Allotropy


Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element; the atoms of the element are bonded together in a different manner. For example, the allotropes of carbon include diamond, graphite, graphene, and fullerenes. The term allotropy is used for elements only, not for compounds. The more general term, used for any crystalline material, is polymorphism. Allotropy refers only to different forms of an element within the same phase ; the differences between these states would not alone constitute examples of allotropy.
For some elements, allotropes have different molecular formulae despite difference in phase; for example, two allotropes of oxygen can both exist in the solid, liquid and gaseous states. Other elements do not maintain distinct allotropes in different phases; for example, phosphorus has numerous solid allotropes, which all revert to the same P4 form when melted to the liquid state.

History

The concept of allotropy was originally proposed in 1841 by the Swedish scientist Baron Jöns Jakob Berzelius. The term is derived. After the acceptance of Avogadro's hypothesis in 1860, it was understood that elements could exist as polyatomic molecules, and two allotropes of oxygen were recognized as O2 and O3. In the early 20th century, it was recognized that other cases such as carbon were due to differences in crystal structure.
By 1912, Ostwald noted that the allotropy of elements is just a special case of the phenomenon of polymorphism known for compounds, and proposed that the terms allotrope and allotropy be abandoned and replaced by polymorph and polymorphism. Although many other chemists have repeated this advice, IUPAC and most chemistry texts still favour the usage of allotrope and allotropy for elements only.

Differences in properties of an element's allotropes

Allotropes are different structural forms of the same element and can exhibit quite different physical properties and chemical behaviours. The change between allotropic forms is triggered by the same forces that affect other structures, i.e., pressure, light, and temperature. Therefore, the stability of the particular allotropes depends on particular conditions. For instance, iron changes from a body-centered cubic structure to a face-centered cubic structure above 906 °C, and tin undergoes a modification known as tin pest from a metallic form to a semiconductor form below 13.2 °C. As an example of allotropes having different chemical behaviour, ozone is a much stronger oxidizing agent than dioxygen.

List of allotropes

Typically, elements capable of variable coordination number and/or oxidation states tend to exhibit greater numbers of allotropic forms. Another contributing factor is the ability of an element to catenate.
Examples of allotropes include:

Non-metals

Metalloids

Metals

Among the metallic elements that occur in nature in significant quantities, almost half are allotropic at ambient pressure: Li, Be, Na, Ca, Ti, Mn, Fe, Co, Sr, Y, Zr, Sn, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Yb, Hf, Tl, Th, Pa and U. Some phase transitions between allotropic forms of technologically relevant metals are those of Ti at 882 °C, Fe at 912 °C and 1394 °C, Co at 422 °C, Zr at 863 °C, Sn at 13 °C and U at 668 °C and 776 °C.
ElementPhase nameSpace groupPearson symbolStructure typeDescription
LithiumRmhR9α-Samarium structureForms below 70 K.
LithiumImmcI2Body-centered cubicStable at room temperature and pressure.
LithiumcF4Face centered cubicForms above 7GPa
LithiumhR1An intermediate phase formed ~40GPa.
LithiumcI16Forms above 40GPa.
BerylliumP63/mmchP2Hexagonal close packedStable at room temperature and pressure.
BerylliumImmcI2Body centered cubicForms above 1255°C.
SodiumRmhR9α-Samarium structureForms below 20 K.
SodiumImmcI2Body centered cubicStable at room temperature and pressure.
SodiumFmmcF4Face centered cubicForms at room temperature above 65 GPa.
SodiumI3dcI16Forms at room temperature, 108GPa.
SodiumPnmaoP8Forms at room temperature, 119GPa.
MagnesiumP63/mmchP2hexagonal close packedStable at room temperature and pressure.
MagnesiumImmcI2Body centered cubicForms above 50 GPa.
Tinα-tin, gray tin, tin pestFdmcF8Diamond cubicStable below 13.2 °C.
Tinβ-tin, white tinI41/amdtI4β-Tin structureStable at room temperature and pressure.
Tinγ-tin, rhombic tinI4/mmmBody-centered tetragonal-
Tinσ-SnBody-centered cubicForms at very high pressure.
TinStanene
Ironα-Fe, ferriteImmcI2Body-centered cubicStable at room temperature and pressure. Ferromagnetic at T<770 °C, paramagnetic from T=770–912 °C.
Ironγ-iron, austeniteFmmcF4Face-centered cubicStable from 912–1,394 °C.
Ironδ-ironImmcI2Body-centered cubicStable from 1,394 – 1,538 °C, same structure as α-Fe.
Ironε-iron, HexaferrumP63/mmchP2Hexagonal close-packedStable at high pressures.
Cobaltα-Cobaltsimple cubicForms above 417 °C.
Cobaltβ-Cobalthexagonal close packedForms below 417 °C.
Poloniumα-Poloniumsimple cubic
Poloniumβ-Poloniumrhombohedral

Lanthanides and actinides

In 2017, the concept of nanoallotropy was proposed by Prof. Rafal Klajn of the Organic Chemistry Department of the Weizmann Institute of Science. Nanoallotropes, or allotropes of nanomaterials, are nanoporous materials that have the same chemical composition, but differ in their architecture at the nanoscale. Such nanoallotropes may help create ultra-small electronic devices and find other industrial applications. The different nanoscale architectures translate into different properties, as was demonstrated for surface-enhanced Raman scattering performed on several different nanoallotropes of gold. A two-step method for generating nanoallotropes was also created.