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Allotropes of sulfur
There are a number of allotropes of sulfur. Sulfur is second only to carbon in this respect.
The most common form found in nature is yellow orthorhombic α-sulfur, which contains puckered rings of S8. Chemistry students may have seen "plastic sulfur", this is not an allotrope but is a mixture that contains long chain polymeric sulfur, two forms of which have been identified as allotropes. In addition to these there are other solid forms that contain sulfur rings of 6, 7, 9-15, 18 and 20 atoms. There are also gases, S2, S3, S4 and S5 and perhaps five or more high pressure forms, two of which are metallic.
Additional recommended knowledge
List of allotropes and forms
Allotropes are bold.
S2 is the predominant species in sulfur vapour above 720°C. at low pressure (1mm Hg) at 530°C it comprises 99% of vapour species. It is a singlet diradical like dioxygen with an S-S bond length of 188.7 pm. The blue colour of burning sulfur is due to emission of light by the S2 molecule produced in the flame.
S3 is found in sulfur vapour, comprising 10% of vapour species at 440°C and 10mm Hg. It is cherry red in colour. Its structure is bent, similar to ozone, O3.
This has been detected in the vapour phase but has not been fully characterised. Various forms, (e.g chains, branched chains and rings) have been proposed. The latest view, based on theoretical calculations is that it has a ring structure.
Solid cyclo-sulfur allotropes
This has not been isolated, but has been detected in molten sulfur.
This was first prepared by M.R. Engel in 1891 by reacting concentrated 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
The sulfur ring in cyclo-S6 has a "chair" conformation, reminiscent of the chair form of cyclohexane. All of the sulfur atoms are essentially equivalent, in other rings S7, S9 and above, the sulfur bond lengths and angles vary within the ring.
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.
It is a bright yellow solid. Four (α-, β-, γ-, δ-) forms of cyclo-heptasulfur are known. Two forms (γ-, δ-)have been characterised. The cyclo-S7 ring has an unusual range of bond lengths of 199.3pm - 218.1pm.
α-sulfur 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 35.5g/100g solvent at 25°C. It has a rhombohedral 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 206pm and the S-S-S angles are 108° with a dihedral angle of 98°. At 95.3°C α-sulfur converts to β-sulfur.
This is a yellow solid with a monoclinic crystal form. 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 less dense than the α-form. 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 ca. 119°C melting point has been termed the "ideal melting point" and the typical lower value (114.5°) when decomposition occurs, the "natural melting point".
This form was first prepared by F.W Muthmann in 1890, 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, Sn−5Cl2
or alternatively reacting a dichlorosulfane, SxCl2 and a polysulfane, H2Sy
S12, S18 and S20 can alternatively be prepared from S8
Cyclo-S12 is the second most stable cyclo- allotrope after cyclo-S8. 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.
Solid catena sulfur 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.
This form, also called (confusingly) fibrous sulfur or ω1-sulfur, has been well characterised. It has a density of 2.01g cm-3 (α-sulfur 2.069g 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 95pm. The S-S bond length is 206.6pm, the S-S-S bond angle is 106° and the dihedral angle is 85.3°. (Comparable figures for α-sulfur are 203.7pm, 107.8° and 98.3°)
This has not been well characterised 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.
This is the quenched product of molten sulfur (>160°C). 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.
This is obtained by washing quenched liquid sulfur with CS2. It is sometimes called polymeric sulfur, μ-S or ω-S.
φ-sulfur, fibrous sulfur
This is a mixture of the allotropic ψ- form and γ-cycloS8.
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.
This name is given to the molten sulfur immediately after melting. Cooling this gives predominantly β-sulfur.
This name is applied to solid insoluble sulfur and the melt prior to quenching.
Dark coloured liquid formed when λ-sulfur is left to stay molten. Contains mixture of Sn rings .
This term is applied to biradical catena- chains in sulfur melts or the chains in the solid.
High pressure forms
The P-T phase diagram of sulfur is complex. Some researchers have used laser illumination of samples and found that perhaps 3 forms can be photo-induced below 20-30 GPa. In a high pressure study at ambient temperatures, four forms, termed S-II, S-III, S-IV, S-V have been characterised (α-sulfur being S-I) . S-II and S-III are polymeric forms, S-IV and S-V are metallic and are superconductors below 10°K and 17°K respectively.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Allotropes_of_sulfur". A list of authors is available in Wikipedia.|