Chemical substance consisting of a lattice that traps or contains molecules
Aclathrate is achemical substance consisting of alattice that traps or contains molecules. The wordclathrate is derived from theLatinclathratus (clatratus), meaning 'with bars,latticed'.[1] Most clathrate compounds arepolymeric and completely envelop the guest molecule, but in modern usage clathrates also includehost–guest complexes andinclusion compounds.[2] According toIUPAC, 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."[3] The term refers to many molecular hosts, includingcalixarenes andcyclodextrins and even some inorganic polymers such aszeolites.
Clathrate cavities. For example, 512 (dodecahedral) and 51262 (tetrakaidecahedral) make up a Type I (sI) structure.[4]
Clathrates can be divided into two categories:clathrate hydrates 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 asdodecahedral,tetrakaidecahedral, andhexakaidecahedral cavities.Unlike hydrates,inorganic clathrates have acovalently bonded framework of inorganic atoms with guests typically consisting ofalkali oralkaline earth metals. 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 followZintl 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 usingball milling 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 (Ba8Si46). A common property of inorganic clathrates that has attracted researchers is lowthermal 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 scattersphonons that transport heat.[4]
Crystal structure of Na8Si46. Example of a Type I clathrate consisting of dodecahedral (orange) and tetrakaidecahedral (yellow) silicon cavities containing sodium atoms.[4]
Portion of the lattice of the clathrate xenon-paraquinol.[5]
Clathrates have been explored for many applications including: gas storage, gas production, gas separation,desalination,thermoelectrics,photovoltaics, and batteries.
Clathrate compounds with formulaA8B16X30, whereA is analkaline earth metal,B is agroup III element, andX is an element fromgroup IV have been explored for thermoelectric devices. Thermoelectric materials follow a design strategy called thephonon glass electron crystal concept.[6][7] Lowthermal conductivity and high electrical conductivity is desired to produce theSeebeck 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 exhibitelectron crystal.
Methane clathrates feature the hydrogen-bonded framework contributed by water and the guest molecules of methane. Large amounts ofmethane naturally frozen in this form exist both inpermafrost formations and under the ocean sea-bed.[8] Other hydrogen-bonded networks are derived fromhydroquinone,urea, andthiourea. A much studied host molecule isDianin's compound.
Hofmann clathrates arecoordination polymers with the formula Ni(CN)4·Ni(NH3)2(arene). These materials crystallize with small aromatic guests (benzene, certain xylenes), and this selectivity has been exploited commercially for the separation of these hydrocarbons.[2]Metal organic frameworks (MOFs) form clathrates.
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