Light-induced changes in molecular arsenic sulfides: State of the art and new evidence by single-crystal X-ray diffraction

Light-induced structural changes in single crystals belonging to the β-As 4 S 4 -As 8 S 9 series and in a crystal of synthetic β-As 4 S 3 were monitored step by step by determining the unit-cell dimensions. A marked increase of unit-cell volume as a function of exposure time was observed for all the crystals belonging to the β-As 4 S 4 -As 8 S 9 series except for stoichiometric alacranite (As 8 S 9 ). No significant change upon long exposures to light was observed for the synthetic β-As 4 S 3 crystal. Crystal structure refinements were carried out for crystals with different composition at selected steps of the light-induced process. The structural results clearly showed that the percentage of the As 4 S 5 molecule in the structure increases when a crystal is exposed to light. Therefore, the increment of the unit-cell volume induced by light exposure appears to be related to a random replacement of As 4 S 5 for As 4 S 4 in the structure according to the reaction 5As 4 S 4 + 3O 2 → 4As 4 S 5 + 2As 2 O 3 . The results obtained in the present study combined with a critical review of data previously published indicate that the As 4 S 4 molecule is able to incorporate sulfur to convert to As 4 S 5 upon exposure to light, whereas either As 4 S- or As 4 S 5 molecules do not go undergo any modification. It appears that the extent of sulfur incorporation is strictly controlled by the type of molecular packing as well as by the kind of molecule. A final, complete conversion to pararealgar was observed only for pure β-As 4 S 4 , whereas nonstoichiometric As 8 S 9-x crystals initially containing variable amounts of β-As 4 S 4 microdomains convert only partially to pararealgar upon light exposure.