The room-temperature reflection spectra of Mg₂Si, Mg₂Ge, and Mg₂Sn have been measured at wave-lengths between 20 and 50{μ}. Reststrahl reflectivity peaks were observed at 36 and 45 {μ} for Mg₂Si and Mg₂Ge, respectively, and a rising reflectivity in Mg₂Sn is estimated to peak at about 54 {μ}. The occurrence of peaks indicates that these compounds are partly ionic. The reflection spectra are more like those of the alkali halides than those of the III-V semiconducting compounds. If a simple damped harmonic oscillator is used to describe the reststrahl mode, the three Mg compounds have the same force constant for this mode. Values of dielectric constants and effective charges are estimated for Mg₂Si, and are used to evaluate a previous analysis of electron mobility in this compound. Effects of free carriers on the reflectivity of Mg₂Ge indicate that the effective masses are in rough agreement with the results of transport measurements.
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McWilliams et al. (1963) studied this question.
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