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Solid Sulfur Allotropes 67 Table 25 Correlation of the photo-induced amorphization of a-S8 with the aser energy and the pressure tuned absorption edge [205, 216] Laser Transition pressure (GPa)a Absorption edge (eV)b 2.37 2.14 1.86 - 1.70 Wavelength Energy Loading (nm) (eV) Unloading 1.9 (2.7–1.0) 2.8 (4.1–1.5) 9.3 (10.2–8.3) - 488.0 2.541 514.5 2.410 600.0 2.067 632.8 1.960 3.5 (2.6–4.3) 4.0 c 4.7 (3.8–5.5) 5.7 c 10.1 (8.8–11.4) 12.0 c a Mean values of the transition (pressure range in brackets) obtained from single crystal samples. Error<€0.1 GPa b Absorption edge energy at the onset pressure of amorphization based on the optical absorption data given by Peanasky et al. [229], by Lorenz and Orgzall [214], and by Syassen [230]. Data have partly been adjusted to the indirect absorption edge of a-S8 at STP conditions reported by Abass et al. [231] c Pressure values at which the lines of a-S8 disappeared in the Raman spectra [216]. These values correspond to those reported [205] at which the amorphization transition was completed 3.3.3 High-Pressure High-Temperature Phases (p<20 GPa, T>300 K) Only a few in-situ X-ray structural studies have been performed at high- pressure and high-temperature conditions [135, 225, 226]. By laser-heating of sulfur at 11 GPa a mixture of two phases was observed which consisted of room-temperature sulfur (a-S8) and a phase which was not indexed by the authors [225]. However, by evaluation of the diffraction lines it was conclud- ed that this phase cannot be interpreted as S6 as had previously been sug- gested on the basis of Raman studies [211]. In the pressure-temperature range up to 16 GPa and 1170 K three phases of sulfur were found as “stable”, namely in the pressure ranges of 3–8 GPa, 8–15 GPa, and above 15 GPa, respectively [226]. All phase transitions were reported to occur only at temperatures above 570 K. In addition, the phases were found to be unquenchable to STP conditions. The structure of the low- er pressure phase was automatically indexed as a hexagonal unit cell with the parameters a=697.6(1) and c=429.1(2) pm at 4.2 GPa and 300 K, contain- ing nine atoms. The crystal structure was compared with those of hexagonal selenium and tellurium, and therefore a helical structure of sulfur was as- sumed. The phase at 8–15 GPa was interpreted as S6 with a hexagonal unit cell containing 18 atoms. The phase above 15 GPa remained unsolved in these studies. All three phases showed a large hysteresis. Independently, a two-chain helical structure was found at 3 GPa and 673 K by Chrichton et al. [135]. A hexagonal unit cell with a=709.08(5) and c=430.28(6) pm has been obtained which was refined to a trigonal latticePDF Image | Topics in Current Chemistry
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