and ${\mathrm{Mg}}_2$Sn at room dry-ice, and liquid-nitrogen temperatures is reported. The Raman frequencies obtained (${\Gamma }_{{25}^{\prime }}$ optical phonon) decrease with increasing temperature. The room-temperature results for ${\mathrm{Mg}}_2$Sn are in excellent agreement with recent neutron-scattering data; such data are not available for ${\mathrm{Mg}}_2$Ge and ${\mathrm{Mg}}_2$Si. In these cases, the Raman frequencies obtained are compared with lattice-dynamical calculations based on the reststrahlung frequencies and the elastic constants. The Raman lines observed show considerable sharpening (a factor of 2 to 3) between room and liquid-nitrogen temperature. Weaker, possibly two-phonon, spectra have also been observed.