Measurement of local structural configurations associated with reversible photostructural changes in arsenic trisulfide films

Extended x-ray-absorption fine-structure measurements have been made on three reversible and reproducible cycles of thermally annealed and light-soaked amorphous ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ films. Associated with the light-soaked material are (1) a very small increase in the population of wrong bonds in the first shell, (2) an enlarged As—S—As bond angle with an expansion of As—As distance in the second shell, (3) a larger spread in the distribution of As—S—As bond angles, and (4) an absence of any change in the third As—S shell. From these data, we present the first quantitative correlation between observed local atomic structural changes and measured macroscopic properties that are associated with photodarkening. Our data demonstrate that the photoinduced structural changes mainly involve bonding alterations at S atoms as well as a change in the dihedral angle relationship between adjacent ${\mathrm{AsS}}_3$ pyramids joined at S atoms.