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Solid Sulfur Allotropes 47 Table 20 Crystal structure parameters of “2nd fibrous sulfur” (Sy0 =Sw1) and of “laminar sul- fur” (Sc=Sw2) [54, 115]. The molecular parameters were claimed to be very similar to those of fibrous sulfur, Sy Allotrope Crystal system Space group Lattice onstants a (pm) b (pm) c (pm) 2nd fibrousa Orthorhombic Pccn–D2h10 (no. 56) 902 833 458 (~1380/3)b Laminar Tetragonal (body-centered) I4 S24 (no. 82) 458 (~1380/3)b (a=b) 1632 a Data have been derived from partly stretched samples. Strong extended fibers gave dif- fraction pattern of fibrous sulfur, Sy b Coincides with one third of the 10S3 helix axis of repetition length 1380 pm at sample temperatures of 80–110 C [116] and by Steudel et al. by DSC and other experiments [55]. On the other hand, Sw2 is obtained by heating samples, “home-made” by quenching, cautiously to 80 C [54, 115], or by quenching sulfur vapor fol- lowed by extraction, or by extraction of commercially available polymeric sulfur with CS2 at low temperatures (~195 K) [140]. In addition, Tuinstra re- ported the diffraction pattern of both forms, Sw1 and Sw2, to be complemen- tary and to coincide with that of the untreated Crystex. X-ray studies of Crystex demonstrated that this form is at least partially crystalline [116]. The diffraction lines are broadened in comparison with re- crystallized material (a-S8). The diffraction pattern of commercial Crystex is identical to that of a mixture of Sw1 and Sw2 with the latter being the predom- inant component [116, 117, 140]. These results are in accordance with Ra- man and infrared spectra of Crystex and sublimed sulfur [116, 117, 141, 142] which are identical to those of quenched melts with the exception of the lines of a-S8 [106, 143]. Michaud et al. reported the decomposition of polymeric sulfur to a-S8 by recording the time-dependent evolution of the Raman spectrum [140]. From the coincidence of the X-ray layer lines of Sw1 and Sw2 with those of Sy Tuinstra concluded that these polymers have the same molecular confor- mation. Since the diffraction patterns of Sw1 and Sw2 were much poorer than that of Sy the exact positions of the atoms of the helices remained uncertain [54, 115]. However, the gross arrangement of the helices in the units of Sw1 and Sw2 could be determined. Accordingly, the structure of Sw1 is an ortho- rhombic lattice which has nearly the same size as the unit cell of Sy on a pseudo-orthorhombic setting (Table 20). In these structures the helices are arranged parallel to each other. Since the densities of both were found to be the same the unit cell was expected to be build by four helices as in the case of Sy. A maximum of interlocking of the helices was estimated for the direc- tion of the b-axis which consists of alternating helices with opposite turns (see Fig. 18). Because of the similarity with the fibrous sulfur allotrope Sy,PDF Image | Topics in Current Chemistry
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