Allotropy

File:Diamond and graphite.jpg

Diamond and graphite are two allotropes of carbon: pure forms of the same element that differ in crystalline structure.

Allotropy or allotropism (Template:Ety) 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 different manners.<ref>Template:GoldBookRef</ref> For example, the allotropes of carbon include diamond (the carbon atoms are bonded together to form a cubic lattice of tetrahedra), graphite (the carbon atoms are bonded together in sheets of a hexagonal lattice), graphene (single sheets of graphite), and fullerenes (the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations).

The term allotropy is used for elements only, not for compounds. The more general term, used for any compound, is polymorphism, although its use is usually restricted to solid materials such as crystals. Allotropy refers only to different forms of an element within the same physical phase (the state of matter, such as a solid, liquid or gas). The differences between these states of matter would not alone constitute examples of allotropy. Allotropes of chemical elements are frequently referred to as polymorphs or as phases of the element.

For some elements, allotropes have different molecular formulae or different crystalline structures, as well as a difference in physical phase; for example, two allotropes of oxygen (dioxygen, O₂, and ozone, O₃) can both exist in the solid, liquid and gaseous states. Other elements do not maintain distinct allotropes in different physical 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 1840 by the Swedish scientist Baron Jöns Jakob Berzelius (1779–1848).<ref>See:

• Template:Cite book From p. 14: "Om det ock passar väl för att uttrycka förhållandet emellan myrsyrad ethyloxid och ättiksyrad methyloxid, så är det icke passande för de olika tillstånd hos de enkla kropparne, hvari dessa blifva af skiljaktiga egenskaper, och torde för dem böra ersättas af en bättre vald benämning, t. ex. Allotropi (af αλλότροπος, som betyder: af olika beskaffenhet) eller allotropiskt tillstånd." (If it [i.e., the word isomer] is also well suited to express the relation between formic acid ethyl oxide [i.e., ethyl formate] and acetic acid methyloxide [i.e., methyl acetate], then it [i.e., the word isomers] is not suitable for different conditions of simple substances, where these [substances] transform to have different properties, and [therefore the word isomers] should be replaced, in their case, by a better chosen name; for example, Allotropy (from αλλότροπος, which means: of different nature) or allotropic condition.)
• Republished in German: Template:Cite journal From p. 13: "Wenn es sich auch noch gut eignet, um das Verhältniss zwischen ameisensaurem Äthyloxyd und essigsaurem Methyloxyd auszudrücken, so ist es nicht passend für ungleiche Zustände bei Körpern, in welchen diese verschiedene Eigenschaften annehmen, und dürfte für diese durch eine besser gewählte Benennung zu ersetzen sein, z. B. durch Allotropie (von αλλότροπος, welches bedeutet: von ungleicher Beschaffenheit), oder durch allotropischen Zustand." (Even if it [i.e., the word isomer] is still well suited to express the relation between ethyl formate and methyl acetate, then it is not appropriate for the distinct conditions in the case of substances where these [substances] assume different properties, and for these, [the word isomer] may be replaced with a better chosen designation, e.g., with Allotropy (from αλλότροπος, which means: of distinct character), or with allotropic condition.)
• Merriam-Webster online dictionary: Allotropy</ref><ref name=Jensen>Template:Citation.</ref> The term is derived Template:Ety.<ref>Template:Citation.</ref> 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.<ref name=Jensen/> 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.<ref>Template:Cite book From p. 104: "Substances are known which exist not only in two, but even in three, four or five different solid forms; no limitation to the number is known to exist. Such substances are called polymorphous. The name allotropy is commonly employed in the same connexion, especially when the substance is an element. There is no real reason for making this distinction, and it is preferable to allow the second less common name to die out."</ref><ref name=Jensen/> Although many other chemists have repeated this advice, IUPAC and most chemistry texts still favour the usage of allotrope and allotropy for elements only.<ref>Jensen 2006, citing Addison, W. E. The Allotropy of the Elements (Elsevier 1964) that many have repeated this advice.</ref>

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 (ferrite) to a face-centered cubic structure (austenite) above 906 °C, and tin undergoes a modification known as tin pest from a metallic form to a semimetallic form below 13.2 °C (55.8 °F). As an example of allotropes having different chemical behaviour, ozone (O₃) is a much stronger oxidizing agent than dioxygen (O₂).

