Allotropes of sulfur
The allotropes of sulfur refers to the many allotropes of the element sulfur. In terms of large number of allotropes, sulfur is second only to carbon. In addition to the allotropes, each allotrope often exists in polymorphs, delineated by Greek prefixes (α, β, etc.).
Furthermore, because elemental sulfur has been an item of commerce for centuries, its various forms are given traditional names. Early workers identified some forms that have later proved to be single or mixtures of allotropes. Some forms have been named for their appearance, e.g. "mother of pearl sulfur", or alternatively named for a chemist who was pre-eminent in identifying them, e.g. "Muthmann's sulfur I" or "Engel's sulfur".
The most commonly encountered form of sulfur is the orthorhombic polymorph of S
8, which adopts a puckered ring – or "crown" – structure. Two other polymorphs are known, also with nearly identical molecular structures. In addition to S8, sulfur rings of 6, 7, 9–15, 18 and 20 atoms are known. At least five allotropes are uniquely formed at high pressures, two of which are metallic.
The number of sulfur allotropes reflects the relatively strong S−S bond of 265 kJ/mol. Furthermore, unlike most elements, the allotropes of sulfur can be manipulated in solutions of organic solvents and is amenable to analysis by HPLC.
The pressure-temperature (P-T) phase diagram for sulfur is complex (see image). The region labeled I (a solid region), is α-sulfur. See the legend for further identifications and information, and see the section on high pressure forms for information on these phases.
In a high-pressure study at ambient temperatures, four new solid forms, termed II, III, IV, V have been characterized, where α-sulfur is form I. Solid forms II and III are polymeric, while IV and V are metallic (and are superconductive below 10 K and 17 K, respectively). Laser irradiation of solid samples produces three sulfur forms below 200–300 kbar (20–30 GPa).
Two methods exist for the preparation of the cyclo-sulfur allotropes. One of the methods, which is most famous for preparing hexasulfur, is to react hydrogen polysulfides with polysulfur dichloride:
A second strategy uses titanocene pentasulfide as a source of the S52− unit. This complex is easily made from polysulfide solutions:
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