Superstructures of perovskite

Numerous structures are found for compounds ABO4, many of which are polymorphic at atmospheric pressure and also adopt different structures at higher pressures. These compounds range from those with silica-like structures, with tetrahedral coordination of A and B to oxy-salts containing well-defined oxy-ions, BO4, and high coordination number of A. Some of these structures are listed in Table 13.8, and we comment here only on a number of general points. (For uranates CaU04 etc. see Chapter 28.) 

Some series of compounds illustrate the change in environment of A with its size, as in chromates of divalent metals  

The statistical rutile structure is the normal form of many M M 04 compounds, for example  

The structure of AlNb04 (and the isostructural GaNb04) is mentioned later (p. 504). 

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This account of these structures has been based on the mode of packing of the c.p. layers. For the alternative description in terms of the way in which the octahedral BX groups are joined together by sharing X atoms, see Chapter 5. The structures based on cubic closest-packing are normally illustrated and described in terms of the cubic unit cell. We noted earlier that the cryolite structure is a superstructure of perovskite the relation between the perovskite, cryolite, and K2PtQg structures is described in Chapter 10 (p. 388). 

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