Rhombic and monoclinic sulphur are known as the two most important allotropes of sulphur. Both these forms are differ from each other. Eight-membered ring form of rhombic form is more stable than monoclinic. These eight membered rings are not efficiently packed in monoclinic sulphur which is formed when sulphur solidifies at melting point.
There are several changes related to appearance and viscosity, occurred on melting the sulphur which is done for forming mobile liquid. When continuously heated, it turned into red liquid which is viscous. This viscosity is responsible for the interruption of eight-membered rings, which are linked to each other in a chain form. When heated on a temperature above 190° C, the liquid turns into the mobile form due to the breakdown of these chains of eight-membered rigns. If this liquid is cooled, the anhistic substance is known as plastic sulphur.
The sulphur vapor also includes diatomic sulphur along with the eight-membered rings. Its blue color is caused by the diatomic sulphur.
Various molecular allotropes of sulphur are classified on the behalf of S-S bond angles (90° to 120°), bond lengths (180pm to 260pm) and bond strength. One S-S bond has high bond energy of 265 kJmol-1 . Allotropes are also considered the pure forms which are different from the mixtures. The names of some allotropes are based on their appearance and the name of person who identified them first. For example- "mother of pearl sulphur" or "Muthmann's sulphur I" etc. Most names uses Greek suffixes like ?, ?. This system is used generally for the form that are synthesised and not prepared from elemental sulphur.
S2 is a strong species of sulphur vapour above 720°C. It comprises 99% vapour species at 530°C. S-S bond length it contains is 188.7 pm. This S2 is the main reason behind the blue colour of burning sulphur .
Tetrasulphur has been found in the vapour phase but its characters are not clearly defined. As per the latest view, it has rings structure.
Found in molten sulphur, has not been isolated yet.
This form was discovered in 1891 by M.R. Engel. It known by the names- Engel's sulphur, Aten's sulphur, ρ-sulphur and ε-sulphur. Prepared by reacting thiosulfate with concentrated HCI, cyclo hexasulphur has orange red color and it has form of rhombohedral crystal.
This Cyclo S6 is also prepared by reacting polysulfane with sulphur monochloride-
H2S4 + S2Cl2 → cyclo-S6 +2HCl
In Cyclo S6, eight-membered rings has chair conformation which includes equivalent sulphur atoms in all rings. Although, S-S bond length and inner bond angles are differ in each rings.
Cyclo-S6.Cyclo-S10adduct: It's a solutiion of cyclo hexasulphur and cyclo-S10 in CS2. Its crystal contain alternate layers of cyclo-S10 and cyclo-S6. The density of this product is in the range between cyclo-S6 to cylco-S10. This is only allotropes which consists different sizes molecules.
This product has an unusual range of S-S bond (199.3pm to 218pm). Yellow colored solid. Cyclo-heptasulphur has four known forms- α-, β-, γ-, δ-, out of which two have been characterised (- γ- and δ-).
Greenish yellow colored form of sulphur, which is found most commonly in nature- "roll sulphur", "milk of sulphur", "flowers of sulphur". This product is a good electrical insulator. It has poor thermal conductivity and it's practically insoluble in water. It has rhombohedral crystal structure. α-sulphur is soluble in carbon disulfide of 35.5g solvent at a temperature of 25°C. It has crown shape. S-S bond lengths are 206pm, and entire S-S-S angles are 108°. α-sulphur converted into γ-sulphur at 95.3°C.
This is yellow solid product which remains stable only above 95.3° C. Below this temperature, it converts into α-sulphur. It has monoclinic crystal form. It contains the same puckered S8 rings as of α-sulphur, but different say of packing in the crystal. Preparation method includes crystallising at 100°C and fast cooling in order to slow down the formation of α-sulphur. The ideal melting point of β-sulphur is 119°C but natural melting point, when decomposition occurs is 114.5°C.
This product is found in nature in the form of rosickyite. It was discovered in 1890, by F.W Muthmann and named as "mother of pearl sulphur". Also it is called "nacreous sulphur" sometimes. It contains puckered S8 rings same as of α-sulphur and β-sulphur but the way of packing of rings is found different in this form. This form is crystallises in monoclinic needles. γ-sulphur can be prepared by heating the molten sulphur at a temperature of 150°C, thereafter slowly cooled. Also it can use cooling by carbon disulfide, ethyl alcohol etc.
cyclo-Sn, (n=9-15, 18, 20)
Various methods used for preparing this allotropes are given below-
S12, S18 and S20 can be prepared by S8.
Cyclo S9- Two forms are known, one is characterised.
Cyclo S12- Second most stable cyclo-allotrope after S8. Containing sulphur in three planes in a sequence of 3-6-3.
CycloS18- Two forms are known.
Their production is found extremely difficult.
Ψ-sulphur or ω1-sulphur
It's also called fibrous sulphur, this form is well characterised. It has melting point of 104°C and density around 2.01g cm^-3. The S-S bond length is 206.6pm and S-S-S bond angle is 106° with dihedral angle 85.3°.
Also called ω2-sulphur or Χ-sulphur, this form is not well characterised yet. It consist of criss-crossed helices.
It contains mixture of cyclo forms and catena-sulphure. The form of this product changes from plastic to glassy. It is called vitreous sulphur, Χ-sulphur. Its quenched product of molten sulphur.
Also being called as polymeric sulphur, it obtained by washing quenced sulphur with CS2. Other names- μ-sulphur or ω-sulphur.
Φ-sulphur, fibrous sulphur
Mixture of γ-cycloS8 and ψ- form allotropes.
This form is a dark coloured liquid which contain mixture of Sn rings. Prepared by leaving γ-sulphur to remain molten.
This is the name of sulphur just melted. Cooling γ-sulphur gives β-sulphur.
Prepared from amorphous sulphur, this is a product which is commercially available. "Crystex" is the well known commerical form. Some other names of this sulphur are supersublimated sulphur and "white sulphur of Das". It's a mixture of lamina sulphur and ψ-sulphur. Mainly used in vulcanization of rubber.
High pressure forms
Four forms, termed S-II, S-III, S-IV, S-V have been characterised in the study of high pressure at ambient temperatures. Out of which two (S-II and S-III) are polymeric forms, and two (S-IV and S-V) are metallic forms. They are superconductors below 10°K and 17°K.