LIGHT-INDUCED VARIATIONS IN REALGAR AND BETA-AS4S4 - X-RAY-DIFFRACTION AND RAMAN STUDIES

Transformations induced by light in realgar and beta-As4S4 have been s tudied by means of X-ray diffraction (powder and single-crystal) and R aman laser spectroscopy. Both polychromatic and monochromatic light al ter realgar and the beta phase into pararealgar. By lowering the react ion rate Gong-wavelength pass filter, low laser power) an intermediate product, corresponding to the so-called ''chi phase,'' can be observe d during the alteration process from beta-As4S4 to pararealgar (polych romatic filtered light and laser light) and that from realgar to parar ealgar (only with polychromatic filtered light). The powder pattern of the chi phase is fairly similar to that of beta-As4S4 but differs fro m realgar, and it can be indexed on the basis of an expanded beta-phas e unit cell. Lattice parameters are as follows: a = 9.758(5), b = 9.52 2(5), c = 9.074(5) Angstrom, beta = 100.84(5)degrees and a = 9.757(4), b = 9.512(7), c = 9.089(4) Angstrom, beta = 100.97(3)degrees for the chi phase obtained from alteration of realgar and beta-As4S4, respecti vely. The formation of the chi phase is preceded by a strong anisotrop ic increase of the unit-cell volume of realgar and the beta phase. The effects of light exposure on lattice parameters were studied using si ngle-crystal X-ray diffraction: In realgar a and c sin beta increase l inearly with increasing exposure times, whereas b remains substantiall y unchanged. In the beta-As4S4 crystals, b and c sin beta increase whi le a decreases so that, after 1560 min, the unit-cell dimensions match those of the chi phase. Raman spectroscopy, which allows monitoring o f the alteration process in real time, indicates the presence of the s ame As4S4 cage molecule in the chi phase as in realgar and beta-As4S4 and confirms the similarity between the structure of chi and that of b eta. However, a less-ordered crystal structure and a lower molecular s ymmetry is suggested from analysis of the spectroscopic results.