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54 Ralf Steudel · Bodo Eckert low the melting point of the a-phase at 115 C, does not result in any decom- position of the eight-membered rings, but as soon as a melt has formed equilibration with other ring sizes takes place [49]. High purity liquid sulfur shows a strong tendency for supercooling: it can be kept liquid in bulk at about 50 C for 20 min or so. At temperatures closer to the melting point this period of supercooling is even extended [24]. g-S8 is metastable at all temperatures but in a clean environment it survives at 20 C for longer peri- ods of time. However, in the laboratory atmosphere it was reported to con- vert fairly rapidly to a-S8 even at 20 C [31]; the rate of transformation into a-S8 reaches a maximum at 80 C. These phase transitions are catalyzed by traces of carbon disulfide but hindered by the presence of polymeric sulfur [158]. S9: if solid a-S9 is heated with a rate of 2–20 K min1 to the melting point of 63 C and the melt is rapidly cooled to room temperature, it consists mainly of S9 with traces of S7 and S8. At 25 C S9 is about as stable as S6 [33]. S10: if S10 is heated at a rate of 5 K min1 in a DSC apparatus one observes the melting at 92 C and an exothermic polymerization reaction in the tem- perature range 95–119 C followed by depolymerization to mainly S8 and some S7 together with traces of other sulfur rings. If rapidly heated to 120 C, S10 decomposes within 1 min [55]. S6·S10: This allotrope can be stored at 20 C for several days, at 30 C for several months without decomposition [35]. S11: at 20 C S11 can be handled for several days without decomposition. On heating it polymerizes to S1 [38]. S12: on heating at a rate of 5 K min1 S12 decomposes within the tempera- ture range of 140–155 C with simultaneous formation of S1, S8, and some S7. At room temperature S12 can be stored for indefinite periods of time without decomposition [55]. S13: as in the case of other sulfur allotropes S13 polymerizes on heating and a well separated melting process cannot be observed. Instead, the two processes take place more or less simultaneously. At 20 C crystalline S13 does not decompose for several days [38, 55]. S14: at 25 C crystalline S14 is stable for several days. On heating to the melting point of 117 C it decomposes to S7 and S8 while S6 is not formed. Probably the dissociation into two molecules of S7 is the first step followed by the interconversion of S7 to S8 [46]. S15: at 23 C solid S15 can be kept for several hours without decomposi- tion. The thermal behavior at higher temperatures has not been reported [47]. endo-S18 (a-S18): this allotrope can be kept at room temperature for ex- tended periods of time without decomposition but no DSC measurements have so far been reported, and no information is available about the thermal behavior of exo-S18 (b-S18). S20: like crystalline S12 and S18 this allotrope can be kept at 25 C for indef- inite periods of time without decomposition. On heating S20 melts near 121 C and simultaneously polymerizes followed by depolymerization of thePDF Image | Topics in Current Chemistry
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