Interpretation of the Raman spectra of the glassy states of SixS1−x and SixSe1−x

We use the density-functional theory to make models of SixSy and SixSey for the values of x,y = 1–6. The vibrational frequencies are calculated for each model. The stable clusters are selected on the basis of positive vibrational frequencies. In the case of SixS1−x, the values of the vibrational frequencies calculated from the first principles for Si2S(triangular)cluster of atoms, 364.1 cm−1 and 380.8 cm−1, agree with the experimentally measured values of 367 cm−1 and 381 cm−1, indicating that Si2S clusters occur in the glassy state of SiS. The calculated values of the vibrational frequencies of SiSe4 (pyramidal) which agree with the experimental Raman frequencies of glassy SixSe1−x are 114, 166 and 361 cm−1. The calculated values for Si2Se4 (bipyramidal) which agree with the experimental data of SixSe1−x are 166 and 464 cm−1. In Si4Se (pyramidal) the values 246 and 304 cm−1 agree with the measured values. In Si4Se2 (bipyramidal), the calculated values 162, 196 and 304 cm−1 agree with the measured values. The calculated values of 473 cm−1 for Si6Se2 (bipyramidal) also agree with the experimentally measured values. We thus find that pyramidal structures are present in the amorphous SixSe1−x glassy state.