Measurements of the first-order Raman scattering of Mg2X (X = Si, Ge, Sn) as a function of temperature and pressure are presented. The volume contribution (implicit effect) to the frequency shift with temperature is computed and compared to the lattice-anharmonicity contribution (explicit effect). The latter contribution turns out to be larger than the former. The values of the implicit fraction (ratio of the volume to the total effect) for all three materials are found to be smaller than 0.5 and to depend on temperature. These findings are indicative of a predominantly covalent bonding in these materials.
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Raptis et al. (1987) studied this question.
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