Origin of the effective charges of amorphous silicon and germanium

Abstract

The infrared absorption of materials like amorphous silicon and germanium shows that the atoms have first-order effective charges although in the corresponding crystals they do not. This paper demonstrates that these charges originate in the second-order effective charges of the crystals, which are converted in the amorphous phases to first-order charges by the distortion of the interatomic coordinates. The intensities of the amorphous-phase fundamental spectra are therefore related to the intensities of the binary combinations of the crystals and the mean-square displacements of the atoms relative to their neighbors, and they can be used to determine the displacements. The difference of mean-square frequency between the crystalline and amorphous phases can be related to the displacements and a sum over the fourth-order anharmonic constants. The root-mean-square atomic displacements in amorphous silicon and germanium calculated from the crystalline and amorphous spectra are 0.59 and ∼ 0.25-0.32 Å, respectively. The structures of amorphous silicon and germanium therefore differ significantly.