  High-resolution neutron-scattering measurements have been performed on bulk
glasses of As2S3 and As2S3I1.65 at a spallation neutron source. For the case of
As2S3, an isotopic-substitution experiment involving the S isotope has allowed
some of the various pair correlations contributing to the second peak in the
radial distribution function to be determined by the method of first
differences. For the ternary glass, it has been confirmed that iodine bonds
preferentially to As atoms, and that on average each As atom is coordinated to
one iodine and two sulphur atoms in the first coordination shell.
