Structural models and vibrational spectra of tetrahedral chalcogenide crystals and glasses

Cluster models of Ge${\mathrm{}(\mathrm{S},\mathrm{S}\mathrm{e})\mathrm{}}_2$ containing 60 or 132 atoms were constructed. The vibrational spectra of these clusters were calculated and examined in detail in the region of the $A_1$ (symmetric breathing) mode. Comparison with experiment shows generally good agreement and permits detailed analysis of the Raman spectra of the crystals, the compound glasses, and the alloys of Ge${\mathrm{S}}_2$ and Ge${\mathrm{Se}}_2$.