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Solid Sulfur Allotropes 27 Table 7 Crystal structure data of b-S8 at two temperaturesa (standard deviations are in brackets) Space group P21/cC2h5 (no. 14) P21C22 (no. 4) Six molecules per unit cell (four in ordered general positions, two in disordered pseudocen- tric sites, T>198 K), site symmetry: 1C1 Lattice parameters a (pm) b (pm) c (pm) b () Temperature (K) Reference 1092.6(2) 1085.2(2) 1079.0(3) 95.92(2) 297 [81] 1079.9(2) 1068.4(2) 1066.3(2) 95.71(1) 113 [27] a From the temperature dependence of lattice parameters a volume expansion of about 2104 K1 can be estimated Fig. 7 Unit cell of monoclinic b-S8 projected along the crystal b-axis. The twofold disor- der of one-third of the molecules is indicated [8] 2.2.1.3.3 Monoclinic g-S8 This allotrope, first described by several authors in the late nineteenth cen- tury, occurs as light-yellow, nacreous-glittering, needle-like prismatic crys- tals (for details see [8, 78]). There was some confusion in the literature about the crystallographic structure until Watanabe in 1974 reported a complete determination of the structure of g-S8 [8, 29]. The most recent and accurate structure data of chemically prepared g-S8 are those of Gallacher and Pinker- ton in 1993 [30a] and of natural g-S8 (rosickyite) are those of Meisser et al. [30b]. The “sheared penny roll” arrangement of the S8 molecules in this allo- trope (see Fig. 8), which has been presented earlier by De Haan [92], wasPDF Image | Topics in Current Chemistry
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