Multimodal Structural Characterization of Ge-S-I Glasses by Combination of DFT Calculation and IR and Polarized Raman Spectroscopy

From a dual experimental-theoretical vibrational analysis, we propose a new rationalized structural description of Ge-S-I chalcogenides glasses at the nanoscale. A vibrational multipolar approach based on a simultaneous deconvolution of infrared (IR) and polarized Raman spectra (RS-VV and RS-HV) has been applied on these glasses. According to recent results on the amorphous GeS2 structure by X-Ray and neutron diffraction and to our spectral analyses, we suggest that the local structure of the glass backbone is effectively described by a combination of alpha-GeS2 nanolayers, edge-sharing GeS4 tetrahedra (ES-Td, ca similar to 50%), and corner sharing GeS4 tetrahedra (CS-Td, ca similar to 50%). We have then compared the experimental spectra to the calculated IR and polarized Raman spectra of some selected GexSyIz structural units obtained by density functional theory calculation. The stretching modes of the Ge-S-I occurring in the high frequency spectral range (300-450 cm(-1)) are essentially those of the GeS2 glass backbone and have been revisited. In addition, through a careful analysis of the vibrational multipolar activities of stoichiometric and over-stoichiometric sulfur glasses between 180 and 280 cm(-1), we propose new assignments for the seven modes that have been identified by our trimodal spectral analysis. We finally suggest that there is a competition between the insertion of atomic iodine as a glass modifier which involves the Ge-S-I clusters and molecular diiodine as a spectator encaged between two alpha-GeS2 nanolayers.