129I-Mössbauer spectroscopy study of MI–As2Se3 (M=Ag, Cu) superionic chalcohalide glasses

129I-Mössbauer spectroscopy was used to study the local iodine environment in the superionic xAgI·(1−x)As2Se3 and mixed conducting (Cu+, hole) xCuI·(1−x)As2Se3 glassy systems (x=0.05–0.50). Well-resolved quadrupole-split multiplets, also characterised by a Gaussian-like distribution of the nuclear quadrupole interaction constant νQ, as well as distinctly different values of the isomer shift S compared to that of c-AgI and c-CuI (0.3≤ΔS≤0.6 mm s−1), clearly indicate fundamental differences of the iodine local environment in the MI–As2Se3 glasses and crystalline superionic α-phases of these metal iodides. Hyperfine interaction parameters of both the glassy systems as a function of x exhibit, however, different trends for the AgI- and CuI-containing glasses. In contrast, the two systems show a remarkable common decrease of the absorber recoil-free fraction fA by a factor of 3–5 and, respectively, of the local Debye temperature ΘD with increasing x. These and other spectroscopic features are consistent with the recent high-resolution neutron diffraction results and two drastically different ion transport regimes at low and high mobile ion content and will be discussed in the present contribution.