Allotropy

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

Allotropy or allotropism (from

Ancient Greek ἄλλος (allos) 'other', and τρόπος (tropos) 'manner, form') 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.[1]
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

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).[2][3] The term is derived from Greek άλλοτροπἱα (allotropia) 'variability, changeableness'.[4] 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.[3]
In the early 20th century, it was recognized that other cases such as carbon were due to differences in crystal structure.

By 1912,

IUPAC and most chemistry texts still favour the usage of allotrope and allotropy for elements only.[6]

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.,

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

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
Nitrogen
Phosphorus
  • White phosphorus
    – crystalline solid of tetraphosphorus (P4) molecules
  • Red phosphorusamorphous polymeric solid
  • Scarlet phosphorus
  • 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
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

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
Germanium
  • α-germanium – semimetallic, with the same structure as diamond
  • 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
  • Gray arsenic, polymeric As (metallic, though heavily anisotropic)
  • 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
  • Black antimony (non-metallic and amorphous, only stable as a thin layer)
Tellurium
  • Amorphous tellurium – gray-black or brown powder[7]
  • Crystalline tellurium – hexagonal crystalline structure (metalloid)

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 1394 °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 R3m hR9 α-Sm Forms below 70 K.[8]
β-Li Im3m cI2 W Stable at room temperature and pressure.
Fm3m cF4 Cu Forms above 7GPa
R3m hR1 α-Hg An intermediate phase formed ~40GPa.[9]
I43d cI16 Forms above 40GPa.[9]
oC88 Forms between 60 and 70 GPa.[10]
oC40 Forms between 70 and 95 GPa.[10]
oC24 Forms above 95 GPa.[10]
Beryllium α-Be P63/mmc hP2 Mg Stable at room temperature and pressure.
β-Be Im3m cI2 W Forms above 1255 °C.
Sodium α-Na R3m hR9 α-Sm Forms below 20 K.
β-Na Im3m cI2 W Stable at room temperature and pressure.
Fm3m cF4 Cu Forms at room temperature above 65 GPa.[11]
I43d cI16 Forms at room temperature, 108GPa.[12]
Pnma oP8 MnP Forms at room temperature, 119GPa.[13]
tI19* A host-guest structure that forms above between 125 and 180 GPa.[10]
hP4 Forms above 180 GPa.[10]
Magnesium P63/mmc hP2 Mg Stable at room temperature and pressure.
Im3m cI2 W Forms above 50 GPa.[14]
Aluminium α-Al Fm3m cF4 Cu Stable at room temperature and pressure.
β-Al P63/mmc hP2 Mg Forms above 20.5 GPa.
Potassium Im3m cI2 W Stable at room temperature and pressure.
Fm3m cF4 Cu Forms above 11.7 GPa.[10]
I4/mcm tI19* A host-guest structure that forms at about 20 GPa.[10]
P63/mmc hP4 NiAs Forms above 25 GPa.[10]
Pnma oP8 MnP Forms above 58GPa.[10]
I41/amd tI4 Forms above 112 GPa.[10]
Cmca oC16 Formas above 112 GPa.[10]
Iron α-Fe, ferrite Im3m cI2
Body-centered cubic
Stable at room temperature and pressure. Ferromagnetic at T<770 °C, paramagnetic from T=770–912 °C.
γ-iron, austenite Fm3m cF4
Face-centered cubic
Stable from 912 to 1,394 °C.
δ-iron Im3m 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[15] 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 Im3m cI2 W Stable at room temperature and pressure.
cF4 Forms above 7 GPa.[10]
oC52 Forms above 13 GPa.[10]
tI19* Forms above 17 GPa.[10]
tI4 Forms above 20 GPa.[10]
oC16 Forms above 48 GPa.[10]
Tin α-tin,
gray tin, tin pest
Fd3m 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.[16]
γ'-Sn Immm oI2 MoPt2 Forms above 30 GPa.[16]
σ-Sn, γ"-Sn Im3m cI2 W Forms above 41 GPa.[16] Forms at very high pressure.[17]
δ-Sn P63/mmc hP2 Mg Forms above 157 GPa.[16]
Stanene
Polonium α-Polonium simple cubic
β-Polonium
rhombohedral

Most stable stable under standard conditions.
Structures stable below room temperature.
Structures stable above room temperature.
Structures stable above atmospheric pressure.

Lanthanides and actinides

Phase diagram of the actinide elements.

Nanoallotropes

In 2017, the concept of nanoallotropy was proposed.

surface-enhanced Raman scattering performed on several different nanoallotropes of gold.[19] A two-step method for generating nanoallotropes was also created.[20]

See also

  • Isomer
  • Polymorphism (materials science)

Notes

  1. ^ See:
    • Berzelius, Jac. (1841). Årsberättelse om Framstegen i Fysik och Kemi afgifven den 31 Mars 1840. Första delen [Annual Report on Progress in Physics and Chemistry submitted March 31, 1840. First part.] (in Swedish). Stockholm, Sweden: P.A. Norstedt & Söner. p. 14. 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: Berzelius, Jacob; Wöhler, F., trans. (1841). "Jahres-Bericht über die Fortschritte der physischen Wissenschaften" [Annual Report on Progress of the Physical Sciences]. Jahres Bericht Über die Fortschritte der Physischen Wissenschaften (in German). 20. Tübingen, (Germany): Laupp'schen Buchhandlung: 13.{{cite journal}}: CS1 maint: multiple names: authors list (link) 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
  2. ^ .
  3. ^ "allotropy", A New English Dictionary on Historical Principles, vol. 1, Oxford University Press, 1888, p. 238.
  4. ^ Ostwald, Wilhelm; Taylor, W.W., trans. (1912). Outlines of General Chemistry (3rd ed.). London, England: Macmillan and Co., Ltd. p. 104.{{cite book}}: CS1 maint: multiple names: authors list (link) 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."
  5. ^ Jensen 2006, citing Addison, W. E. The Allotropy of the Elements (Elsevier 1964) that many have repeated this advice.
  6. . Retrieved January 6, 2017.
  7. .
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  19. ^ a b c "Materials That Don't Exist in Nature Might Lead to New Fabrication Techniques". israelbds.org. Archived from the original on 2017-12-09. Retrieved 2017-12-08.

References

External links