Ab initio calculation of vibrational frequencies of Ge0.25S0.75−yIy glass

The vibrational frequencies of Ge4, S4, I4, SI3, Ge2I2, S2I2, Ge2S2, Ge2SI, GeS3, GeI2S, Ge3S and GeI3 are computed using first principles. These frequencies are compared with those found experimentally in the Raman spectra of Ge0.25S0.75−xIx glass. In this way, it is found that Ge4, GeI2S and GeI3 tetrahedra occur in the glass. These tetrahedra are of different sizes so they stack to form the floppy phase of the glass.