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Solid Sulfur Allotropes 69 Fig. 24 Proposed structures for the observed high-pressure phases of sulfur above ca. 80 GPa [8]. For the more simple sc, bcc, and fcc structures, see textbooks on solid state physics/chemistry phase is amorphous; however, nothing is known about its structure. It is not clear yet if this pressure-induced amorphous sulfur has any relation to the photo-induced amorphous sulfur observed in high-pressure Raman studies. Indeed, as in the case of the photo-induced transformations (a-S8!a-S!p-S) the pressure-induced amorphization is an intermediate state between two crystalline forms towards close-packing upon further compression [236]. At 34 GPa, Akahama et al. [201b] recorded diffraction patterns indicating the presence of a single crystal since the Bragg reflections were observed as “spotty”. The transition was interpreted as a process of recrystallization and reorientation. Luo et al. [200] reported a transition from the amorphous phase at 37 GPa at which a single intense peak appeared in the diffraction spectra. A second intense peak was observed at 42 GPa and the transition was found to be accomplished at 75 GPa. In both investigations the struc- tures were not analyzed. However, there might be some relation to the trigo- nal phase observed by Krüger et al. above 32 GPa [217]. The diffraction lines of this unknown phase, termed S-II by Akahama et al., became weaker with increasing pressure, and at 89€2 GPa, the lines had completely vanished [201]. Amaya et al. reported a stepwise change of the electrical resistivity of sul- fur at room temperature and pressures of about 40 and 90 GPa, respectively [178b]. The changes in electrical resistivity can be reasonably interpreted as insulator-to-semiconductor and semiconductor-to-metal transitions. At 82–84 GPa, in the studies of Akahama et al. [201] and Luo et al. [200], respectively, new diffraction lines appeared which were indexed as a base- centered orthorhombic (bco) lattice (Fig. 24). On theoretical grounds, such a structure should be metallic [190, 237]. Luo et al. stated that the transforma- tion from the unknown phase to the bco phase was not completed until about 100 GPa because of the persistence of a diffraction peak from the pre- vious phase. On the other hand, Akahama et al. reported a transition pres- sure of about 89 GPa. The bco phase showed a hysteresis of about 6 GPa upon lowering the pressure [201]. Further evidence for the transition to a metallic phase comes from the optical reflectivity measurements of Luo et al. [204]. Below 88 GPa, the reflectivity was reported to be low (typicallyPDF Image | Topics in Current Chemistry
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