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

Element	Allotropes
Carbon	Diamond – an extremely hard, transparent crystal, with the carbon atoms arranged in a tetrahedral lattice. A poor electrical conductor. An excellent thermal conductor. Lonsdaleite – also called hexagonal diamond. Graphene – is the basic structural element of other allotropes, nanotubes, charcoal, and fullerenes. Q-carbon – a ferromagnetic, tough, and brilliant crystal structure that is harder and brighter than diamonds.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= Template:Fix }} Graphite – a semimetallic, soft, black, flaky solid, a good electrical conductor. The C atoms are bonded in flat hexagonal lattices (graphene), which are then layered in sheets. Linear acetylenic carbon (carbyne) Amorphous carbon Fullerenes, including buckminsterfullerene, also known as "buckyballs", such as C60. Carbon nanotubes – allotropes having a cylindrical nanostructure. Schwarzites Cyclocarbon Glassy carbon Superdense carbon allotropes – proposed allotropes
Nitrogen	Dinitrogen – by far the most common and stable form of nitrogen, found in the air. Hexazine Octaazacubane Tetranitrogen Trinitrogen Solid nitrogen
Phosphorus	White phosphorus – crystalline solid of tetraphosphorus (P4) molecules Red phosphorus – amorphous polymeric solid Scarlet phosphorus Violet phosphorus with monoclinic crystalline structure Black phosphorus – semiconductor, analogous to graphite Diphosphorus – gaseous form composed of P2 molecules, stable between 1200 °C and 2000 °C; created e.g. by dissociation of P4 molecules of white phosphorus at around 827 °C
Oxygen	Dioxygen, O2 – colorless (faint blue liquid and solid) Ozone, O3 – blue Tetraoxygen, O4 – metastable Octaoxygen, O8 – red
Sulfur	Cyclo-Pentasulfur, Cyclo-S5 Cyclo-Hexasulfur, Cyclo-S6 Cyclo-Heptasulfur, Cyclo-S7 Cyclo-Octasulfur, Cyclo-S8
Selenium	"Red selenium", cyclo-Se8 Gray selenium, polymeric Se Black selenium, irregular polymeric rings up to 1000 atoms long Monoclinic selenium, dark red transparent crystals
Spin isomers of hydrogen	Orthohydrogen, H2 with nuclear spins aligned parallel Parahydrogen, H2 with nuclear spins aligned antiparallel These nuclear spin isomers have sometimes been described as allotropes, notably by the committee which awarded the 1932 Nobel prize to Werner Heisenberg for quantum mechanics and singled out the "allotropic forms of hydrogen" as its most notable application.<ref>Werner Heisenberg – Facts Nobelprize.org</ref>

Metalloids

Element	Allotropes
Boron	Amorphous boron – brown powder – B12 regular icosahedra α-rhombohedral boron β-rhombohedral boron γ-orthorhombic boron α-tetragonal boron β-tetragonal boron High-pressure superconducting phase
Silicon	Amorphous silicon α-silicon, a semiconductor, diamond cubic structure β-silicon - metallic, with the BCC similar to molybdenum and beta-tin (High Pressure Phase) Q-Silicon - a ferromagnetic (Similar to Q-Carbon) and highly conductive phase of silicon (similar to graphite) <ref name="google">{{#invoke:citation/CS1|citation	CitationClass=web }}</ref> Silicene, buckled planar single layer Silicon, similar to Graphene
Germanium	Amorphous germanium α-germanium – semimetallic element or semiconductor, with the same structure as diamond (similar chemical properties with sulfur and silicon) β-germanium – metallic, with the same structure as beta-tin Germanene – Buckled planar Germanium, similar to graphene
Arsenic	Yellow arsenic – molecular non-metallic As4, with the same structure as white phosphorus (Similar chemical properties with nitrogen and phosphorus) Gray arsenic, polymeric As (metallic, though heavily anisotropic) (similar to aluminum and antimony in chemical properties) Black arsenic – molecular and non-metallic, with the same structure as red phosphorus
Antimony	Blue-white antimony – stable form (metallic), with the same structure as gray arsenic (similar to arsenic in chemical properties) Black antimony (non-metallic and amorphous, only stable as a thin layer)
Tellurium	Amorphous tellurium – gray-black or brown powder<ref>Template:Cite book</ref> Crystalline tellurium – hexagonal crystalline structure (metalloid) (similar chemical properties with selenium)

