1
The ABX3 Perovskite Structure
1.1 Perovskites
Perovskite is a mineral of formula CaTiO3. It was discovered in 1839 by the Prussian mineralogist Gustav Rose in mineral deposits in the Ural Mountains and named after the Russian mineralogist Count Lev Aleksevich von Petrovski. Natural crystals have a hardness of 5.5–6 and a density of 4000–4300 kg m−3. They are usually dark brown to black, due to impurities, but when pure are clear with a refractive index of approximately 2.38. The crystal structure of this compound, initially thought to be cubic, was later shown to be orthorhombic (Table 1.1).
Table 1.1 Representative ABX3 perovskite phasesa
| Phase | Space groupb | Unit cell |
| AgMgF3 | C, Pm m (221) | 0.41162 | | |
| CsPbI3 | C, Pm m (221) | 0.62894 | | |
| KCuF3 | T, I4/mcm (140) | 0.56086 | | 0.76281 |
| KMgF3 | C, Pm m (221) | 0.39897 | | |
| NaMgF3 | O, Pbnm (62) | 0.48904 | 0.52022 | 0.71403 |
| NaFeF3 | O, Pnma (62) | 0.56612 | 0.78801 | 0.54836 |
| NH4ZnF3 | C, Pm m (221) | 0.41162 | | |
| KTaO3 | C, Pm m (221) | 0.40316 | | |
| KNbO3 | O, Amm2 (38) | 0.3971 | 0.5697 | 0.5723 |
| SrTiO3 | C, Pm m (221) | 0.3905 | | |
| BaTiO3 | T, P4mm (99) | 0.39906 | | 0.40278 |
| CaTiO3 | O, Pbmn (62) | 0.54035 | 0.54878 | 0.76626 |
| BaSnO3 | C, Pm m (221) | 0.4117 | | |
| CdSnO3 | O, Pnma (62) | 0.52856 | 0.74501 | 0.51927 |
| CaIrO3 | O, Pbnm (62) | 0.52505 | 0.55929 | 0.76769 |
| PbTiO3 | T, P4mm (99) | 0.3902 | | 0.4143 |
| PbZrO3 | O, Pbam (55) | 0.58822 | 1.17813 | 0.82293 |
| SrCoO3 | C, Pm m (221) | 0.3855 | | |
| SrMoO3 | C, Pm m (221) | 0.39761 | | |
| SrRuO3 | O, Pnma (62) | 0.55328 | 0.78471 | 0.55693 |
| (Fe,Mg)SiO3 | O, Pnma (62) | 0.5020 | 0.6900 | 0.4810 |
| BiFeO3 | Tr, R3c (161) | 0.55798 | | 1.3867 |
| BiInO3 | O, Pnma (62) | 0.59546 | 0.83864 | 0.50619 |
| ErCoO3 | O, Pbnm (62) | 0.51212 | 0.54191 | 0.73519 |
| GdFeO3 | O, Pbnm (62) | 0.53490 | 0.56089 | 0.76687 |
| HoCrO3 | O, Pnma (62) | 0.5518 | 0.7539 | 0.5245 |
| LaAlO3 | Tr, R3c (161) | 0.53644 | | 1.31195 |
| LaCoO3 | Tr, R c (167) | 0.54437 | | 1.30957 |
| LaMnO3 | O, Pbnm (62) | 0.55367 | 0.57473 | 0.76929 |
| LaTiO3 | O, Pbnm (62) | 0.5576 | 0.5542 | 0.7587 |
| NdAlO3 | Tr, R c (167) | 0.53796 | | 1.31386 |
| PrRuO3 | O, Pnma (62) | 0.58344 | 0.77477 | 0.53794 |
| YbMnO3 | O, Pbnm (62) | 0.52208 | 0.58033 | 0.73053 |
aMany of these phases are polymorphic, and lattice parameters vary with temperature and pressure.
bThe crystal system, here and throughout the other tables in this book, is abbreviated thus: C, cubic; H, hexagonal; M, monoclinic; O, orthorhombic; T, tetragonal; Tr, trigonal (often specified in terms of a hexagonal unit cell); Tri, triclinic.
As with many minerals, Perovskite has given its name to a family of compounds called perovskites, which have a general formula close to or derived from the composition ABX3. At present many hundreds of compounds are known that adopt the perovskite structure. In fact a perovskite structure mineral, Bridgmanite (Fe,Mg)SiO3, is the most abundant solid phase in the Earth’s interior, making up 38% of the total. The phase occurs between depths of approximately 660–2900 km but is only stable at high temperatures and pressures so that it is not found at the surface of the Earth.
To some extent the multiplicity of phases that belong to the perovskite family can be rationalised by assuming that perovskites are simple ionic compounds, where A is usually a large cation, B is usually a medium-sized cation and X is an anion. Naturally the overall ionic structure must be electrically neutral. If the charges on the ions are written as qA, qB and qX, then
Frequently encountered (but not exclusive) combinations are
The importance of perovskites became apparent with the discovery of the valuable dielectric and ferroelectric properties of barium titanate, BaTiO3, in the 1940s. This material was rapidly employed in electronics in the form of capacitors and transducers. In the decades that followed, attempts to improve the material properties of BaTiO3 lead to intensive research on the structure – property relations of a large number of nominally ionic ceramic perovskite-related phases with overall compositions ABO3, with a result that vast numbers of new phases were synthesised.
It was soon realised that, as a group, these materials possessed very useful physical and chemical properties far broader than those shown by BaTiO3, and research widened to include a range of structures and phases that could all be related structurally to the perovskite family, including nominally ionic nitrides and oxynitrides. In addition, a number of materials which are better described as alloys, of formula A3BX, where A and B are metals and X is an anion or semimetal, typically C, N, O and B are known. These are often said to adopt the so-called antiperovskite or inverse perovskite structure, because the metal A atoms occupy the positions corresponding to the anions in the ionic perovskites and the B and X atoms occupy sites corresponding to those occupied by the cations. The flexibility of the perovskite framework also allows it to include cations such as NH4+, which can often be considered to be spherical at normal temperatures. More complex phases, such as the inorganic–organic hybrid compounds (CH3NH3)PbX3, where X is typically Cl, Br, I or a combination of these anions, have also been synthesised.
As well as phases with an ABX3 composition, large numbers of modular structures have been prepared, all of which are built up, at least in part, from fragments, usually slabs, of perovskite-like structure. The formulae of these are not easily reconciled with a composition of ABX3 until the structural building principles have been found and the nature of the interfaces between the various slabs is clarified. For example, Bi2Ca2Sr2Cu3O10+δ, a superconducting oxide, is built from slabs of perovskite type separated by slabs of composition Bi2O2.
As would be expected, there is a close correlation between chemical and physical properties in these complex materials. It is this flexibility that makes the perovskites as a group, important, as the facile replacement of any of the atoms in this range of structures can be used to modify important physical properties in a controlled way. The flexibility comes at a...
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