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A '''plastic crystal''' is a [[crystal]] composed of weakly interacting molecules that possess some orientational or conformational degree of freedom. The name plastic crystal refers to the mechanical softness of such phases: they resemble waxes and are easily deformed. If the internal degree of freedom is molecular rotation, the name '''rotor phase''' or '''rotatory phase''' is also used. Typical examples are the [[Polymorphism (materials science)|modification]]s Methane I and Ethane I. 
In addition to the conventional molecular plastic crystals, there are also emerging ionic plastic crystals, particularly organic ionic plastic crystals (OIPCs) and protic organic ionic plastic crystals (POIPCs).<ref name="1,2,4-Triazolium perfluorobutanesulfonate as an archetypal pure protic organic ionic plastic crystal electrolyte for all-solid-state fuel cells"/><ref name="Imidazolium methanesulfonate as a high temperature proton conductor">
{{cite journal
 |author=Jiangshui Luo
 |author2=Olaf Conrad
 |author3=Ivo F. J. Vankelecom 
 |name-list-style=amp
 |date=2013
 |title=Imidazolium methanesulfonate as a high temperature proton conductor
 |journal=[[Journal of Materials Chemistry A]]
 |volume=1
 |issue=6
 |pages=2238–2247
 |doi=10.1039/C2TA00713D
|url=https://lirias.kuleuven.be/bitstream/123456789/392330/2/JMCA_%20Imidazolium%20methanesulfonate%20as%20a%20high%20temperature%20proton%20conductor.pdf
 }}</ref> POIPCs are solid protic organic salts formed by proton transfer from a Brønsted acid to a Brønsted base and in essence are [[protic ionic liquid]]s in the molten state, have found to be promising solid-state proton conductors for high temperature [[proton-exchange membrane fuel cell]]s.<ref name="1,2,4-Triazolium perfluorobutanesulfonate as an archetypal pure protic organic ionic plastic crystal electrolyte for all-solid-state fuel cells"/> Examples include 1,2,4-[[triazolium]] [[perfluorobutanesulfonate]]<ref name="1,2,4-Triazolium perfluorobutanesulfonate as an archetypal pure protic organic ionic plastic crystal electrolyte for all-solid-state fuel cells"/> and [[imidazolium]] [[methanesulfonate]].<ref name="Imidazolium methanesulfonate as a high temperature proton conductor"/>

If the internal degree of freedom freezes in a disordered way, an [[orientational glass]] is obtained.

The orientational degree of freedom may be an almost free rotation, or it may be a jump diffusion between a restricted number of possible orientations, as was shown for [[carbon tetrabromide]].<ref>
{{cite journal |title=Coupled orientational and displacive degrees of freedom in the high-temperature plastic phase of the carbon tetrabromide α-CBr4 |author1=Jacob C. W. Folmer |author2=Ray L. Withers |author3=T. R. Welberry |author4=James D. Martin |journal=Physical Review B |volume=77 |issue=14 |at=144205 |date=2008 |doi=10.1103/PhysRevB.77.144205|bibcode=2008PhRvB..77n4205F }}
</ref>

X- ray [[diffraction]] patterns of plastic crystals are characterized by strong diffuse intensity in addition to the sharp Bragg peaks.<ref name="1,2,4-Triazolium perfluorobutanesulfonate as an archetypal pure protic organic ionic plastic crystal electrolyte for all-solid-state fuel cells">
{{cite journal
 |display-authors=4
 |author=Jiangshui Luo
 |author2=Annemette H. Jensen
 |author3=Neil R. Brooks
 |author4=Jeroen Sniekers
 |author5=Martin Knipper
 |author6=David Aili
 |author7=Qingfeng Li
 |author8=Bram Vanroy
 |author9=Michael Wübbenhorst
 |author10=Feng Yan
 |author11=Luc Van Meervelt
 |author12=Zhigang Shao
 |author13=Jianhua Fang
 |author14=Zheng-Hong Luo
 |author15=Dirk E. De Vos
 |author16=Koen Binnemans
 |author17=Jan Fransaer
 |name-list-style=amp
 |date=2015
 |title=1,2,4-Triazolium perfluorobutanesulfonate as an archetypal pure protic organic ionic plastic crystal electrolyte for all-solid-state fuel cells
 |journal=[[Energy & Environmental Science]]
 |volume=8
 |issue=4
 |pages=1276–1291
 |doi=10.1039/C4EE02280G
|url=http://orbit.dtu.dk/en/publications/124triazolium-perfluorobutanesulfonate-as-an-archetypal-pure-protic-organic-ionic-plastic-crystal-electrolyte-for-allsolidstate-fuel-cells(d5647472-6868-4873-ba5a-ddd28a3fe195).html
 }}</ref> In a powder pattern this intensity appears to resemble an amorphous background as one would expect for a liquid,<ref name="1,2,4-Triazolium perfluorobutanesulfonate as an archetypal pure protic organic ionic plastic crystal electrolyte for all-solid-state fuel cells" /> but for a single crystal the diffuse contribution reveals itself to be highly structured. The Bragg peaks can be used to determine an average structure but due to the large amount of disorder this is not very insightful. It is the structure of the diffuse scattering that reflects the details of the constrained disorder in the system. Recent advances in two-dimensional detection at synchrotron beam lines facilitate the study of such patterns.

