Allotropes of sulfur

The element sulfur exists as many allotropes. In number of allotropes, sulfur is second only to carbon.^[1] In addition to the allotropes, each allotrope often exists in polymorphs (different crystal structures of the same covalently bonded S_n molecules) delineated by Greek prefixes (α, β, etc.).^[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. 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".^[2][3]

The most commonly encountered form of sulfur is the orthorhombic polymorph of S₈, which adopts a puckered ring – or "crown" – structure. Two other polymorphs are known, also with nearly identical molecular structures.^[4] In addition to S₈, sulfur rings of 6, 7, 9–15, 18, and 20 atoms are known.^[5] At least five allotropes are uniquely formed at high pressures, two of which are metallic.^[6]

The number of sulfur allotropes reflects the relatively strong S−S bond of 265 kJ/mol.^[1] Furthermore, unlike most elements, the allotropes of sulfur can be manipulated in solutions of organic solvents and are analysed by HPLC.^[7]

The pressure-temperature (P-T) phase diagram for sulfur is complex (see image). The region labeled I (a solid region), is α-sulfur.^[11]

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.^[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).^[13]

Two methods exist for the preparation of the cyclo-sulfur allotropes. One of the methods, which is most famous for preparing hexasulfur, is to treat hydrogen polysulfides with polysulfur dichloride:

H₂S_x + S_yCl₂ → cyclo-S_x+y + 2 HCl

A second strategy uses titanocene pentasulfide as a source of the S2−5 unit. This complex is easily made from polysulfide solutions:^[14]

[NH₄]₂S₅ + (η⁵-C₅H₅)₂TiCl₂ → (η⁵-C₅H₅)₂TiS₅ + 2 [NH₄]Cl

Titanocene pentasulfide reacts with polysulfur chloride:^[15]

(η⁵-C₅H₅)₂TiS₅ + S_yCl₂ → cyclo-S_y+5 + (η⁵-C₅H₅)₂TiCl₂

This allotrope was first prepared by M. R. Engel in 1891 by treating thiosulfate with HCl.^[5] Cyclo-S₆ is orange-red and forms a rhombohedral crystal.^[16] It is called ρ-sulfur, ε-sulfur, Engel's sulfur and Aten's sulfur.^[2] Another method of preparation involves the reaction of a polysulfane with sulfur monochloride:^[16]

H₂S₄ + S₂Cl₂ → cyclo-S₆ + 2 HCl (dilute solution in diethyl ether)

The sulfur ring in cyclo-S₆ has a "chair" conformation, reminiscent of the chair form of cyclohexane. All of the sulfur atoms are equivalent.^[16]

It is a bright yellow solid. Four (α-, β-, γ-, δ-) forms of cyclo-heptasulfur are known.^[17] Two forms (γ-, δ-) have been characterized. The cyclo-S₇ ring has an unusual range of bond lengths of 199.3–218.1 pm. It is said to be the least stable of all of the sulfur allotropes.^[18]

Octasulfur contains puckered S₈ rings, and is known in three forms that differ only in the way the rings are packed in the crystal.

α-Sulfur is the form most commonly found in nature.^[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. It is quite soluble in carbon disulfide: 35.5 g/100 g solvent at 25 °C. It has an orthorhombic crystal structure.^[4] α-Sulfur is the predominant form found in "flowers of sulfur", "roll sulfur" and "milk of sulfur".^[19] It contains S₈ puckered rings, alternatively called a crown shape. The S–S bond lengths are all 203.7 pm and the S-S-S angles are 107.8° with a dihedral angle of 98°.^[16] At 95.3 °C, α-sulfur converts to β-sulfur.^[4]

β-Sulfur is a yellow solid with a monoclinic crystal form and is less dense than α-sulfur. It is unusual because it is only stable above 95.3 °C; below this temperature it converts to α-sulfur. β-Sulfur can be prepared by crystallising at 100 °C and cooling rapidly to slow down formation of α-sulfur.^[5] It has a melting point variously quoted as 119.6 °C^[20] and 119.8 °C but as it decomposes to other forms at around this temperature the observed melting point can vary. The 119 °C melting point has been termed the "ideal melting point" and the typical lower value (114.5 °C) when decomposition occurs, the "natural melting point".^[20]

γ-Sulfur was first prepared by F.W. Muthmann in 1890. It is sometimes called "nacreous sulfur" or "mother of pearl sulfur" because of its appearance. It crystallises in pale yellow monoclinic needles. It is the densest form of the three. It can be prepared by slowly cooling molten sulfur that has been heated above 150 °C or by chilling solutions of sulfur in carbon disulfide, ethyl alcohol or hydrocarbons.^[5] It is found in nature as the mineral rosickyite.^[21] It has been tested in carbon fiber-stabilized form as a cathode in lithium-sulfur (Li-S) batteries and was observed to stop the formation of polysulfides that compromise battery life.^[22]

These allotropes have been synthesised by various methods for example, treating titanocene pentasulfide and a dichlorosulfane of suitable sulfur chain length, S_n−5Cl₂:^[17]