Metals

Among the metallic elements that occur in nature in significant quantities (56 up to U, without Tc and Pm), almost half (27) 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 1,394 °C, Co at 422 °C, Zr at 863 °C, Sn at 13 °C and U at 668 °C and 776 °C.

Element	Phase name(s)	Space group	Pearson symbol	Structure type	Description
Lithium	α-Li	RTemplate:Overlinem	hR9	α-Sm	Forms below 70 K.<ref>Template:Cite journal</ref>
	β-Li	ImTemplate:Overlinem	cI2	W	Stable at room temperature and pressure.
		FmTemplate:Overlinem	cF4	Cu	Forms above 7GPa
		RTemplate:Overlinem	hR1	α-Hg	An intermediate phase formed ~40GPa.<ref name=Hanfland2000 />
		ITemplate:Overline3d	cI16		Forms above 40GPa.<ref name=Hanfland2000>Template:Cite journal</ref>
			oC88		Forms between 60 and 70 GPa.<ref name=Degtyareva2014>Template:Cite journal</ref>
			oC40		Forms between 70 and 95 GPa.<ref name=Degtyareva2014 />
			oC24		Forms above 95 GPa.<ref name=Degtyareva2014 />
Beryllium	α-Be	P63/mmc	hP2	Mg	Stable at room temperature and pressure.
	β-Be	ImTemplate:Overlinem	cI2	W	Forms above 1255 °C.
Sodium	α-Na	RTemplate:Overlinem	hR9	α-Sm	Forms below 20 K.
	β-Na	ImTemplate:Overlinem	cI2	W	Stable at room temperature and pressure.
		FmTemplate:Overlinem	cF4	Cu	Forms at room temperature above 65 GPa.<ref>Template:Cite journal</ref>
		ITemplate:Overline3d	cI16		Forms at room temperature, 108GPa.<ref>Template:Cite journal</ref>
		Pnma	oP8	MnP	Forms at room temperature, 119GPa.<ref>Template:Cite journal</ref>
			tI19*		A host-guest structure that forms above between 125 and 180 GPa.<ref name=Degtyareva2014 />
			hP4		Forms above 180 GPa.<ref name=Degtyareva2014 />
Magnesium		P63/mmc	hP2	Mg	Stable at room temperature and pressure.
		ImTemplate:Overlinem	cI2	W	Forms above 50 GPa.<ref>Template:Cite journal</ref>
Aluminium	α-Al	FmTemplate:Overlinem	cF4	Cu	Stable at room temperature and pressure.
	β-Al	P63/mmc	hP2	Mg	Forms above 20.5 GPa.
Potassium		ImTemplate:Overlinem	cI2	W	Stable at room temperature and pressure.
		FmTemplate:Overlinem	cF4	Cu	Forms above 11.7 GPa.<ref name=Degtyareva2014 />
		I4/mcm	tI19*		A host-guest structure that forms at about 20 GPa.<ref name=Degtyareva2014 />
		P63/mmc	hP4	NiAs	Forms above 25 GPa.<ref name=Degtyareva2014 />
		Pnma	oP8	MnP	Forms above 58GPa.<ref name=Degtyareva2014 />
		I41/amd	tI4		Forms above 112 GPa.<ref name=Degtyareva2014 />
		Cmca	oC16		Formas above 112 GPa.<ref name=Degtyareva2014 />
Iron	α-Fe, ferrite	ImTemplate:Overlinem	cI2	Body-centered cubic	Stable at room temperature and pressure. Ferromagnetic at T<770 °C, paramagnetic from T=770–912 °C.
	γ-iron, austenite	FmTemplate:Overlinem	cF4	Face-centered cubic	Stable from 912 to 1,394 °C.
	δ-iron	ImTemplate:Overlinem	cI2	Body-centered cubic	Stable from 1,394 – 1,538 °C, same structure as α-Fe.
	ε-iron, Hexaferrum	P63/mmc	hP2	Hexagonal close-packed	Stable at high pressures.
Cobalt<ref>Template:Cite journal</ref>	α-Cobalt			hexagonal-close packed	Forms below 450 °C.
	β-Cobalt			face centered cubic	Forms above 450 °C.
	ε-Cobalt	P4132		primitive cubic	Forms from thermal decomposition of [Co2CO8]. Nanoallotrope.
Rubidium	α-Rb	ImTemplate:Overlinem	cI2	W	Stable at room temperature and pressure.
			cF4		Forms above 7 GPa.<ref name=Degtyareva2014 />
			oC52		Forms above 13 GPa.<ref name=Degtyareva2014 />
			tI19*		Forms above 17 GPa.<ref name=Degtyareva2014 />
			tI4		Forms above 20 GPa.<ref name=Degtyareva2014 />
			oC16		Forms above 48 GPa.<ref name=Degtyareva2014 />
Tin	α-tin, gray tin, tin pest	FdTemplate:Overlinem	cF8	d-C	Stable below 13.2 °C.
	β-tin, white tin	I41/amd	tI4	β-Sn	Stable at room temperature and pressure.
	γ-tin, rhombic tin	I4/mmm	tI2	In	Forms above 10 GPa.<ref name=Deffrennes2022>Template:Cite journal</ref>
	γ'-Sn	Immm	oI2	MoPt2	Forms above 30 GPa.<ref name=Deffrennes2022 />
	σ-Sn, γ"-Sn	ImTemplate:Overlinem	cI2	W	Forms above 41 GPa.<ref name=Deffrennes2022 /> Forms at very high pressure.<ref>Template:Cite journal</ref>
	δ-Sn	P63/mmc	hP2	Mg	Forms above 157 GPa.<ref name=Deffrennes2022 />
	Stanene
Polonium	α-Polonium			simple cubic
	β-Polonium			rhombohedral

