Allotropes of sulfur

Phase diagram for sulfur

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.

High pressure solid allotropes

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).

Solid cyclo allotrope preparation

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:

H₂S_x + S_yCl₂ → cyclo-S_x+y + 2 HCl

A second strategy uses titanocene pentasulfide as a source of the S₅²⁻ unit. This complex is easily made from polysulfide solutions:

[NH₄]₂[S₅] + (C₅H₅)₂TiCl₂ → (C₅H₅)₂TiS₅ + 2 NH₄Cl

Then the resulting pentasulfur-titanocene complex is allowed to react with polysulfur dichloride to give the desired cyclo-sulfur in the series:

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

Cyclo-Pentasulfur, cyclo-S5

This has not been isolated, but has been detected in the vapour phase.

Cyclo-hexasulfur, cyclo-S6

This was first prepared by M. R. Engel in 1891 by reacting HCl with thiosulfate, HS₂O₃⁻. Cyclo-S₆ is orange-red and forms a rhombohedral crystal. It is called ρ-sulfur, ε-sulfur, Engel's sulfur and Aten's sulfur. Another method of preparation involves reacting a polysulfane with sulfur monochloride:

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.

Cyclo-heptasulfur, cyclo-S7

It is a bright yellow solid. Four (α-, β-, γ-, δ-) forms of cyclo-heptasulfur are known. 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.

α-Sulfur

α-Sulfur, see image, is the form most commonly found in nature. 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. This is the predominant form found in "flowers of sulfur", "roll sulfur" and "milk of sulfur". 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°. At 95.3 °C, α-sulfur converts to β-sulfur.

β-Sulfur

This is a yellow solid with a monoclinic crystal form and is less dense than α-sulfur. Like the α- form it contains puckered S₈ rings and only differs from it in the way the rings are packed in the crystal. It is unusual because it is only stable above 95.3 °C, below this it converts to α-sulfur.It can be prepared by crystallising at 100 °C and cooling rapidly to slow down formation of α-sulfur. It has a melting point variously quoted as 119.6 °C and 119.8 °C but as it decomposes to other forms at around this temperature the observed melting point can vary from this. 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".

γ-Sulfur

This form, first prepared by F.W. Muthmann in 1890, is sometimes called "nacreous sulfur" or "mother of pearl sulfur" because of its appearance. It crystallises in pale yellow monoclinic needles. It contains puckered S₈ rings like α-sulfur and β-sulfur and only differs from them in the way that these rings are packed. 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. It is found in nature as the mineral rosickyite.

Cyclo-Sn (n = 9–15, 18, 20)

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

(η⁵-C₅H₅)₂TiS₅ + S_n−5Cl₂ → cyclo-S_n

or alternatively reacting a dichlorosulfane, S_n−mCl₂ and a polysulfane, H₂S_m:

S_n−mCl₂ + H₂S_m → cyclo-S_n

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

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.

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

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-.

Cyclo-S6.cyclo-S10 adduct

This 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. For all the elements this may be the only allotrope which contains molecules of different sizes.

Solid catena allotropes

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.

ψ-Sulfur

This form, also called fibrous sulfur or ω1-sulfur, 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°).

Lamina sulfur

This has not been well characterized but is believed to consist of criss-crossed helices. It is also called χ-sulfur or ω2-sulfur.

Catena sulfur forms

The naming of the different forms is very confusing and care has to be taken to determine what is being described as the same names are used interchangeably.

Amorphous sulfur

This is the quenched product of sulfur melts above 160 °C (at this point the properties of the liquid melt change remarkably, e.g. large increase in viscosity). Its form changes from an initial plastic form gradually to a glassy form, hence its other names of plastic, glassy or vitreous sulfur. It is also called χ-sulfur. It contains a complex mixture of catena-sulfur forms mixed with cyclo-forms.

Insoluble sulfur

This is obtained by washing quenched liquid sulfur with CS₂. It is sometimes called polymeric sulfur, μ-S or ω-S.

Fibrous (φ-) sulfur

This is a mixture of the allotropic ψ- form and γ-cycloS₈.

ω-Sulfur

This 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 samples history. One well known commercial form is "Crystex". ω-sulfur is used in the vulcanization of rubber.

λ-Sulfur

This name is given to the molten sulfur immediately after melting, cooling this gives predominantly β-sulfur.

μ-Sulfur

This name is applied to solid insoluble sulfur and the melt prior to quenching.

π-Sulfur

Dark-coloured liquid formed when λ-sulfur is left to stay molten. Contains mixture of S_n rings.

Biradical catena (S∞) chains

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

List of allotropes and forms

Allotropes are in Bold.

Disulfur, S2

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. It is a triplet diradical (like dioxygen and sulfur monoxide), with an S-S bond length of 188.7 pm. The blue colour of burning sulfur is due to the emission of light by the S₂ molecule produced in the flame.

The S₂ molecule has been trapped in the compound [S₂I₄][EF₆]₂ (E = As, Sb) for crystallographic measurements, produced by reacting elemental sulfur with excess iodine in liquid sulfur dioxide. 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.

Trisulfur, S3

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.

Tetrasulfur, S4

This has been detected in the vapour phase but has not been fully characterized; various forms, (e.g. chains, branched chains and rings) have been proposed. A primary research result based on theoretical calculations suggests that it has a ring structure.