Allotropes of sulfur

[2] 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.

[2][3] The most commonly encountered form of sulfur is the orthorhombic polymorph of S8, which adopts a puckered ring – or "crown" – structure.

[1] Furthermore, unlike most elements, the allotropes of sulfur can be manipulated in solutions of organic solvents and are analysed by HPLC.

[11] Solid forms II and III are polymeric, while IV and V are metallic (and are superconductive below 10 K and 17 K, respectively).

[12] Laser irradiation of solid samples produces three sulfur forms below 200–300 kbar (20–30 GPa).

One of the methods, which is most famous for preparing hexasulfur, is to treat hydrogen polysulfides with polysulfur dichloride: A second strategy uses titanocene pentasulfide as a source of the S2−5 unit.

This complex is easily made from polysulfide solutions:[14] Titanocene pentasulfide reacts with polysulfur chloride:[15] This allotrope was first prepared by M. R. Engel in 1891 by treating thiosulfate with HCl.

[4] When pure it has a greenish-yellow colour (traces of cyclo-S7 in commercially available samples make it appear yellower).

It is practically insoluble in water and is a good electrical insulator with poor thermal conductivity.

[4] β-Sulfur is a yellow solid with a monoclinic crystal form and is less dense than α-sulfur.

β-Sulfur can be prepared by crystallising at 100 °C and cooling rapidly to slow down formation of α-sulfur.

Its structure can be visualised as having sulfur atoms in three parallel planes, 3 in the top, 6 in the middle and three in the bottom.

[30] ω-Sulfur is a commercially available product prepared from amorphous sulfur that has not been stretched prior to extraction of soluble forms with CS2.

[31] μ-Sulfur is the name applied to solid insoluble sulfur and the melt prior to quenching.

[29] π-Sulfur is a dark-coloured liquid formed when λ-sulfur is left to stay molten.

[20] This term is applied to biradical catena-chains in sulfur melts or the chains in the solid.

Complicating factors include the purity of the starting material and the thermal history of the sample.

[34] Disulfur, S2, is the predominant species in sulfur vapour above 720 °C (a temperature above that shown in the phase diagram); at low pressure (1 mmHg) at 530 °C, it comprises 99% of the vapor.

[citation needed] It is a triplet diradical (like dioxygen and sulfur monoxide), with an S−S bond length of 188.7 pm.

[citation needed] The blue colour of burning sulfur is due to the emission of light by the S2 molecule produced in the flame.

Cyclo -octasulfur ( cyclo - S 8 or cyclooctasulfane), the most prevalent allotrope of sulfur in nature.
A historic phase diagram of sulfur. A phase diagram from 1975, presenting data through 1970. The ordinate is pressure in kilobars (kbar). and the abscissa is temperature in kelvins (K). (The temperatures 200, 400, 600, and 800 K correspond to the approximate temperatures of −73, 127, 327, and 527 °C, respectively.) The Roman numerals I-XII refer to known solid phases identified by "volumetric, optical, and electrical resistance techniques," and letters A-E to putative distinct liquid "phases" identified by differential thermal analysis . Phase information is based on the work of G. C. Vezzoli, et al., as reviewed by David Young; as Young notes, "The literature on the allotropy of sulfur presents the most complex and confused situation of all the elements." [ 8 ] [ 9 ] Phase information are limited to ≤50 kbar and thus omitting metallic phases. [ 10 ]
Cyclo -hexasulfur, cyclo - S 6
Structure of cyclo - S 7 .
Cyclo -dodecasulfur, cyclo - S 12
Two parallel monatomic sulfur chains grown inside a single-wall carbon nanotube (CNT, a) Zig-zag (b) and straight (c) S chains inside double-wall CNTs. [ 33 ]