Light-induced degradation dynamics in realgar: in situ structural investigation using single-crystal X-ray diffraction study and X-ray photoelectron spectroscopy

Light-induced degradation in realgar (arsenic sulfide) has been studied by means of four-circle single-crystal X-ray diffraction and X-ray photoelectron spectroscopy. Because of the alteration of realgar exposed to light, the a lattice parameter and c sinβ value increase linearly from 9.327 to 9.385 Å and fro|m 6.320 to 6.364 Å, respectively. In contrast, the b lattice parameter remains substantially constant. Anisotropic variations of the lattice parameters engender a continuous increase of the unit-cell volume from 799.5 to 810.4 Å 3 . Nevertheless, no correlation exists between the continuous increase of the unit-cell volume and the bond distance variations in As 4 S 4 molecules because the As 4 S 4 molecule in the unit cell expands very little during light exposure. The most pronounced change was in the distance between centroids UiAs 4 S 4 cages. The spread of As 4 S 4 intermolecular distances increases continuously from 5.642 to 5.665 Å, which directly affects the unit-cell volume expansion of realgar. In addition, the O1s peak increases rapidly after light exposure. The result substantiates the following reaction proposed by Biudi et al. (2003): 5As 4 S 4 + 3O 2 -> 4As 4 S 5 + 2As 2 O 5 . That is, realgar is transformed into pararealgar if oxygen exists and produces the As 4 S 5 molecule. The additional S atom contributes to anisotropic expansion for the a and c axes because the direction of the additional S atom points toward [41̅4] in the unit cell. Furthermore, an S atom in the As 4 S 5 molecule is released from one of the equivalent As-S-As linkages in As 4 S 5 which becomes the As 4 S 4 molecular of pararealgar. After the As 4 S 5 molecule is divided into an S atom (radical) and the As 4 S 4 (pararealgar type) molecule, the free S atom is re-attached to another As 4 S 4 (realgar type) molecule, and reproduces an As 4 S 5 molecule. The reproduced As 4 S 5 molecule is again divided into an S atom (radical) and an As 4 S 4 (pararealgar type) molecule. This cycle whereby realgar is indirectly transformed into pararealgar via the As 4 S 5 molecule is promoted by light and repeated during light exposure.