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=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.