Allotropes of sulfur
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The allotrope of the element, sulfur, most prevalent in nature, cyclo-octasulfur (cyclo-S8).
The element sulfur exists as many allotropes. In terms of large number of allotropes, sulfur is
second only to carbon.[1] In addition to the allotropes, each allotrope often exists in polymorphs,
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 S
8, 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 is amenable to analysis by HPLC.[7]
Contents
[hide]
1Phase diagram for sulfur
2High pressure solid allotropes
3Solid cyclo allotrope preparation
4Solid cyclo-sulfur allotropes
o 4.1Cyclo-Pentasulfur, cyclo-S5
o 4.2Cyclo-hexasulfur, cyclo-S6
o 4.3Cyclo-heptasulfur, cyclo-S7
o 4.4Cyclo-octasulfur, cyclo-S8
4.4.1α-Sulfur
4.4.2β-Sulfur
4.4.3γ-Sulfur
o 4.5Cyclo-Sn (n = 9–15, 18, 20)
o 4.6Cyclo-S6.cyclo-S10 adduct
5Solid catena allotropes
o 5.1ψ-Sulfur
o 5.2Lamina sulfur
6Catena sulfur forms
o 6.1Amorphous sulfur
o 6.2Insoluble sulfur
o 6.3Fibrous (φ-) sulfur
o 6.4ω-Sulfur
o 6.5λ-Sulfur
o 6.6μ-Sulfur
o 6.7π-Sulfur
o 6.8Biradical catena (S∞) chains
7List of allotropes and forms
8High-temperature gaseous allotropes
o 8.1Disulfur, S2
o 8.2Trisulfur, S3
o 8.3Tetrasulfur, S4
9References
10Bibliography
Phase diagram for sulfur[edit]
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, while complete to this date, are incomplete as
of 2015, since limited to ≤50 kbar and thus omitting metallic phases. [10]
The pressure-temperature (P-T) phase diagram for sulfur is complex (see image). The region
labeled I (a solid region), is α-sulfur.[11] 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[edit]
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]
Solid cyclo allotrope preparation[edit]
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
A second strategy uses titanocene pentasulfide as a source of the S52− unit. This complex is
easily made from polysulfide solutions:[14]
[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:[15]
(C5H5)2TiS5 + SyCl2 → cyclo-Sy+5 + (C5H5)2TiCl2
Solid cyclo-sulfur allotropes[edit]
Cyclo-Pentasulfur, cyclo-S5[edit]
This has not been isolated, but has been detected in the vapour phase. [16]
Cyclo-hexasulfur, cyclo-S6[edit]
Main article: Hexasulfur
Cyclo-hexasulfur, cyclo-S6
This was first prepared by M. R. Engel in 1891 by reacting HCl with thiosulfate,
HS2O3−.[5] Cyclo-S6 is orange-red and forms a rhombohedral crystal.[17] It is called ρ-
sulfur, ε-sulfur, Engel's sulfur and Aten's sulfur. [2] Another method of preparation
involves reacting a polysulfane with sulfur monochloride:[17]
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. [17]
Cyclo-heptasulfur, cyclo-S7[edit]
It is a bright yellow solid. Four (α-, β-, γ-, δ-) forms of cyclo-heptasulfur are
known.[18] 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.[19]
Cyclo-octasulfur, cyclo-S8[edit]
Main article: Octasulfur
α-Sulfur[edit]
α-Sulfur, see image, 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] This is the predominant form found in "flowers of sulfur", "roll sulfur"
and "milk of sulfur".[20] 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°.[17] At 95.3 °C, α-sulfur converts to β-sulfur.[4]
β-Sulfur[edit]
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. [5] It
has a melting point variously quoted as 119.6 °C[21] 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". [21]
γ-Sulfur[edit]
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 S 8 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.[5] It is found in nature as the
mineral rosickyite.[22]
Cyclo-Sn (n = 9–15, 18, 20)[edit]
Cyclo-dodecasulfur, cyclo-S12
These allotropes have been synthesised by various methods for example,
reacting titanocene pentasulfide and a dichlorosulfane of suitable sulfur chain
length, Sn−5Cl2:[18]
(η5-C5H5)2TiS5 + Sn−5Cl2 → cyclo-Sn
or alternatively reacting a dichlorosulfane, Sn−mCl2 and a polysulfane, H2Sm:[18]
Sn−mCl2 + H2Sm → cyclo-Sn
S12, S18 and S20 can also be prepared from S8.[21] With the exception of
cyclo-S12, the rings contain S-S bond lengths and S-S-S bond angle that
differ one from another.[17]
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-
Sn compounds refer to different packings of essentially the
same conformer, these two conformers have been termed endo- and
exo-.[25]
Cyclo-S6.cyclo-S10 adduct[edit]
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.[26]
Solid catena allotropes[edit]
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.[27]
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[edit]
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°).[28]
Lamina sulfur[edit]
This has not been well characterized but is believed to consist of criss-
crossed helices. It is also called χ-sulfur or ω2-sulfur. [2]
Catena sulfur forms[edit]
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.[2]
Amorphous sulfur[edit]
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[28]). 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.[2] It contains a complex mixture of catena-sulfur
forms mixed with cyclo-forms.[29]
Insoluble sulfur[edit]
This is obtained by washing quenched liquid sulfur with CS2.[30] It is
sometimes called polymeric sulfur, μ-S or ω-S.[2]
Fibrous (φ-) sulfur[edit]
This is a mixture of the allotropic ψ- form and γ-cycloS 8.[31]
ω-Sulfur[edit]
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.[20]
λ-Sulfur[edit]
This name is given to the molten sulfur immediately after melting,
cooling this gives predominantly β-sulfur. [32]
μ-Sulfur[edit]
This name is applied to solid insoluble sulfur and the melt prior to
quenching.[30]
π-Sulfur[edit]
Dark-coloured liquid formed when λ-sulfur is left to stay molten.
