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Perovskites and their Potential use in Solar Energy Applications

1. Introduction A material may be described as having a perovskite structure1 if it has same type of crystal structure as perovskite--calcium titanium oxide (CaTi[O.sub.3])--does (Figure 1). Perovskite was first discovered in 1839 by Gustav Rose, in the Ural mountains in Russia, and is named after the Russian mineralogist L. A. Perovski (1792-1856), who first characterised the material. The general formula for perovskites is [ABX.sub.3], with 'A' and 'B' being two cations of significantly different sizes ('A'>'B'), while X is an anion that binds with both cations. The perovskite structure is adopted by many oxides that have an elemental composition: AB[O.sub.3]. The ideal cubic-structure has the 'B' cation in a 6-fold coordination, surrounded by an octahedron of anions, with the 'A' cation in a 12-fold cuboctahedral coordination. Cations 'A' occupy the cube corner positions (0, 0, 0), while cations 'B' occupy the body centred positions (1/2, 1/2, 1/2) with oxygen anions 'O' being located at face centred positions (1/2, 1/2, 0). Figure 1 shows edges for an equivalent unit cell with 'A' in body centre, 'B' at the comers, and 'O' in mid-edge. The requirements of relative ionic radii are quite exacting to maintain a stable cubic structure, meaning that even relatively minor degrees of buckling and distortion can result in a number of alternative versions with lower symmetry, in which the coordination numbers of either the 'A' cations, 'B' cations, or both, are reduced. Tilting of the B06 octahedron reduces the coordination of a too-small 'A' cation from 12 down to as low as 8. Conversely, when a small 'B' cation is brought off-centre, within its octahedral coordination, a stable bonding arrangement can be obtained. Such distortions can create an electric dipole and it is for this reason that perovskites such as BaTi03, which distort in this manner, exhibit the property of ferroelectricity. The most usual non-cubic forms of perovskites are the orthorhombic and tetragonal variants. There are also some more complex perovskite structures which contain two different 'B'-site cations, with the result that ordered and disordered variants are possible. [FIGURE 1 OMITTED] Under the high pressure conditions of the Earth's lower mantle, the pyroxene enstatite, MgSi[O.sub.3], is converted to a more dense perovskite-type polymorph, and indeed it is speculated that this particular phase of the material might be the most common mineral in the Earth (2). It has a perovskite structure, with an orthorhombic distortion, and is stable at pressures from ~24 GPa to ~110 GPa. [For comparison, the pressure at the centre of the Earth is ca 300 GPa.] However, it is stable only at depths of several hundred kilometres and could not be transported back to the Earth's surface without reforming into less dense materials. At yet greater pressures, MgSi[O.sub.3] perovskite undergoes a transformation to form post-perovskite. Although the most common perovskite compounds contain oxygen, perovskites containing fluoride anions are known, e.g. NaMg[F.sub.3]. Metallic perovskite compounds also exist (1), with the general formula [RT.sub.3]M, where R represents a rare-earth or other relatively large cation, T is a transition metal ion and M represents light metalloids (anions) which occupy the octahedrally coordinated 'B' sites, e.g. [RPd.sub.3]B, [RRh.sub.3]B and Ce[Ru.sub.3]C. MgC[Ni.sub.3] is a metallic perovskite compound, and is of particular interest on account of its superconducting properties. A further category has mixed oxide-aurides of Cs and Rb, such as [Cs.sub.3]AuO, which contain large alkali metal cations in the traditional "anion" sites, bonded to [O.sup.2-] and [Au.sup.-] anions. 2. Properties of perovskites As noted, the perovskite structure is imparted with an appreciable element of structural pliancy, and the ideal cubic structure (Figure 1) can be distorted in many different ways. Thus, the octahedra may become tilted, the cations can be displaced from the centres of their coordination polyhedra, and the octahedra might be distorted at the behest of electronic factors (e. …