Allotropes of sulfur
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The electronic structure of this atom results in a large number of allotropes of sulfur, in number, second only to those observed for carbon. Sulfur's [Ne] 3s2 3p4 electronic structure gives rise to a wide range of available bond lengths and angles, and results in a high S−S bond energy (and so, strong S−S bonds), all of which manifest in sulfur's many allotropic forms. A common naming system for the allotropes uses Greek suffixes (α, β, etc.), while some remain known by traditional names based on appearance (e.g. "mother of pearl sulfur") or an individual pre-eminent in their discovery (e.g. "Engel's sulfur"). The most common sulfur allotrope in nature is the yellow orthorhombic α-sulfur allotrope, which exhibits a puckered S
8 ring. In addition, sulfur rings of 6, 7, 9–15, 18 and 20 atoms are known solid forms, and gaseous species from 2 to 5 atoms in length have been studied or detected. Finally, there are perhaps five or more high-pressure forms, two of which are metallic. The complexity of forms has led to an interesting history of study and discovery, sometimes determining that materials under study were not allotropes but mixtures, in other cases adding in number to the known allotropes.
- 1 Introduction
- 2 Gaseous allotropes
- 3 Solid cyclo allotrope preparation
- 4 Solid cyclo-sulfur allotropes
- 5 Solid catena allotropes
- 6 Catena sulfur forms
- 7 High-pressure forms
- 8 List of allotropes and forms
- 9 References
There are a large number of allotropes of sulfur. In this respect, sulfur is second only to carbon. The most common form found in nature is yellow orthorhombic α-sulfur, which contains puckered rings of S
8. Chemistry students may have seen "plastic sulfur"; this is not an allotrope but a mixture of long chain polymeric sulfur forms, two 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; some species only detected in the vapour phase, S4 and S5 and perhaps five or more high-pressure forms, two of which are metallic.
The range of molecular allotropes possessed by sulfur can in part be ascribed to the wide range of bond lengths (180–260 pm) and bond angles (90–120°) exhibited by the S−S bond and its strength (the unrestrained S−S single bond has a high bond energy of 265 kJ/mol).
Early workers identified some forms that have later proved to be allotropes, i.e. pure forms, whilst others have proved to be mixtures. 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". A commonly used naming system uses Greek suffixes (α, β, etc.); however, this system predates the discovery of the new forms that have been synthesised rather than prepared from elemental sulfur.
Disulfur, S2, is the predominant species in sulfur vapour above 720 °C. At low pressure (1 mmHg) at 530 °C, it comprises 99% of the vapour. 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. This is 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]2+ ion donates the unpaired electron in the π* molecular orbital to an vacant orbital of the S2 molecule.
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. The latest view, based on theoretical calculations is that it has a ring structure.
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:
- H2Sx + SyCl2 → cyclo-Sx+y + 2 HCl
- [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 + SyCl2 → cyclo-Sy+5 + (C5H5)2TiCl2
Solid cyclo-sulfur allotropes
This has not been isolated, but has been detected in the vapour phase.
This was first prepared by R.C. 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)
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, 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 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 206 pm 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 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".
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)
- (η5-C5H5)2TiS5 + Sn−5Cl2 → cyclo-Sn
- Sn−mCl2 + H2Sm → cyclo-Sn
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.
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-Sn compounds refer to different packings of essentially the same conformer, these two conformers have been termed endo- and exo-.
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.
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°).
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 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.
Fibrous (φ-) 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.
Biradical catena (S∞) chains
This term is applied to biradical catena- chains in sulfur melts or the chains in the solid.
The pressure-temperature (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 characterized (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.
List of allotropes and forms
Allotropes are in Bold.
|Formula/name||Common name||Other names||Notes|
|S2||disulfur||A diatomic gas with a triplet ground state like dioxygen.|
|S3||trisulfur||A cherry red triatomic gas with a bent ozone-like structure.|
|S4||tetrasulfur||Structure not determined but calculations indicate it to be cyclo-S4.|
|cyclo-S5||cyclo-pentasulfur||Not yet isolated, only detected in sulfur vapour.|
|cyclo-S6||ρ-sulfur||cyclo-hexasulfur, "ε-sulfur", "Engel's" sulfur, "Aten's sulfur"||The ring adopts a chair form in the solid.|
|cyclo-S6/cyclo-S10 adduct||A mixed crystal with alternating layers of cyclo-S6 and cyclo-S10.|
|cyclo-S7||α-, β-, γ-, δ- cycloheptasulfur||Four forms known, two(γ-, δ- ) characterized.|
|cyclo-S8||α-sulfur||"orthorhombic sulfur" "rhombic sulfur", "flowers of sulfur", "roll sulfur" "milk of sulfur", "Muthmann's sulfur I"||Yellow solid consisting of S8 puckered rings. The thermodynamically stable form at ordinary temperatures.|
|cyclo-S8||β-sulfur||"monoclinic sulfur" "prismatic sulfur" "Muthmann's sulfur II"||Yellow crystalline solid, consisting of S8 puckered rings. Only stable above 95.6 °C, it reverts to α-sulfur at room temperature.|
|cyclo-S8||γ-sulfur||"nacreous sulfur" "mother of pearl sulfur" "Gernez’s sulfur" or "Muthmann's sulfur III".||Light yellow solid, crystal monoclinic, consisting of S8 puckered rings. Found in nature as the rare mineral rosickyite.|
n = 9-15, 18, 20
|cyclo-(nona; deca; undeca; dodeca; trideca; tetradeca; pentadeca; octadeca; eicosa)sulfur||Pure forms all allotropes, cyclo-S9 has four forms, cyclo-S18 has two forms. Generally synthesised rather than obtained by treatment of another form of elemental sulfur.|
|catena-Sx||fibrous (ψ) sulfur||Well characterized, contains parallel helical sulfur chains and is difficult to obtain pure.|
|catena-Sx||lamina sulfur||Not well characterized, contains helical chains partially crossed.|
|amorphous sulfur||Quenched molten sulfur plastic sulfur at first crystallises to amorphous or glassy sulfur. Consists of a mixture of catena sulfur and cyclo sulfur.|
|insoluble sulfur||Quenched liquid sulfur with soluble species extracted with CS2. Sometimes called polymeric sulfur, μ-S or ω-S.|
|φ-sulfur||A mixture of allotropic ψ-sulfur and cyclo forms mainly γ-cyclo-S8.|
|ω-sulfur||insoluble sulfur||A mixture of chains with a minimum of soluble species.|
|λ-sulfur||Light yellow mobile liquid formed when β-sulfur first melts at 119.6 °C. Consists of S8 rings.|
|μ-sulfur||The dark-coloured viscous liquid formed when π-sulfur is heated and the solid when cooled. Contains a mixture of polymeric chains.|
|π-sulfur||Dark-coloured liquid that develops as λ-sulfur is left molten. Contains mixture of Sn rings.|
|High-pressure forms of α-sulfur||S-II, S-III, S-IV, S-V and others||Four high-pressure phases (at ambient temperature) including two that are metallic and become superconducting at low temperature and some additional phases photo-induced below 20–30 GPa.|
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