Contains mixture of Sn rings.[21]
Biradical catena (S∞) chains[edit]
This term is applied to biradical catena- chains in sulfur melts or the
chains in the solid.[33]
List of allotropes and forms[edit]
Allotropes are in Bold.
Formula/ Common
Other names[2] Notes
name name
A diatomic gas with a
S2 disulfur triplet ground state like
dioxygen.[34]
A cherry red triatomic
S3 trisulfur gas with a bent ozone-
like structure.[28]
Structure not
determined but
S4 tetrasulfur
calculations indicate it
to be cyclo-S4.[35]
Not yet isolated, only
cyclo-
cyclo-S5 detected in sulfur
pentasulfur
vapour.[16]
cyclo-
hexasulfur, "ε-
The ring adopts a chair
cyclo-S6 ρ-sulfur sulfur",
form in the solid.[5]
"Engel's" sulfur,
"Aten's sulfur"
cyclo-S6/ A mixed crystal with
cyclo- alternating layers of
S10adduct cyclo-S6 and cyclo-S10.
[26]
α-, β-, γ-, δ- Four forms known,
cyclo-S7 cycloheptasulf two(γ-, δ- )
ur characterized.[19]
cyclo-S8 α-sulfur "orthorhombic Yellow solid consisting
sulfur" "rhombic of S8 puckered rings.
sulfur", "flowers The
of sulfur", "roll thermodynamically
sulfur" "milk of stable form at ordinary
sulfur", temperatures.[4]
"Muthmann's sul
fur I"
"monoclinic Yellow crystalline
sulfur" solid, consisting of
"prismatic S8 puckered rings. Only
cyclo-S8 β-sulfur
sulfur" stable above 95.3 °C, it
"Muthmann's sul reverts to α-sulfur at
fur II" room temperature.[5]
"nacreous sulfur" Light yellow solid,
"mother of pearl crystal monoclinic,
sulfur" consisting of
cyclo-S8 γ-sulfur "Gernez’s S8 puckered rings.
sulfur" or [5]
Found in nature as
"Muthmann's the rare
sulfur III". mineral rosickyite.[22]
Pure forms all
allotropes, cyclo-S9 has
four forms, cyclo-
cyclo-Sn cyclo-(nona; deca; undeca; S18 has two forms.
n = 9–15, 18, dodeca; trideca; tetradeca; Generally synthesised
20 pentadeca; octadeca; eicosa)sulfur rather than obtained by
treatment of another
form of elemental
sulfur.[23]
Well characterized,
fibrous (ψ) contains parallel helical
catena-Sx sulfur chains and is
sulfur
difficult to obtain pure.
[28]
Not well characterized,
catena-Sx lamina sulfur contains helical chains
partially crossed.
Quenched molten
sulfur at first solidifies
amorphous to amorphous or glassy
"plastic sulfur"
sulfur sulfur. Consists of a
mixture of catena
sulfur and cyclo sulfur.
insoluble Quenched liquid sulfur
sulfur with soluble species
extracted with CS2.
Sometimes called
polymeric sulfur, μ-S
or ω-S.
A mixture of allotropic
ψ-sulfur and cyclo
φ-sulfur
forms mainly γ-cyclo-
S8.[31]
A mixture of chains
ω-sulfur insoluble sulfur with a minimum of
soluble species.[30]
Light yellow mobile
liquid formed when β-
λ-sulfur sulfur first melts at
119.6 °C. Consists of
S8rings.[21]
The dark-coloured
viscous liquid formed
when π-sulfur is heated
μ-sulfur and the solid when
cooled. Contains a
mixture of polymeric
chains.[21]
Dark-coloured liquid
that develops as λ-
π-sulfur sulfur is left molten.
Contains mixture of
Sn rings.[21]
Four high-pressure
phases (at ambient
temperature) including
High- two that are metallic
S-II, S-III, S-
pressure and
IV, S-V and
forms of α- become superconductin
others
sulfur g at low temperature[11]
[12]
and some additional
phases photo-induced
below 20–30 GPa.
High-temperature gaseous allotropes[edit]
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Disulfur, S2[edit]
Main article: Disulfur
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.[34]
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.[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]
Trisulfur, S3[edit]
Main article: Trisulfur
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.[34]
Tetrasulfur, S4[edit]
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.[citation needed] A primary research result based on
theoretical calculations suggests that it has a ring structure. [35][non-primary source
needed]
