Structure determination of Ag-Ge-S glasses using neutron diffraction

The structure of the superionic glass system (Ag2S)(x)(GeS2)(1-x), for three compositions x = 0.3, 0.4, 0.5, has been studied using neutron diffraction, and isotopic-substitution neutron-diffraction experiments have been performed on three silver isotope-substituted (Ag-107,Ag-nat,Ag-109) samples of the composition (Ag2S)(0.5)(GeS2)(0.5). The average short-range orderings of Ge-S, Ag-S, and Ge-Ag correlations were identified in the radial distribution functions for the isotopically substituted system of (Ag2S)(0.5)(GeS2)(0.5). From the first and second differences in the three sets of isotopic-substitution neutron-diffraction data, the other three partial correlations (Ag-Ag, Ge-Ge, and S-S), were also identified. By examining unusually broad peaks in the Ag-Ag correlation function, it was concluded that the Ag-Ag distribution was rather homogeneous. We were also able to obtain further information by combining the first and second difference analyses, resulting in a structural model of a slightly elongated GeS4 tetrahedron with the local environment of Ag+ ions being threefold coordination by nonbridging sulphur ions. The medium-range order of the host framework was found to be a chainlike structure of linked corner-sharing GeS4 tetrahedra. Substantial changes in the first and second peaks in the distinct scattering functions i(Q) were found with composition and also with isotopic substitution. It was possible to explain the trends in the changes of the heights of these peaks in the structure factor by applying the void model for the first sharp diffraction peak.