Perovskite oxides lattice distortions

Several perovskite compounds have been proposed as pigments (Table 12.1). They are both oxide and non-oxide phases with crystal symmetry spanning from ideal cubic perovskite to increasingly distorted hexagonal, tetragonal, and orthorhombic types, down to peculiar cases of monoclinic and triclinic lattices. 

For an ideal perovskite, t is unity however, the perovskite structure is also found for lower t-values (0.75 < t < 1). In such cases, the structure distorts to tetragonal, rhombohedral or other lower symmetries. Indeed, many perovskite oxides are polymorphs. In ideal perovskite structure, the atoms are in the following positions A 000, B A Vi Vi X Vi Vi 0, Vi 0 Vi, 0 Vi Vi. Moreover, the above given geometric relations are not a sufficient condition for the stabilization of the perovskite lattice, since the A and B cations must, in themselves, be stable in twelvefold and sixfold eoordination respectively. This latter condition sets lower limits for the cationie radii. In oxide systems these limits are rA > 0.90 A and re > 0.51 A [19]. 

The ideal perovskite stracture (Figure 3.32) is related to that of ReOj and may be thought of as a cep array of o.xide ions with one fourth of the oxygens missing. Ti occupies one fourth of the octahedral sites with Ca located in the oxide ion vacancy. The TiO octahedra share comers to give the characteristic three-dimensional framework. This gives a CN of 12 for Ca however, distortions of the lattice reduce this to 8. 

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