Phase transitions in crystalline sulfur at p>8 GPa, observed by Raman spectroscopy in a diamond anvil cell

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

“…Above 10 GPa, S-II transforms to rhombohedral S-VI and further to tetragonal S-III with square-shaped chains above 12 GPa. , Upon further compression, S-III transforms to body centered orthorhombic (bco) S-IV at 83 GPa and further to rhombohedra S-V (β-Po) above 157 GPa, both of which become superconductors at low temperatures . Even among the previous Raman studies, ,− the evolution of phases vary depending upon the laser energy and laser power density used in those experiments. This is in part because sulfur becomes photochemically sensitive above 3 GPa .…”

“…Among these allotropes, the most stable form of sulfur at ambient condition is S-I (or αS 8 ), which is a molecular crystal with crown shaped S 8 puckered rings . Sulfur has a complex phase diagram, consisting of several polymorphs whose stabilities and transformations strongly vary depending on various measurements reported. ,− Figure , for example, summarizes the phase diagram to 20 GPa, determined by the previous Raman studies. , According to this phase diagram, S-I transforms to amorphous sulfur ( a -S) above 3 GPa, which then transforms to trigonal S-II above 6 GPa. Above 10 GPa, S-II transforms to rhombohedral S-VI and further to tetragonal S-III with square-shaped chains above 12 GPa. , Upon further compression, S-III transforms to body centered orthorhombic (bco) S-IV at 83 GPa and further to rhombohedra S-V (β-Po) above 157 GPa, both of which become superconductors at low temperatures .…”

“…For instance, if a low power laser is used, S-I bypasses the transitions to S-II and S-VI, and transforms to a -S and then to tetragonal S-III. As a result, the previously reported phase diagrams of sulfur are greatly diverse, depending on the experimental methods. ,− …”

Raman Studies of Sulfur Crystal (α-S8) at High Pressures and Low Temperatures

“…The pressure dependence of the Raman modes at 300 K was first investigated by Zallen and later by Slade et al., who extended the pressure range up to 9 GPa. Several other spectroscopic investigations at high pressure have been reported in recent years. − Different pressure and photoinduced phase transitions have been described, but there is still a lack of agreement among these results. This may be because the phase transitions in sulfur depend upon the incident laser energy and upon a threshold power of the laser beam, as well as upon other parameters such as the type of pressure medium, chemical environment, impurities, and the type of sample.…”

Pressure and Photoinduced Phase Transitions of Elemental Sulfur Confined by Open-End Single-Wall Carbon Nanohorns

“…In addition, the stoichiometric ratio, sulfur content, and structural configuration of C–S bonds in such a binary system may also lead to varying structural properties according to theoretical studies. , Raman spectroscopy is also a powerful tool to investigate phase transitions of sulfur under pressure. However, the interpretations of Raman results have not yet reached a consensus and are even self-contradictory, due to strong photothermal-induced phenomena that take place during light exposure and excitation. − Some Raman investigations tentatively concluded a photoinduced transition sequence as follows: α-S 8 → first photoinduced amorphous phase (a-S) → second photoinduced phase (p-S) → S 6 , and the determination of phase diagram is closely dependent on pressure, laser energy, and power density. − On the other hand, nanocarbon/sulfur hybrids have been the subject of intensive research efforts in fields such as nanoenergy engineering . To obtain more insights into the confinement effect on filled molecules encapsulated in nanospaces of CNHs, as well as into the pressure-induced phase transition and photothermal behavior of nanocarbon/sulfur hybrids, S@SWCNHs are designed and are measured using X-ray diffraction and Raman spectroscopy upon cold compression.…”