Abstract
Light-induced degradation in realgar has been studied by means of four-circle single-crystal X-ray diffraction and X-ray photoelectron spectroscopy. Due to the alteration of realgar exposed to light, the a lattice parameter and c sinβ value increases linearly, whereas the b lattice parameter remains substantially constant. The anisotropic variations of the lattice parameters led to a continuous increase of the unit cell volume. But there is no correlation between the continuous increase of the cell volume and the bond distance variations in As4S4 molecules, since As4S4 molecule in the unit cell expands itself during light exposure. The most pronounced change was observed in the distance between centroids in As4S4 cage. That is, the spread of As4S4 intermolecular distances is responsible for the unit cell volume expansion. In addition, the O 1s peak becomes apparently higher and greater as the increase of light exposure time. This result substantiates the following reaction; 5As4S4 + 3O2 -> 4As4S5 + 2As2O3, and the formation of As4S5 molecule with the light exposure. The results obtained seem to indicate that the additional sulfur at As4S4 molecule affects geometrically the anisotropic expansion for a and c axis, and the As4S4 molecule in realgar transform to pararealgar via As4S5 molecule.
