A
perovskite structure is any material with the same type of
crystal structureIn mineralogy and crystallography, a crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
as calcium titanium oxide (CaTiO
3), known as the
perovskite structure, or
XIIA
2+VIIB
4+X
2-3 with the oxygen in a
fccThe cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
. Perovskites take their name from this compound, which was first discovered in the
UralThe Ural Mountains are a mountain range that runs roughly north-south through western Russia. They are usually considered the natural boundary between Europe and Asia....
mountains of
RussiaRussia , officially known as both Russia and the Russian Federation , is a country in northern Eurasia . It is a semi-presidential republic, comprising 83 federal subjects...
by Gustav Rose in 1839 and is named after Russian mineralogist, L.
A
perovskite structure is any material with the same type of
crystal structureIn mineralogy and crystallography, a crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
as calcium titanium oxide (CaTiO
3), known as the
perovskite structure, or
XIIA
2+VIIB
4+X
2-3 with the oxygen in a
fccThe cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
. Perovskites take their name from this compound, which was first discovered in the
UralThe Ural Mountains are a mountain range that runs roughly north-south through western Russia. They are usually considered the natural boundary between Europe and Asia....
mountains of
RussiaRussia , officially known as both Russia and the Russian Federation , is a country in northern Eurasia . It is a semi-presidential republic, comprising 83 federal subjects...
by Gustav Rose in 1839 and is named after Russian mineralogist, L. A. Perovski (1792-1856). The general chemical formula for perovskite compounds is ABX
3, 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
octahedronIn geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex.It is a 3-dimensional cross polytope.-Dimensions:...
of anions, and the A cation in 12-fold
cuboctahedralIn geometry, a cuboctahedron is a polyhedron with eight triangular faces and six square faces. A cuboctahedron has 12 identical vertices, with two triangles and two squares meeting at each, and 24 identical edges, each separating a triangle from a square. As such it is a quasiregular polyhedron,...
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
perovskiteA perovskite structure is any material with the same type of crystal structure as calcium titanium oxide , known as the perovskite structure, or
XIIA
2+VIIB
4+X
2-3 with the oxygen in a fcc...
, loparite, and silicate perovskite.
Structure
The perovskite structure is adopted by many
oxideAn oxide is a chemical compound containing at least one oxygen atom as well as at least one other element. Most of the Earth's crust consists of oxides. Oxides result when elements are oxidized by oxygen in air. Combustion of hydrocarbons affords the two principal oxides of carbon, carbon...
s that have the chemical formula ABO
3.
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 BO
6 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
ferroelectricityFerroelectricity is a spontaneous electric polarization of a material that can be reversed by the application of an external electric field....
and shown by perovskites such as BaTiO
3 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 O
2- ions compress so
olivineThe mineral olivine is a magnesium iron silicate with the formula
2SiO
4...
adopts a
spinelThe spinels are any of a class of minerals of general formulation A
2+B
23+O
42- which crystallise in the cubic crystal system, with the oxide anions arranged in a cubic close-packed lattice and the cations A and B occupying some or all of the...
structure, then a perovskite structure and then a
periclasePericlase occurs naturally in contact metamorphic rocks and is a major component of most basic refractory bricks. It is a cubic form of magnesium oxide ....
structure.
At the high pressure conditions of the Earth's
lower mantleThe mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is about 2,970 km thick rocky shell that...
, the
pyroxeneThe pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
enstatiteEnstatite is the magnesium endmember of the pyroxene silicate mineral series enstatite - ferrosilite . The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks...
, MgSiO
3, transforms into a denser perovskite-structured
polymorphPolymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to elemental solids...
; 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, MgSiO
3 perovskite transforms to
post-perovskitePost-perovskite is a high-pressure phase of magnesium silicate . It is composed of the prime oxide constituents of the Earth's rocky mantle , and its pressure and temperature for stability imply that it is likely to occur in portions of the lowermost few hundred km of Earth's mantle...
.
Although the most common perovskite compounds contain oxygen, there are a few perovskite compounds that form without oxygen. Fluoride perovskites such as NaMgF
3 are well known. A large family of metallic perovskite compounds can be represented by RT
3M (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. RPd
3B, RRh
3B and CeRu
3C are examples. MgCNi
3 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 Cs
3AuO, which contain large alkali cations in the traditional "anion" sites, bonded to O
2- and Au
1- anions.
Material Properties
Perovskite materials exhibit many interesting and intriguing properties from both the theoretical and the application point of view.
Colossal magnetoresistanceColossal magnetoresistance is a property of some materials, mostly manganese-based perovskite oxides, that enables them to dramatically change their electrical resistance in the presence of a magnetic field....
,
ferroelectricityFerroelectricity is a spontaneous electric polarization of a material that can be reversed by the application of an external electric field....
,
superconductivitySuperconductivity is a phenomenon occurring in certain materials generally at very low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field . It was discovered by Heike Kamerlingh Onnes in 1911. Like ferromagnetism and atomic spectral...
,
charge orderingCharge ordering is second order phase transition that occurs when atoms with different oxidation states form an ordered superlattice .-Examples of 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
spintronicsSpintronics , also known as magnetoelectronics, is an emerging technology that exploits the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices.-History:The research field of Spintronics emerged from experiments...
applications.
Many superconducting
ceramicA ceramic is an inorganic, non-metallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
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 includes
Yttrium barium copper oxideYttrium barium copper oxide, often abbreviated YBCO, is a crystalline chemical compound with the formula YBa
2Cu
3O
7...
which can be insulating or superconducting depending on the oxygen content.
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