Editing Clathrate compound (section)

{{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" />]]