Infrared Reflectivities of Magnesium Silicide, Germanide, and Stannide

Abstract

The room-temperature reflection spectra of ${\mathrm{Mg}}_2$Si, ${\mathrm{Mg}}_2$Ge, and ${\mathrm{Mg}}_2$Sn have been measured at wave-lengths between 20 and 50μ. Reststrahl reflectivity peaks were observed at 36 and 45 μ for ${\mathrm{Mg}}_2$Si and ${\mathrm{Mg}}_2$Ge, respectively, and a rising reflectivity in ${\mathrm{Mg}}_2$Sn 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 ${\mathrm{Mg}}_2$Si, and are used to evaluate a previous analysis of electron mobility in this compound. Effects of free carriers on the reflectivity of ${\mathrm{Mg}}_2$Ge indicate that the effective masses are in rough agreement with the results of transport measurements.