Perovskite
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Perovskite is a class of materials that form in a cubic structure and in general contain oxygen as a major chemical component. Perovskite (CaTiO3, the first known example) was discovered in the Ural mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist, L. A. Perovski (1792-1856). CaTiO3 (calcium titanium oxide) can be taken as a classic example for this family of compounds. The representative chemical formula for perovskite compounds is ABO3. In the cubic unit cell of this 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 'A' atoms are larger than the 'B' atoms. The structure is stabilized by the 6-fold coordination of the B cation (octahedron) and 12-fold coordination of the A cation. Though the representative structure of perovskite compounds is cubic, the compounds in this family may possess some distortion. The orthorhombic and tetragonal phases are most common variants.
Formed under the high pressure conditions of the Earth's mantle, the pyroxene enstatite is a perovskite-structured polymorph of MgSiO3, and may be the most common mineral in the Earth.[1]. Although the most common perovskite compounds contain oxygen, there are a few perovskite compounds that form without oxygen. Metallic perovskite compounds belong to such a category. One of the family of such compounds can be represented by RT3M (R: rare-earth ion, T: transition metal ion and M: light metalloids). RPd3B, RRh3B and CeRu3C are members of this family. MgCNi3 is a metallic perovskite compound and has received lot of attention because of its superconducting properties.
Perovskite materials exhibit lot of interesting and intriguing properties both from theoretical point of view as well as application point of view. Colossal magnetoresistance, charge ordering, spin dependent transport, interplay of structural, magnetic and transport properties are commonly observed features in this family. These compounds are used as catalyst electrodes in certain types of fuel cells and are candidates for memory devices and spintronics applications .
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[edit] Structure
The perovskite structure is adopted by many oxides that have the chemical formula ABO3. The structure is very versatile having many useful technological applications such as ferroelectrics, catalysts, sensors, thermopower and superconductors.
Although the primitive cube is the idealized structure, differences in radius between the A and B cations can alter the structure to a number of different so-called distortions, of which tilting is the most common one. With perovskite tilt the BO6 octahedron twists along one or more axes to accommodate the difference.
Complex perovskite structures contain two different B-site cations. This results in ordered and disordered variants.
The perovskite structure shares the property of ferroelectricity with garnet and olivine.
Many superconducting ceramic materials (the high temperature superconductors) have perovskite-like structures often with 3 or more metals including copper, and a deficit of oxygen.
[edit] Occurrences
Perovskite is found in contact metamorphic rocks and associated mafic intrusives.
It is found in some silica-undersaturated igneous rocks, such as nepheline syenite and melilitite, and rare carbonatites.
Perovskite also is a common mineral in the Ca-Al-rich inclusions (CAIs) found in some chondritic meteorites.
[edit] See also
[edit] References
- ^ John Lloyd; John Mitchinson. "What's the commonest material in the world", QI: The Book of General Ignorance. Faber & Faber. ISBN 0-571-23368-6.
- Moty Schultz and Lior Klein, Physical Review B 73, 085109 (2006).
- Tejuca, Luis G (1993). Properties and applications of perovskite-type oxides. New York: Dekker, 382. ISBN 0-8247-8786-2.
- Mitchell, Roger H (2002). Perovskites modern and ancient. Thunder Bay, Ontario: Almaz Press, 318. ISBN 0-9689411-0-9.
