Clathrate compound

A clathrate is a unique chemical substance defined by its structure. It consists of a lattice that traps or contains other molecules. The name comes from the Latin word "clathrum," which means "with bars" or "latticed."

This structure is often described as a host-guest system. The host is the large framework that forms the cage. The guest is the smaller molecule trapped inside that cage. According to IUPAC, these are inclusion compounds where a guest molecule resides within a cage. These cages can be formed by a single host molecule or a large lattice of host molecules.

There are two primary categories of clathrates: clathrate hydrates and inorganic clathrates. Each type relies on a specific framework to hold its guests. In clathrate hydrates, the framework is built from water molecules. These water molecules are held together by hydrogen bonds.

Methane clathrates are a famous example of this type. In these, methane molecules are trapped within the water framework. These can be found naturally in permafrost or under the ocean sea-bed. Other hydrogen-bonded networks can be formed using substances like urea, thiourea, or hydroquinone.

Inorganic clathrates work differently than hydrates. Instead of hydrogen bonds, they use a covalently bonded framework of inorganic atoms. This covalent bonding is much stronger than the bonds in hydrates. Because of this strength, the cages in inorganic clathrates are often smaller. The guest atoms in these structures are usually alkali or alkaline earth metals. These guest atoms interact with the host through ionic or covalent bonds. To keep the overall charge of the compound conserved, the substitution of guest atoms follows Zintl rules.

Scientists use different methods to create inorganic clathrates. One common way is through direct reaction using ball milling. This process requires high temperatures or high pressures to work. Another method is crystallization from a melt. Because the host and guest species can vary so much, inorganic clathrates are chemically diverse. They can possess a wide range of physical properties. For instance, some inorganic clathrates can act as insulators. Others, such as Ba8Si46, can function as superconductors.

One of the most interesting properties of inorganic clathrates is low thermal conductivity. This means they do not conduct heat very well. This happens because of the way the guest atoms move inside the cages. The guest atoms can "rattle" within the host framework. This freedom of movement allows the atoms to scatter phonons. Phonons are the particles or waves that transport heat through a material.

By scattering these phonons, the guest atoms effectively slow down the flow of heat.

This ability to control heat and electricity is useful for thermoelectric devices. Researchers use a design strategy called the phonon glass electron crystal concept. In this concept, scientists want a material to behave like a glass for phonons and a crystal for electrons. They aim for low thermal conductivity and high electrical conductivity. This combination is necessary to produce the Seebeck Effect. To achieve this, the guest and host framework must be appropriately tuned. Some clathrates used for this purpose have the formula A8B16X30. In this formula, A is an alkaline earth metal, B is a group III element, and X is a group IV element.

Clathrates are also used for many practical industrial applications. They are explored for gas storage, gas production, and gas separation. They can also be used in desalination, batteries, and photovoltaics. Hofmann clathrates are a notable example used in industry. These are coordination polymers with the formula Ni(CN)4·Ni(NH3)2(arene). They can crystallize with small aromatic guests like benzene or certain xylenes. This specific selectivity allows them to be used commercially for the separation of these hydrocarbons.

Even metal organic frameworks, known as MOFs, can form clathrates.