Reversible Photochemical Transformation of S and H2 Mixture to (H2S)2H2 at High Pressures

We report the.reversible phOtochemical transformation of sulfur hydrogen mixture to-(H2S)(2)H-2, which occurs through an expanded state.of,photeactive a-S phase of sulfur at 4:0Pa. Upon,further compression, the photoprocInct,(H2S)(2)H-2 undergoes a phase transition at 17 GPa, and 40 GPa. The pressure-induced Raman changes indicate that the phase transition from phase Ito II at 17 GPa is associated with the ptoton-ordering process in (H2S)(2)H-2, evident by the profound splitting of S-H and H-H Vibrational modes, whereas the transition at 40 GPa is accompanied by the disappearaiice of all the S-H stretching and bending modes and partial dissociation to sulfur. Withincrease in pressure, the molar volume of (H2S)(2)H-2 is substantially larger than that of S + H-2 mixtures, suggesting the significance of photochemical effect in order to driVe the reaction from S + H-2 to (H2S)(2)H-2. In addition, we, have also provided the thermal- arid pressure-induced effect in the. mixtures using,,confocal Raman-spectroscopy. From our-reaults, it is clear that the effect:of pressure" and photochemistry can be coupled to tirive the reaction at room teniperatire and lower pressure,-rather ithan having to drive 'the-reaction thermally or ineehanicallyiundetscoring the significance of the photochemical effect in understanding the path-dependent transformations of sulfur and sulfur-containing materials: