Lattice dynamics in intermetallic Mg2Ge and Mg2Si

The lattice dynamics of polycrystalline Mg2Ge and Mg2Si are compared using both microscopic and macroscopic measurements as well as theoretical calculations. The volume thermal expansion coefficient between 200 and 300K was found to be 4.37(5) . 10(-5) K-1 in Mg2Ge, compared to 3.69(5) . 10(-5) K-1 in Mg2Si. Inelastic neutron scattering measurements yield densities of phonon states which are in line with theoretical calculations. The microscopic data were corroborated with macroscopic calorimetry measurements and provide quantified values for anharmonicity. The estimated macroscopic Gruneisen parameter is, gamma(Mg2Si) = 1.17(5) and gamma(Mg2Ge) = 1.46(5) at 295 K, in excellent agreement with Raman scattering data. Although the element specific mean force constants are practically the same, in Mg2Ge and Mg2Si, a mass homology relation alone cannot reproduce the difference in the partial densities of vibrational states in these compounds and differences in elemental bonding should be taken into account.