Perovskite

A perovskite is any material with the same type of crystal structure as calcium titanium oxide (CaTiO₃, known as the "perovskite structure". Perovskites take their name from this compound, which was first discovered in the Ural mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist, L. A. Perovski (1792-1856). The general chemical formula for perovskite compounds is ABX₃, where 'A' and 'B' are two cations of very different sizes, and X is an anion that bonds to both. The 'A' atoms are larger than the 'B' atoms. The ideal cubic-symmetry structure has the B cation in 6-fold coordination, surrounded by an octahedron of anions, and the A cation in 12-fold cuboctahedral coordination. The relative ion size requirements for stability of the cubic structure are quite stringent, so slight buckling and distortion can produce several lower-symmetry distorted versions, in which the coordination numbers of A cations, B cations or both are reduced.

At the high pressure conditions of the Earth's lower mantle, the pyroxene enstatite, MgSiO₃, transforms into a denser perovskite-structured polymorph; this phase may be the most common mineral in the Earth. [citebook|title=QI: The Book of General Ignorance|last=John Lloyd|authorlink=John Lloyd (writer)|coauthors=John Mitchinson|chapter=What's the commonest material in the world|isbn=0-571-23368-6|publisher=Faber & Faber] . However, it cannot be transported from depths of several hundred km to the Earth's surface without transforming back into less dense materials.

Although the most common perovskite compounds contain oxygen, there are a few perovskite compounds that form without oxygen. Fluoride perovskites such as NaMgF₃ are well known. A large family of metallic perovskite compounds can be represented by RT₃M (R: rare-earth or other relatively large ion, T: transition metal ion and M: light metalloids). The metalloids occupy the octahedrally coordinated "B" sites in these compounds. RPd₃B, RRh₃B and CeRu₃C are examples. MgCNi₃ is a metallic perovskite compound and has received lot of attention because of its superconducting properties. An even more exotic type of perovskite is represented by the mixed oxide-aurides of Cs and Rb, such as Cs₃AuO, which contain large alkali cations in the traditional "anion" sites, bonded to O^2- and Au^1- anions.

Perovskite materials exhibit many interesting and intriguing properties from both the theoretical and the application point of view. Colossal magnetoresistance, ferroelectricity, superconductivity, charge ordering, spin dependent transport, high thermopower and the interplay of structural, magnetic and transport properties are commonly observed features in this family. These compounds are used as sensors and catalyst electrodes in certain types of fuel cells and are candidates for memory devices and spintronics applications .

tructure

The perovskite structure is adopted by many oxides that have the chemical formula ABO₃.

In the idealized cubic unit cell of such a compound, type 'A' atom sits at cube corner positions(0, 0, 0), type 'B' atom sits at body centre position (1/2, 1/2, 1/2) and oxygen atoms sit at face centred positions (1/2, 1/2, 0). (The diagram shows edges for an equivalent unit cell with B at the corners, A in body centre, and O in mid-edge).

The relative ion size requirements for stability of the cubic structure are quite stringent, so slight buckling and distortion can produce several lower-symmetry distorted versions, in which the coordination numbers of A cations, B cations or both are reduced. Tilting of the BO₆ octahedra reduces the coordination of an undersized A cation from 12 to as low as 8. Conversely, off-centering of an undersized B cation within its octahedron allows it to attain a stable bonding pattern. The resulting electric dipole is responsible for the property of ferroelectricity and shown by perovskites such as BaTiO₃ that distort in this fashion.

The orthorhombic and tetragonal phases are most common non-cubic variants.

Complex perovskite structures contain two different B-site cations. This results in the possibility of ordered and disordered variants.

Many superconducting ceramic materials (the high temperature superconductors) have perovskite-like structures, often with 3 or more metals including copper, and a some oxygen positions left vacant.

References

*Moty Schultz and Lior Klein, Physical Review B 73, 085109 (2006).
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External links

* [http://cst-www.nrl.navy.mil/lattice/struk/e2_1.html "Cubic Perovskite Structure"] , including a [http://cst-www.nrl.navy.mil/lattice/struk.jmol/e2_1.html Java applet where the structure can be interactively rotated]
* [http://wikis.lib.ncsu.edu/index.php/Perovskite perovskites at NCSU]

See also

* Diamond anvil
* List of minerals
* List of minerals named after people