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

Abstract

We report the reversible photochemical transformation of sulfur–hydrogen mixture to (H2S)2H2, which occurs through an expanded state of photoactive a-S phase of sulfur at 4 GPa. Upon further compression, the photoproduct (H2S)2H2 undergoes a phase transition at 17 GPa, and 40 GPa. The pressure-induced Raman changes indicate that the phase transition from phase I to II at 17 GPa is associated with the proton-ordering process in (H2S)2H2, evident by the profound splitting of S–H and H–H vibrational modes, whereas the transition at 40 GPa is accompanied by the disappearance of all the S–H stretching and bending modes and partial dissociation to sulfur. With increase in pressure, the molar volume of (H2S)2H2 is substantially larger than that of S + H2 mixtures, suggesting the significance of photochemical effect in order to drive the reaction from S + H2 to (H2S)2H2. In addition, we have also provided the thermal- and pressure-induced effect in the mixtures using confocal Raman spectroscopy. From our results, it is clear that the effect of pressure and photochemistry can be coupled to drive the reaction at room temperature and lower pressure, rather than having to drive the reaction thermally or mechanically, underscoring the significance of the photochemical effect in understanding the path-dependent transformations of sulfur and sulfur-containing materials.

Properties and phase diagram of (H2S)2H2

“…;[15,16] at low temperatures we find at least seven narrow S-H stretching modes (∼2250-2580 cm -1 ) and observe two doublets of H-H Raman vibrons (ν IIa and ν IIb , Figs. 5 and 6).…”

“…DACs with culets of 50-200 μm, containing crystalline sulfur suspended in fluid H 2 , were heated to 373 K for 1 h (below 0.5 GPa), then cooled to 300 K, producing discrete fluid domains of H 2 S and H 2 . Samples were then compressed above 4.8 GPa to form phase I of (H 2 S) 2 H 2 [15,16] (Fig. 4).…”

“…Given the intense interest in the sulfur-hydrogen system, there are surprisingly few direct experiments on (H 2 S) 2 H 2 [6,[15][16][17]. The first experimental observation of superconductivity in this system used pure H 2 S as a precursor but claims a maximum T c of 203 K [1].…”

“…In (H 2 S) 2 H 2 the underlying S-atom configurations are 68 similar across all known experimental and predicted phases, 69 making them difficult to distinguish on the basis of diffrac-70 tion alone [13,15]. Transitions between high-pressure phases 71 in (H 2 S) 2 H 2 at room temperature are primarily driven by 72 hydrogen bonding (H bonding) and are characterized by the 73 orientation of H 2 S molecules and the nature of the H 2 sites 74 within the structure, producing unique vibrational spectra 75 [6,[15][16][17]. In particular the number of H 2 vibrational modes 76 (vibrons) and the effect of compression on their respective 77 frequencies can be utilized to distinguish phase changes and 78 shifts in bond orientation of the surrounding H 2 S molecules.…”

“…In particular the number of H 2 vibrational modes 76 (vibrons) and the effect of compression on their respective 77 frequencies can be utilized to distinguish phase changes and 78 shifts in bond orientation of the surrounding H 2 S molecules. 79 Therefore Raman spectroscopy combined with infrared (IR) spectroscopy presents a powerful diagnostic combination for (H 2 S) 2 H 2 at high pressure [15,16], being sensitive to both inter-and intramolecular interactions [23][24][25].…”