Perovskite (structure)

File:Perovskite.jpg

Structure of a perovskite with a chemical formula ABX₃. The red spheres are X atoms (usually oxygens), the blue spheres are B-atoms (a smaller metal cation, such as Ti⁴⁺), and the green spheres are the A-atoms (a larger metal cation, such as Ca²⁺). Pictured is the undistorted cubic structure; the symmetry is lowered to orthorhombic, tetragonal or trigonal in many perovskites.

A perovskite structure is any material with the same type of crystal structure as calcium titanium oxide (CaTiO₃), known as the perovskite structure, or ^XIIA^2+VIB⁴⁺X²⁻₃ with the oxygen in the face centers. 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.

Natural compounds with this structure are perovskite, loparite, and silicate perovskite.

Structure

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.

Common occurrence

As pressure increases, the O²⁻ ions compress so olivine transforms to the spinel structure, which at still higher pressure converts to a perovskite structure and a coexisting periclase structure.

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. This phase has the orthorhombically distorted perovskite structure (GdFeO₃-type structure) that is stable at pressures from ~24 GPa to ~110 GPa. However, it cannot be transported from depths of several hundred km to the Earth's surface without transforming back into less dense materials. At higher pressures, MgSiO₃ perovskite transforms to post-perovskite.

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 O²⁻ and Au⁻ anions.

Material properties

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.

Many superconducting ceramic materials (the high temperature superconductors) have perovskite-like structures, often with 3 or more metals including copper, and some oxygen positions left vacant. One prime example is yttrium barium copper oxide which can be insulating or superconducting depending on the oxygen content.

Chemical engineers are considering this material as a replacement for platinum in catalytic converters in diesel vehicles.

See also

• spinel
• Diamond anvil
• Post-perovskite

References

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External links

• "Cubic Perovskite Structure", including a Java applet where the structure can be interactively rotated
• perovskites at NCSUca:Perovskita