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Solid Sulfur Allotropes 61 Fig. 22 Schematic phase diagram of sulfur around the a-b-liquid-gas states at low pres- sures (cf. Table 24). Pressure scale is in arbitrary units the sample combined with large gradients of the thermodynamic parameters of state along the shock front have to be taken into account [193]. When the high intensity synchrotron X-ray sources became available a major problem of the former structural investigations was overcome. How- ever, the low X-ray scattering intensity of sulfur makes detailed structural analyses still difficult, particularly in high-pressure apparatuses dealing with extremely small sample volumes of the order of 104 mm3. 3.2 Triple Points in the Vicinity of the Melting Curve The melting curve of sulfur with respect to metastability and experimental procedures has been critically analyzed by Vezzoli and Walsh [194]. The au- thors also discussed the previously reported phase boundaries of the solid high-pressure allotropes in the range up to 4 GPa and from room tempera- ture up to about 670 K [132]. However, the structures of these high-pressure polymorphs are unknown and their probability to exist under equilibrium conditions is still awaiting confirmation. The p-T phase diagram near the a-b transition of crystalline S8 has been studied and the triple-points of the solids in equilibrium with the vapor and the liquid were determined; see Fig. 22. Under higher pressures, liquid sulfur shows several additional transitions [195]. Near the melting point at 12 GPa and 1100 K, the melt transforms to a metallic liquid state the pressure of metallization being much lower than in the solid state (~80–90 GPa at room temperature) [196]. The most reliable critical points in the vicinity of the melting curve of sulfur are listed in Ta- ble 24.PDF Image | Topics in Current Chemistry
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