Editing Clathrate compound

{{Short description|Chemical substance consisting of a lattice that traps or contains molecules}}

A '''clathrate''' is a [[chemical substance]] consisting of a [[lattice (group)|lattice]] that traps or contains molecules.  The word ''clathrate'' is derived from the [[Latin language|Latin]] {{wikt-lang|la|clathratus}} ({{lang|la|clatratus}}), meaning 'with bars, [[Crystal structure|lattice]]d'.<ref>[http://lysy2.archives.nd.edu/cgi-bin/WORDS.EXE?clathrate Latin dictionary] {{webarchive|url=https://web.archive.org/web/20120414202654/http://lysy2.archives.nd.edu/cgi-bin/WORDS.EXE?clathrate |date=2012-04-14 }}</ref> Most clathrate compounds are [[polymer]]ic and completely envelop the guest molecule, but in modern usage clathrates also include [[host–guest complex]]es and [[inclusion compound]]s.<ref name=Ullmann>Atwood, J. L. (2012) "Inclusion Compounds" in ''Ullmann's Encyclopedia of Industrial Chemistry''. Wiley-VCH, Weinheim. {{doi| 10.1002/14356007.a14_119}}</ref> According to [[International Union of Pure and Applied Chemistry|IUPAC]], clathrates are inclusion compounds "in which the guest molecule is in a cage formed by the host molecule or by a lattice of host molecules."<ref>{{GoldBookRef |title=clathrates |file=C01097 }}</ref> The term refers to many molecular hosts, including [[calixarene]]s and [[cyclodextrin]]s and even some inorganic polymers such as [[zeolite]]s.

[[File:Clathrate hydrate cages.svg|thumb|Clathrate cavities. For example, 5<sup>12</sup> (dodecahedral) and 5<sup>12</sup>6<sup>2</sup> (tetrakaidecahedral) make up a Type I (sI) structure.<ref name=":0">{{Cite journal |last1=Krishna |first1=Lakshmi |last2=Koh |first2=Carolyn A. |date=February 2015 |title=Inorganic and methane clathrates: Versatility of guest–host compounds for energy harvesting |journal=MRS Energy & Sustainability |language=en |volume=2 |issue=1 |pages=8 |doi=10.1557/mre.2015.9 |issn=2329-2229|doi-access=free }}</ref>]]Clathrates can be divided into two categories: [[clathrate hydrate]]s and inorganic clathrates. Each clathrate is made up of a framework and guests that reside the framework. Most common clathrate crystal structures can be composed of cavities such as [[Regular dodecahedron|dodecahedral]], [[Tetradecahedron|tetrakaidecahedral]], and [[Hexadecahedron|hexakaidecahedral]] cavities.

Unlike hydrates, [[inorganic]] clathrates have a [[covalently bonded]] framework of inorganic atoms with guests typically consisting of [[alkali]] or [[alkaline earth metal]]s. Due to the stronger covalent bonding, the cages are often smaller than hydrates. Guest atoms interact with the host by ionic or covalent bonds. Therefore, partial substitution of guest atoms follow [[Zintl phase|Zintl]] rules so that the charge of the overall compound is conserved. Most inorganic clathrates have full occupancy of its framework cages by a guest atom to be in stable phase. Inorganic clathrates can be synthesized by direct reaction using [[ball mill]]ing at high temperatures or high pressures. [[Crystallization]] from melt is another common synthesis route. Due to the wide variety of composition of host and guest species, inorganic clathrates are much more chemically diverse and possess a wide range of properties. Most notably, inorganic clathrates can be found to be both an insulator and a superconductor (Ba<sub>8</sub>Si<sub>46</sub>). A common property of inorganic clathrates that has attracted researchers is low [[thermal conductivity]]. Low thermal conductivity is attributed to the ability of the guest atom to "rattle" within the host framework. The freedom of movement of the guest atoms scatters [[phonon]]s that transport heat.<ref name=":0" />[[File:Na8Si46 inorganic clathrate structure with coordination polyhedra.png|thumb|Crystal structure of Na<sub>8</sub>Si<sub>46</sub>. Example of a Type I clathrate consisting of dodecahedral (orange) and tetrakaidecahedral (yellow) silicon cavities containing sodium atoms.<ref name=":0" />]]

