Allotropy
Allotropy or allotropism (from
The term allotropy is used for elements only, not for
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
By 1912,
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.,
List of allotropes
Typically, elements capable of variable
Examples of allotropes include:
Non-metals
Element | Allotropes |
---|---|
Carbon |
|
Nitrogen |
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Phosphorus |
|
Oxygen |
|
Sulfur |
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Selenium |
|
Metalloids
Element | Allotropes |
---|---|
Boron |
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Silicon |
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Germanium |
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Arsenic |
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Antimony |
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Tellurium |
|
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
- 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.[18]
Nanoallotropes
In 2017, the concept of nanoallotropy was proposed.
See also
- Isomer
- Polymorphism (materials science)
Notes
- ^ 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
- ^ .
- ^ "allotropy", A New English Dictionary on Historical Principles, vol. 1, Oxford University Press, 1888, p. 238.
- ^ 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." - ^ Jensen 2006, citing Addison, W. E. The Allotropy of the Elements (Elsevier 1964) that many have repeated this advice.
- ISBN 9788187224037. Retrieved January 6, 2017.
- ISSN 0031-9007.
- ^ S2CID 4303422.
- ^ .
- ISSN 0163-1829.
- PMID 17947379.
- S2CID 29596632.
- PMID 9936412.
- PMID 20632764.
- ^ .
- S2CID 120417927.
- .
- ^ PMID 29074773.
- ^ 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
- Chisholm, Hugh, ed. (1911). . Encyclopædia Britannica (11th ed.). Cambridge University Press.
External links
- Nigel Bunce and Jim Hunt. "The Science Corner: Allotropes". Archived from the original on January 31, 2008. Retrieved January 6, 2017.
- Allotropes – Chemistry Encyclopedia