Vibrational modes and structure of (AgI) (GeS1.5)100− chalcohalide glasses

We report a structural investigation of bulk Ge-rich Ge–S–AgI chalcohalide glasses. A vibrational spectroscopic study of the quaternary system (AgI) _x (GeS _1.5) _{100− x} (0 ⩽ x _AgI⩽ 20) has been undertaken using infrared spectroscopy and Fourier transform Raman scattering. It was found that the GeS _1.5 Raman spectrum is compatible with a glass structure composed of corner- and edge-sharing mixed GeS _nGe _{4− n} ( n = 0–4) tetrahedra where units with n = 2–4 dominate, whilst the fraction of corner-sharing units are significantly lower than the corresponding fraction in the stoichiometric GeS ₂ glass. The addition of AgI has revealed a subtle but systematic effect in the structure of the Ge-rich glass matrix, manifested by mild decrease of the ES units and the concomitant increase of complex GeS _nI _{4− n} or GeS _nGe _mI _{4- n − m} tetrahedra whose vibrational modes form a continuum at low frequencies. Although, AgI seems to cause subtle structural changes due to the formation of Ge–I bonds, it is also evident that AgI does not act as a real modifier that would depolymerize appreciably the Ge–S network structure.