Allotropes of Sulfur - List of Allotropes and Forms

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.
cyclo-Sn
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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