== Mechanical properties ==

Plastic crystals behave like true plastic metals under mechanical stress.<ref name="Metal‐like Ductility in Organic Plastic Crystals: Role of Molecular Shape and Dihydrogen Bonding Interactions in Aminoboranes">
{{cite journal
 |display-authors=4
 |author=Amit Mondal
 |author2=Biswajit Bhattacharya
 |author3=Susobhan Das
 |author4=Surojit Bhunia
 |author5=Rituparno Chowdhury
 |author6=Somnath Dey
 |author7=C Malla Reddy
 |name-list-style=amp
 |date=2020
 |title=Metal‐like Ductility in Organic Plastic Crystals: Role of Molecular Shape and Dihydrogen Bonding Interactions in Aminoboranes
 |journal=[[Angewandte Chemie International Edition]]
 |volume=59
 |issue=27
 |pages=10971–10980
 |doi=10.1002/anie.202001060
|pmid=32087039
 |url=https://onlinelibrary.wiley.com/doi/full/10.1002/anie.202001060
 }}</ref>
 
For example, closer to melting, plastic crystals show high ductility and/or malleability. Plastic crystals can flow through a hole under pressure. For example, aminoborane plastic crystals<ref name="Metal‐like Ductility in Organic Plastic Crystals: Role of Molecular Shape and Dihydrogen Bonding Interactions in Aminoboranes"/> bend, twist and stretch with characteristic necking, under appropriate stress. These crystals can be literally shaped into any possible way, like copper or silver metals.

This way, they are very unique compared to other molecular crystals, which are generally brittle and fragile.

== Plastic crystals versus liquid crystals ==

Like [[liquid crystal]]s, plastic crystals can be considered a transitional stage between real solids and real liquids and can be considered ''soft matter''. Another common denominator is the simultaneous presence of order and disorder. Both types of phases are usually observed between the true solid and liquid phases on the temperature scale:

: {{chem2|true crystal -> plastic crystal -> true liquid}}
: {{chem2|true crystal -> liquid crystal -> true liquid}}

The difference between liquid and plastic crystals is easily observed in [[X-ray diffraction]]. Plastic crystals possess strong long range order and therefore show sharp Bragg reflections.<ref name="1,2,4-Triazolium perfluorobutanesulfonate as an archetypal pure protic organic ionic plastic crystal electrolyte for all-solid-state fuel cells"/> Liquid crystals show none or very broad Bragg peaks because the order is not long range. The molecules that give rise to liquid crystalline behavior often have a strongly elongated or disc like shape. Plastic crystals consist usually of almost spherical objects. In this respect one could see them as opposites.

Certain liquid crystals go through plastic crystal phase before melting. In general, liquid crystals are closer to liquids while plastic crystals are closer to true crystals.

== History ==

Plastic crystals were discovered in 1938 by J. Timmermans by their anomalously low [[melting entropy]]. He found that organic substances having a melting entropy lower than approximately 17 J·K<sup>−1</sup>·mol<sup>−1</sup> (~2Rg) are having peculiar properties. Timmermans named them ''{{lang|fr|molecular globulare}}''.

Michils showed in 1948 that these organic compounds are easily deformed and accordingly named them, ''plastic crystals'' ({{lang|fr|cristaux organiques plastiques}}).<ref>{{cite journal
| author=A. Michils
| title=Recherches stoechiométriques V.VIII. LA PLASTICITÉ D'UN GROUPE PARTICULIER DE CRISTAUX ORGANIQUES
| journal=[[Bulletin des Sociétés Chimiques Belges]]
| volume=57
| number=10–12
| year=1948
| pages=575–617
| language=fr
| doi=10.1002/bscb.19480571013
}}
</ref> [[Perfluorocyclohexane]] for example is plastic to such a degree that it will start to flow under its own weight. 
<ref>{{cite book
 |editor=Peter R. Sahm
 |editor2=Iván Egry
 |editor3=Thomas Volkmann
| title=Schmelze, Erstarrung, Grenzflächen. Eine Einführung in die Physik und Technologie flüssiger und fester Metalle
| publisher = Springer |location=Berlin, Heidelberg
| year   = 1999
|isbn=978-3-540-41566-4
|doi=10.1007/978-3-642-58523-4
}}</ref>

==References==
{{Reflist}}

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[[Category:Crystals|Crystallinity, plastic]]
[[Category:Physical quantities]]
[[Category:Soft matter]]
[[Category:Phases of matter]]