== Examples ==
[[File:Xenon-paraquinol (JAMKEN) clathrate.png|thumb|Portion of the lattice of the clathrate xenon-paraquinol.<ref>{{cite journal |doi=10.1107/S0108270188014556|title=Β-Hydroquinone xenon clathrate|year=1989|last1=Birchall|first1=T.|last2=Frampton|first2=C. S.|last3=Schrobilgen|first3=G. J.|last4=Valsdóttir|first4=J.|journal=Acta Crystallographica Section C Crystal Structure Communications|volume=45|issue=6|pages=944–946|bibcode=1989AcCrC..45..944B }}</ref>]]Clathrates have been explored for many applications including: gas storage, gas production, gas separation, [[desalination]], [[Thermoelectric materials|thermoelectrics]], [[photovoltaics]], and batteries.
* Clathrate compounds with formula ''A''<sub>8</sub>''B''<sub>16</sub>''X''<sub>30</sub>, where ''A'' is an [[alkaline earth metal]], ''B'' is a [[Boron group|group III]] element, and ''X'' is an element from [[Carbon group|group IV]] have been explored for thermoelectric devices. Thermoelectric materials follow a design strategy called the ''phonon glass electron crystal'' concept.<ref>{{Cite journal |last1=Nolas |first1=G. S. |last2=Cohn |first2=J. L. |last3=Slack |first3=G. A. |last4=Schujman |first4=S. B. |date=1998-07-13 |title=Semiconducting Ge clathrates: Promising candidates for thermoelectric applications |journal=Applied Physics Letters |language=en |volume=73 |issue=2 |pages=178–180 |doi=10.1063/1.121747 |bibcode=1998ApPhL..73..178N |issn=0003-6951|doi-access=free }}</ref><ref>{{cite journal | vauthors=((Beekman, M.)), ((Morelli, D. T.)), ((Nolas, G. S.)) | journal=Nature Materials | title=Better thermoelectrics through glass-like crystals | volume=14 | issue=12 | pages=1182–1185 | date=2015 | issn=1476-4660 | doi=10.1038/nmat4461| pmid=26585077 | bibcode=2015NatMa..14.1182B }}</ref> Low [[thermal conductivity]] and high electrical conductivity is desired to produce the [[Thermoelectric effect|Seebeck Effect]]. When the guest and host framework are appropriately tuned, clathrates can exhibit low thermal conductivity, i.e., ''phonon glass'' behavior, while electrical conductivity through the host framework is undisturbed allowing clathrates to exhibit ''electron crystal''. 
* [[Methane clathrate]]s feature the hydrogen-bonded framework contributed by water and the guest molecules of methane.  Large amounts of [[methane]] naturally frozen in this form exist both in [[permafrost]] formations and under the ocean sea-bed.<ref>{{cite news|issue = 2714|url-status = dead|url = https://www.newscientist.com/article/mg20227141-100-ice-on-fire-the-next-fossil-fuel/|title = Ice on fire: The next fossil fuel|last = Pearce|first = Fred|date = 27 June 2009|work = [[New Scientist]]|pages = 30–33|accessdate = July 5, 2009|archive-date = April 13, 2016|archive-url = https://web.archive.org/web/20160413101737/https://www.newscientist.com/article/mg20227141-100-ice-on-fire-the-next-fossil-fuel/}}</ref> Other hydrogen-bonded networks are derived from [[hydroquinone]], [[urea]], and [[thiourea]].  A much studied host molecule is [[Dianin's compound]].
[[File:Cd(CN)2CCl4.jpg|thumb|Cd(CN)<sub>2</sub>&middot;CCl<sub>4</sub>: [[Cadmium cyanide]] clathrate framework (in blue) containing [[carbon tetrachloride]] (C atoms in gray and disordered Cl positions in green) as [[Host–guest chemistry|guest]].]]
* [[Hofmann clathrates]] are [[coordination polymer]]s with the formula Ni(CN)<sub>4</sub>·Ni(NH<sub>3</sub>)<sub>2</sub>(arene).  These materials crystallize with small aromatic guests (benzene, certain xylenes), and this selectivity has been exploited commercially for the separation of these hydrocarbons.<ref name=Ullmann/> [[Metal organic framework]]s (MOFs) form clathrates.
[[File:IRMOF-1 wiki.png|thumb|[[MOF-5]], an example of a [[metal organic framework]]: the yellow sphere represents the guest cavity.]]

== See also ==
* [[Molecular tweezers]]
* [[Clathrate hydrate]]

== References ==
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{{commons category|Clathrates}}

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[[Category:Clathrates| ]]