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Transformations induced by light in realgar and β-As 4 S 4  have been studied by means of X-ray diffraction (powder and single-crystal) and Raman laser spectroscopy. Both poly­chromatic and monochromatic light alter realgar and the β phase into pararealgar. By lowering the reaction rate (long-wavelength pass filter, low laser power) an intermediate product, corresponding to the so-called “χ phase,” can be observed during the alteration process from β-As 4 S 4  to pararealgar (polychromatic filtered light and laser light) and that from realgar to pararealgar (only with polychromatic filtered light). The powder pattern of the χ phase is fairly similar to that of β-As 4 S 4  but differs from realgar, and it can be indexed on the basis of an expanded β-phase unit cell. Lattice parameters are as follows: a = 9.758(5), b = 9.522(5), c = 9.074(5) Å, 0 = 100.84(5)°, and a = 9.757(4), b = 9.512(7), c = 9.089(4) Å, 0 = 100.97(3)° for the χ phase obtained from alteration of realgar and β-As 4 S 4 , respectively. The formation of the χ phase is preceded by a strong anisotropic increase of the unit-cell volume of realgar and the β phase. The effects of light exposure on lattice parameters were studied using single-crystal X-ray diffraction: In realgar a and c sin β increase linearly with increasing exposure times, whereas b remains substantially unchanged. In the β-As 4 S 4  crystals, b and c sin β increase while a decreases so that, after 1560 min, the unit-cell dimensions match those of the χ phase. Raman spectroscopy, which allows monitoring of the alteration process in real time, indicates the presence of the same As 4 S 4  cage molecule in the χ phase as in realgar and β -As 4 S 4  and confirms the similarity between the structure of χ and that of β. However, a less-ordered crystal struc­ture and a lower molecular symmetry is suggested from analysis of the spectroscopic results.

Light-induced variations in realgar and β-AS 4 S 4 :X-ray diffraction and Raman studies