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4 Ralf Steudel · Bodo Eckert diradicalic chains (S1C) which have no end groups while the endgroups in the chain-like components of Sm and Sw are most probably SH or OH. At high pressures, sulfur undergoes several phase transitions towards close-packing. In the low pressure regime (<20 GPa), the phase transitions observed by Raman and X-ray studies are complicated due to photo-induced transformations which have to be attributed to the pressure-tuned red-shift of the optical edge of sulfur. At higher pressures (around 90 GPa), metalliza- tion and superconducting states have been observed. The chemistry of elemental sulfur has been reviewed before [1–7]. In the older literature there are many claims of doubtful sulfur allotropes which have never been characterized properly and which most probably do not consist of pure sulfur or which are identical to the well known allotropes but with a different habitus of the crystals. These materials will not be discussed here [8]. 2 Allotropes at Ambient Pressure 2.1 Preparation 2.1.1 Allotropes Consisting of Cyclic Molecules In the following, convenient methods for the preparation of the homocyclic sulfur allotropes will be described. Which method to use depends on the amount of material needed, on the skills of the experimentalist and on the chemicals and equipment available. Therefore, several alternative prepara- tion procedures are provided. Metastable sulfur allotropes are light-sensitive and should be protected from direct exposure to sun-light or other intense illumination. These mate- rials are also very sensitive towards nucleophiles including alkaline glass surfaces. Therefore, pure and dry solvents should be used and the glassware should be treated with concentrated hydrochloric acid followed by rinsing with water and drying in an oven prior to use. 2.1.1.1 Preparation of S6 cyclo-Hexasulfur S6 forms orange-colored rhombohedral crystals which may be prepared by a variety of methods: 1. Historically, S6 was first prepared from the two inexpensive chemicals sodi- um thiosulfate and hydrochloric acid [9] which, according to more recent results, yield a mixture of mainly S6, S7, and S8 [10]:PDF Image | Topics in Current Chemistry
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