Evidence of the existence of the As4S6 molecule produced by light exposure of alacranite, As8S9

The As4S6 molecule, although theoretically predicted and supposed to occur in amorphous arsenic sulfides, was missing in the reported structures of crystalline As chalcogenides; the thermodynamic stable phase for this stoichiometry, in fact, is that of the mineral orpiment, which shows a layered structural arrangement based on trigonal AsS3 pyramids. Here we report the first possible occurrence of the As4S6 molecule together with the other known As4Sn (n D 3, 4, 5) molecules randomly replacing each other in the crystalline structure of a new monoclinic product obtained by the light-induced alteration of the mineral alacranite, As8S9. Our findings are based on single-crystal X-ray diffraction experiments. The fact that As4S6 formed in a crystalline light-induced alteration product could indicate a heretofore unknown role of this molecule in the photoinduced changes of the physico-chemical properties of both bulk glasses and thin films in the As–S system, widely studied in optics and optoelectronics.