Clathrate compound

A clathrate or clathrate compound is a type of inclusion compound in which a guest atom, ion, or molecule is enclosed in a cage-like cavity formed by a host molecule or by an extended host lattice. The term is derived from the Latin , meaning 'with bars' or 'latticed'. According to the 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".

Clathrates occur in several areas of chemistry and materials science. In clathrate hydrates, water molecules form hydrogen-bonded cages that can trap small gases such as methane, carbon dioxide, nitrogen, or hydrogen sulfide. In organic and supramolecular chemistry, hosts such as hydroquinone, urea, thiourea, cyclodextrins, calixarenes, and Dianin's compound can form inclusion compounds with suitable guest molecules. In solid-state chemistry, inorganic clathrates are extended covalent frameworks, commonly based on group 14 elements such as silicon, germanium, or tin, that enclose guest atoms in polyhedral cages.

The properties of a clathrate depend on both the host framework and the guest species. Guest atoms or molecules may stabilize a framework, occupy only some cages, influence phase stability, scatter phonons, or provide charge balance. Clathrates are therefore relevant to host–guest chemistry, methane hydrates, gas storage and separation, thermoelectric materials, superconductivity, and synthetic phases formed under unusual or extreme conditions. Earlier substances now understood as clathrate hydrates were known before their structures were established; chlorine hydrate, for example, had been described by Humphry Davy and Michael Faraday in the 19th century and was later recognized as a gas hydrate.

Structure and types

Clathrates consist of a host framework and guest species. The host framework defines cavities or cages, while the guest species occupy those cavities. In many clathrates the guest is not strongly bonded to the framework, but interacts through van der Waals forces, hydrogen bonding, electrostatic interactions, or weak covalent interactions. The presence, size, and occupancy of guest species can determine whether a clathrate framework is stable.

The cages of many clathrate structures are described by the number and type of faces in their coordination polyhedra. Common cage types include 5¹², a cage with twelve pentagonal faces; 5¹²6², with twelve pentagonal and two hexagonal faces; and 5¹²6⁴, with twelve pentagonal and four hexagonal faces.

Many inorganic clathrates are Zintl or Zintl-like phases. A common type-I inorganic clathrate has the idealized formula A₈E₄₆, where A is a guest atom and E is a framework element such as silicon, germanium, or tin.

A notable feature of many inorganic clathrates is low lattice thermal conductivity. This is often associated with the motion of guest atoms inside oversized framework cages, sometimes described as "rattling". Such motion can scatter phonons while leaving electronic transport through the framework relatively less affected, making some inorganic clathrates candidates for thermoelectric materials.

Formation and stability

The conditions under which a clathrate forms depend on both the thermodynamics of the host–guest system and the kinetics of nucleation and crystal growth. In clathrate hydrates, stability is commonly represented by pressure–temperature phase boundaries that vary with guest composition, salinity, and the presence of thermodynamic promoters or inhibitors.

Natural and extreme-condition occurrence

Clathrate phases are discussed in planetary science and astrochemistry because water ice and volatile molecules are common in the outer Solar System and in cold astrophysical environments.

A calcium–copper–silicon type-I inorganic clathrate has been identified in red trinitite formed during the 1945 Trinity nuclear test.

Applications and research

Clathrates have been explored for gas storage, gas separation, carbon dioxide capture and sequestration, desalination, refrigeration and cooling, thermoelectric energy conversion, photovoltaics, batteries, and superconducting materials.