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Solid Sulfur Allotropes 65 p-S was suggested. Considering the negative slope of the frequency of one of the S-S stretching vibrations under pressure, the low number of Raman lines and their intensity ratios (stretching vs bending) as well as the kinetics of the transition a chain structure was proposed [205, 207, 213]. The investigation of the high-pressure behavior of sulfur was additionally complicated by the observation of an amorphous phase in the Raman spec- tra around 5 GPa when a laser line of wavelength 515 nm was applied [209]. Later, it was proved that amorphous sulfur (a-S) appears as an intermediate state between two crystalline phases, the orthorhombic a-S8 and the photo- induced structure, p-S [205, 213–215]. Moreover, the amorphous phase was found to be photo-induced, too. Systematic Raman studies on the disappear- ance of the a-S8 lines and on the amorphization transition, which made use of different excitation laser energies, provided strong evidence for a correla- tion of the pressure-tuned red shift of the (indirect) optical absorption edge and the photo-induced ring-opening due to the laser illumination [205, 213, 216]. The controversies in the Raman studies were also reflected by the results of X-ray structural studies. Krüger et al. [217] and Akahama et al. [201] did not observe any transition of a-S8 up to about 35 GPa and 25 GPa, respec- tively. However, Luo and Ruoff [218] could follow the diffraction pattern of a-S8 up to 4.9 GPa, but at 5.3 GPa the diffraction lines indicated a transition to a monoclinic lattice which then was observed up to about 24 GPa. It should be noted that the authors used the 488 nm line of an Ar ion laser for ruby pressure determination. Shortly after, Yoshioka and Nagata [219] re- ported X-ray diffraction patterns of a-S8 up to 7.6 GPa proving that the pat- tern at 5.3 GPa obtained by Luo and Ruoff agrees with that of orthorhombic sulfur at 6 GPa according to their previous results [207]. Furthermore, by application of a higher laser power a transformation of p-S to S6 at a pressure of around 11 GPa could be induced [205, 208, 211, 215, 219, 220]. With increasing pressure, the Raman lines of S6 could be ob- served up to about 45 GPa. However, at about 30–35 GPa new lines appeared in the spectra indicating the formation of a novel high-pressure species [211, 221]. Apart from pressure induced frequency shifts and intensity changes, the Raman spectrum of high-pressure S6 is identical with that of rhombohedral S6 chemically prepared at ambient conditions [34, 211, 222– 224]. Although S6 was considered to be a high-pressure high-temperature phase of sulfur [211], a more recently performed Raman investigation of the high-pressure low-temperature phase diagram has found S6 as the dominant allotrope below 100 K at about 11 GPa (see Fig. 23) [205]. On the other hand, results of in-situ X-ray diffraction studies of sulfur at temperatures much higher than room temperature and at pressures of about 10–13 GPa were in- terpreted controversially with respect to the presence of S6 [225, 226]. In summary, upon compression of orthorhombic sulfur a sequence a- S8!a-S!p-S!S6 with transition pressures of about 5 GPa, 6 GPa, and 11€2 GPa, respectively, were found in Raman studies if laser light with a wavelength of 515 nm was applied and if a certain threshold of the power density of the laser at the sample was exceeded. Since the Raman spectraPDF Image | Topics in Current Chemistry
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