Experimental study of the effect of light intensity on arsenic sulfide (As4S4) alteration

Correlation between light-induced alteration of arsenic sulfide (mineral name: realgar, As4S4) and irradiated light intensity was investigated using in situ single-crystal X-ray diffractometry. The exposure time required for the phase transformation increases gradually as the light intensity decreases. The X-ray diffraction intensities of (2 6 1) and (3 6 1) reflections decrease sharply, indicating that the As–As bonds in As4S4 molecules are broken by insertion of an additional S atom. The As–As bond cleavage and As4S4 molecule deformation are largely produced throughout the crystal structure when the unit cell volume reaches approximately 802 Å3, followed by rearrangement of the As4S4 pararealgar molecular packing. It continues until the unit the cell volume expands continuously to approximately 810 Å3. The feature of the continuous increase of the a cell parameter was observed within the light intensity range. The value of the b cell parameter remained constant during light treatment. Nevertheless, the linear increase in the c sin β value was not confirmed as the light intensity decreased. Variation of the unit cell volume correlates with that of the c sin β value. Consequently, no continuous increase of unit cell volume was apparent in realgar exposed to light of 5 W/m2. Such a discontinuous increase in c sin β value is attributed mainly to the value of the c cell parameter, which varies widely with light exposure. The highest sensitivity for lighting is shown at the a and β angles in unit cell parameters. It increases or decreases linearly up to the loss of crystallinity.