Template:Colorsample Most stable structure under standard conditions.
Template:Colorsample Structures stable below room temperature.
Template:Colorsample Structures stable above room temperature.
Template:Colorsample Structures stable above atmospheric pressure.

Lanthanides and actinides

File:Actinide phases.svg

Phase diagram of the actinide elements.

• Cerium, samarium, dysprosium and ytterbium have three allotropes.
• Praseodymium, neodymium, gadolinium and terbium have two allotropes.
• Plutonium has six distinct solid allotropes under "normal" pressures. Their densities vary within a ratio of some 4:3, which vastly complicates all kinds of work with the metal (particularly casting, machining, and storage). A seventh plutonium allotrope exists at very high pressures. The transuranium metals Np, Am, and Cm are also allotropic.
• Promethium, americium, berkelium and californium have three allotropes each.<ref>Template:Cite journal</ref>

Nanoallotropes

In 2017, the concept of nanoallotropy was proposed.<ref name=":0">Template:Cite journal</ref> Nanoallotropes, or allotropes of nanomaterials, are nanoporous materials that have the same chemical composition (e.g., Au), but differ in their architecture at the nanoscale (that is, on a scale 10 to 100 times the dimensions of individual atoms).<ref name=":1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Such nanoallotropes may help create ultra-small electronic devices and find other industrial applications.<ref name=":1" /> The different nanoscale architectures translate into different properties, as was demonstrated for surface-enhanced Raman scattering performed on several different nanoallotropes of gold.<ref name=":0" /> A two-step method for generating nanoallotropes was also created.<ref name=":1" />

See also

• Isomer
• Polymorphism (materials science)

References

• Template:Cite EB1911

External links

• {{#invoke:citation/CS1|citation

|CitationClass=web }}

• Allotropes – Chemistry Encyclopedia