Property extrema in GeAs sulphide glasses

GeAsS glasses with Ge:As ratios of 2.5:1, 1:1 and 1:2 with S concentrations ranging from 35% to 80%, corresponding to an average coordination number (〈r〉) ranging from 2.35 to 2.98 were synthesized. For glasses with fixed Ge:As ratio, the absorption edge and the glass transition temperature vary monotonically with S content. The thermal expansion coefficient and molar volume, however, attain maxima at 〈r〉 near 2.6, which extrema correspond to a maximum in the relative intensity of a narrow polarized Raman band at 275 cm−1 and a minimum in the frequency of the boson peak. These features shift to smaller 〈r〉 with decreasing Ge:As ratio and are attributed to the development of molecular As4S3-like species in GeAsS glasses with intermediate S deficits.