Experimental study of the effect of light intensity on arsenic sulfide (As 4S 4) alteration

Abstract

Correlation between light-induced alteration of arsenic sulfide (mineral name: realgar, As 4S 4) 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 As 4S 4 molecules are broken by insertion of an additional S atom. The As–As bond cleavage and As 4S 4 molecule deformation are largely produced throughout the crystal structure when the unit cell volume reaches approximately 802 Å 3, followed by rearrangement of the As 4S 4 pararealgar molecular packing. It continues until the unit the cell volume expands continuously to approximately 810 Å 3. 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/m 2. 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.