Editing Clathrate hydrate (section)

== Structure ==
[[Image:HydrateStructures.jpg|thumb|Cages building the different gas hydrate structures]]

Gas hydrates usually form two [[Crystallography|crystallographic]] cubic structures: structure (Type) I (named ''sI'') and structure (Type) II (named ''sII'')<ref>{{cite journal |last1=Stackelberg |first1=M. v |last2=Müller |first2=H. R. |title=Feste Gashydrate II. Struktur und Raumchemie |trans-title=Solid gas hydrates II. Structure and space chemistry |language=de |journal=Zeitschrift für Elektrochemie, Berichte der Bunsengesellschaft für physikalische Chemie |date=1954 |volume=58 |issue=1 |pages=25–39 |doi=10.1002/bbpc.19540580105 |s2cid=93862670 |url=https://onlinelibrary.wiley.com/doi/10.1002/bbpc.19540580105 }}</ref> of space groups <math>Pm\overline{3}n</math>  and <math>Fd\overline{3}m</math> respectively. A third hexagonal structure of space group <math>P6/mmm</math> may also be observed (Type H).<ref>{{cite book |last1=Sloan |first1=E. Dendy |last2=Koh |first2=Carolyn A. |date=2008|title=Clathrate hydrates of natural gases |publisher=CRC Press |orig-date=1st pub. 1998 |pages=45 |chapter=Chapter 2. Molecular Structures and Similarities to Ice |isbn=978-0-8493-9078-4 |name-list-style=amp}}</ref>

The unit cell of Type I consists of 46 water molecules, forming two types of cages – small and large. The unit cell contains two small cages and six large ones. The small cage has the shape of a pentagonal [[dodecahedron]] (5<sup>12</sup>) (which is not a regular dodecahedron) and the large one that of a [[tetradecahedron]], specifically a [[hexagonal truncated trapezohedron]] (5<sup>12</sup>6<sup>2</sup>).  Together, they form a version of the [[Weaire–Phelan structure]]. Typical guests forming Type I hydrates are [[carbon dioxide|CO<sub>2</sub>]] in [[carbon dioxide clathrate]] and [[methane|CH<sub>4</sub>]] in [[methane clathrate]].

The unit cell of Type II consists of 136 water molecules, again forming two types of cages – small and large. In this case there are sixteen small cages and eight large ones in the unit cell.  The small cage again has the shape of a pentagonal dodecahedron (5<sup>12</sup>), but the large one is a [[hexadecahedron]] (5<sup>12</sup>6<sup>4</sup>). Type II hydrates are formed by gases like O<sub>2</sub> and N<sub>2</sub>.

The unit cell of Type H consists of 34 water molecules, forming three types of cages – two small ones of different types, and one "huge". In this case, the unit cell consists of three small cages of type 5<sup>12</sup>, two small ones of type 4<sup>3</sup>5<sup>6</sup>6<sup>3</sup> and one huge of type 5<sup>12</sup>6<sup>8</sup>. The formation of Type H requires the cooperation of two guest gases (large and small) to be stable. It is the large cavity that allows structure H hydrates to fit in large molecules (e.g. [[butane]], [[hydrocarbon]]s), given the presence of other smaller help gases to fill and support the remaining cavities. Structure H hydrates were suggested to exist in the Gulf of Mexico. Thermogenically produced supplies of heavy hydrocarbons are common there.

The molar fraction of [[water]] of most clathrate hydrates is 85%. Clathrate hydrates are derived from organic [[hydrogen-bond]]ed frameworks. These frameworks are prepared from molecules that "self-associate" by multiple hydrogen-bonding interactions. Small molecules or gases (i.e. [[methane]], [[carbon dioxide]], [[hydrogen]]) can be encaged as a guest in hydrates. The ideal guest/host ratio for clathrate hydrates range from 0.8 to 0.9. The guest interaction with the host is limited to [[Van der Waals force|van der Waals]] forces. Certain exceptions exist in ''semiclathrates'' where guests incorporate into the host structure via [[hydrogen bonding]] with the host structure. Hydrates form often with partial guest filling and collapse in the absence of guests occupying the water cages. Like ice, clathrate hydrates are stable at low temperatures and high pressure and possess similar properties like electrical resistivity. Clathrate hydrates are naturally occurring and can be found in the [[permafrost]] and oceanic sediments. Hydrates can also be synthesized through seed crystallization or using amorphous precursors for nucleation.<ref name=":0" />

[[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.