A comparison of the structures of vapour-deposited and bulk arsenic sulphide glasses

Accurate comparative measurements have been made of the X-ray diffraction pattern of a bulk arsenic sulphide glass prepared from the melt and a 250 μm film prepared by vapour deposition of the evaporated bulk. By using Ag Kα radiation, measurements have been made up to $\text{Q = 21}\text{A}\text{?}{}^{\mathrm{?1}}\text{(Q = 4π}\text{sin}\text{θ/λ)}$ so as to give good spatial resolution. Significant differences are found between bulk and film specimens, and in particular a shoulder is found on the high r side of the first peak in the distribution function for the film. Also, the film is richer in arsenic than the parent bulk. The results are compared in detail with a number of models, all of which involve covalent bonding requirements being fully satisfied, and it is concluded that there are more AsAs bonds in the film than the minimum required by stoichiometry and that a significant fraction ($\text{≈}\text{2}\text{3}$) of these are unusually long ($\text{≈2.6}\text{A}\text{?}$ compared to the more usual 2.5 Å). The most probable explanation is that the vapour contains a significant fraction of As₄S₄ species (in which the AsAs distances are $\text{≈2.6}\text{A}\text{?}$) and that the film contains a mixture of molecular species which have undergone considerable polymerization. The extent to which this results in a sheet-like structure similar to that proposed for the bulk glass remains unclear.