Sulphur at High Pressure and Low Temperatures

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.…”

“…Mode Grüneisen parameters have been derived from experiments at T = 300 K. ,− The values are somewhat different because of the different investigated pressure ranges and the different values of compressibility utilized in the calculations (0.095/GPa 26 and 0.139/GPa 27 ).…”