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 sin beta value increase linearly from 9.327 to 9.385 A and from 6.320 to 6.364 angstrom, 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 angstrom(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 in As(4)S(4) cages. The spread of As(4)S(4) intermolecular distances increases continuously from 5.642 to 5.665 angstrom, 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 Bindi et al. (2003): 5A(S4)S(4) + 3O(2) -> 4A(S4)S(5) + 2As(2)O(3). That is, realgar is transformed into pararealgar if oxygen exists and produces the A(S4)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 [4 (1) over bar4] 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.