(η⁵-C₅H₅)₂TiS₅ + S_n−5Cl₂ → cyclo-S_n + (η⁵-C₅H₅)₂TiCl₂

or alternatively treating a dichlorosulfane, S_n−mCl₂ and a polysulfane, H₂S_m:^[17]

S_n−mCl₂ + H₂S_m → cyclo-S_n + 2 HCl

S₁₂, S₁₈, and S₂₀ can also be prepared from S₈.^[20] With the exception of cyclo-S₁₂, the rings contain S–S bond lengths and S-S-S bond angle that differ one from another.^[16]

Cyclo-S₁₂ is the most stable cyclo-allotrope. 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.^[23]

Two forms (α-, β-) of cyclo-S₉ are known, one of which has been characterized.^[24]

Two forms of cyclo-S₁₈ are known where the conformation of the ring is different. To differentiate these structures, rather than using the normal crystallographic convention of α-, β-, etc., which in other cyclo-S_n compounds refer to different packings of essentially the same conformer, these two conformers have been termed endo- and exo-.^[25]

This adduct is produced from a solution of cyclo-S₆ and cyclo-S₁₀ in CS₂. It has a density midway between cyclo-S₆ and cyclo-S₁₀. The crystal consists of alternate layers of cyclo-S₆ and cyclo-S₁₀. This material is a rare example of an allotrope that contains molecules of different sizes.^[26]

The term "Catena sulfur forms" refers to mixtures of sulfur allotropes that are high in catena (polymer chain) sulfur. The naming of the different forms is very confusing and care has to be taken to determine what is being described because some names are used interchangeably.^[2]

Amorphous sulfur is the quenched product from molten sulfur hotter than the λ-transition at 160 °C, where polymerization yields catena sulfur molecules.^[2] (Above this temperature, the properties of the liquid melt change remarkably. For example, the viscosity increases more than 10000-fold as the temperature increases through the transition^[2][27]). As it anneals, solid amorphous sulfur changes from its initial glassy form, to a plastic form, hence its other names of plastic, and glassy or vitreous sulfur. The plastic form is also called χ-sulfur.^[2] Amorphous sulfur contains a complex mixture of catena-sulfur forms mixed with cyclo-forms.^[28]

Insoluble sulfur is obtained by washing quenched liquid sulfur with CS₂.^[29] It is sometimes called polymeric sulfur, μ-S or ω-S.^[2]

Fibrous (φ-) sulfur is a mixture of the allotropic ψ- form and γ-cyclo-S₈.^[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. It sometimes called "white sulfur of Das" or supersublimated sulfur. It is a mixture of ψ-sulfur and lamina sulfur. The composition depends on the exact method of production and the sample's history. One well known commercial form is "Crystex". ω-sulfur is used in the vulcanization of rubber.^[19]

λ-Sulfur is molten sulfur just above the melting temperature. It is a mixture containing mostly cyclo-S₈.^[2] Cooling λ-sulfur slowly gives predominantly β-sulfur.^[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. It contains mixture of S_n rings.^[20]

This term is applied to biradical catena-chains in sulfur melts or the chains in the solid.^[32]

The production of pure forms of catena-sulfur has proved to be extremely difficult. Complicating factors include the purity of the starting material and the thermal history of the sample.

This form, also called fibrous sulfur or ω1-sulfur,^[2] has been well characterized. It has a density of 2.01 g·cm⁻³ (α-sulfur 2.069 g·cm⁻³) and decomposes around its melting point of 104 °C. It consists of parallel helical sulfur chains. These chains have both left and right-handed "twists" and a radius of 95 pm. The S–S bond length is 206.6 pm, the S-S-S bond angle is 106° and the dihedral angle is 85.3°, (comparable figures for α-sulfur are 203.7 pm, 107.8° and 98.3°).^[27]

Lamina sulfur has not been well characterized but is believed to consist of criss-crossed helices. It is also called χ-sulfur or ω2-sulfur.^[2]

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Monatomic sulfur can be produced from photolysis of carbonyl sulfide.^[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.^[35]

The S₂ molecule has been trapped in the compound [S₂I₄]²⁺([EF₆]⁻)₂ (E = As, Sb) for crystallographic measurements, produced by treating elemental sulfur with excess iodine in liquid sulfur dioxide.^{[citation needed]} The [S₂I₄]²⁺ cation has an "open-book" structure, in which each [I₂]⁺ ion donates the unpaired electron in the π^* molecular orbital to a vacant orbital of the S₂ molecule.^{[citation needed]}

S₃ is found in sulfur vapour, comprising 10% of vapour species at 440 °C and 10 mmHg. It is cherry red in colour, with a bent structure, similar to ozone, O₃.^[35]

S₄ has been detected in the vapour phase, but it has not been well characterized. Diverse structures (e.g. chains, branched chains and rings) have been proposed.^{[citation needed]}

Theoretical calculations suggest a cyclic structure.^[36]

Pentasulfur has been detected in sulfur vapours but has not been isolated in pure form.^[37]

Allotropes are in Bold.

• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419.
• Steudel, R., ed. (2004). Elemental sulfur and sulfur-rich compounds I (Topics in current chemistry). Springer. ISBN 3-540-40191-1.

• Media related to Allotropes of sulfur at Wikimedia Commons