Light-induced structural changes in (HgBr2)3(As4S4)2: An X-ray single-crystal diffraction, Raman spectroscopy and ab initio study

To investigate the behaviour of the As4S4 molecule within a crystal-chemical environment differing from realgar, beta-As4S4, and its high-temperature polymorph, beta-As4S4, the effects of the light exposure on the structure of the (HgBr2)3(As4S4)2 adduct have been studied. Differently from the cases previously studied, the action of the light filtered using a 550 nm long-wavelength pass filter did not produce any evident effect on the unit-cell. On the other hand, employing the 440 nm long-wavelength pass filter, remarkable variations of the unit-cell parameters were observed. In particular, an increase of the a, c, and β, and a decrease of the b parameter, producing on the whole an expansion of the unit-cell volume, is observed as a function of the light exposure times. Structure refinements indicated that the increase of the unit-cell volume is to ascribe to the formation of an increasing fraction (up to 20%) of pararealgar-type replacing the realgar-type molecule. Further light-exposure did not cause any further increase of the lattice parameters. On the contrary, a decrease of the unit-cell volume occurred by keeping the crystal in the dark (46 days): due to the loss of the crystallinity, only the core of the crystal, less altered and with smaller unit-cell volume, contributes to the diffraction effects. Micro-Raman spectra were collected on crystals exposed to the above mentioned wavelength light for increasing times. The peak at 275(+/-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 (HgBr2)3(As4S4)2 adduct. Relativistic DFT-GGA ab initio band structure calculations reveal a direct band gap of 2.04 eV and quite flat valence and conduction bands around the Fermi level. According to analyses of the atomic orbital contributions to the electronic band structures the highest occupied states are attributed to non-bonding p-states of As.