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To investigate the behavior of the As 4 S 4  molecule within a crystal-chemical environment differing from realgar, α-As 4 S 4 , and its high-temperature polymorph, β-As 4 S 4 , the effects of the light exposure on the structure of the HgI 2 ·As 4 S 4  adduct have been studied. The structure of this compound consists of a packing of nearly linear HgI 2  molecules and As4S4 cage-molecules. A crystal [V = 1295.9(4) Å 3 ] was exposed to filtered polychromatic light (550 nm long-wavelength pass filter). A marked increase of the unit-cell volume as a function of the exposure time was observed up to V = 1338.9 Å 3  at 3060 min of exposure. Structure refinements indicated that the increase of the unit-cell volume is to ascribe to the formation of an increasing fraction (up to 59%) of pararealgar replacing the realgar-type molecule. After this point, further light-exposure did not cause any further increase of the lattice parameters. On the contrary, a slow, continuous decrease of the unit-cell volume down to 1292.9 Å 3  occurred by keeping the crystal in the dark (39 days). The “reverse” evolution of the unit-cell parameters in the dark almost overlap that observed during the light-induced process and the structural model obtained after the “dark stage” was found to be identical to that of the unaltered crystal, although the diffraction quality was lower and powder-like diffraction rings were observed together with single-crystal reflections. Apart from few peaks belonging to the original unaltered HgI 2 ·As 4 S 4  adduct, most of the collected peaks can be assigned to β-As4S4. This feature could indicate decomposition into micro-crystalline β-As 4 S 4  and HgI 2 ; no diffraction effect ascribable to any HgI 2  phase, however, was observed. Micro-Raman spectra were collected on crystals exposed to the above-mentioned wavelength light for increasing times (up to 3000 min). The peak at 274(±1) cm -1  whose intensity increases as a function of the exposure time confirms the transition from a realgar- to a pararealgar-type molecule in the HgI 2 ·As 4 S 4  adduct.

Light-induced molecular change in HgI 2 ·As 4 S 4 : Evidence by single-crystal X-ray diffraction and Raman spectroscopy