Superstructures, Ordered Defects and Nonstoichiometry in Metal Oxides

The ideal perovskite (AB03) structure which is a simple cubic structure with the space group Pm3m, provides the basis for the structures of a large variety of inorganic solids. The perovskits structure is conventionally described as consisting of a B03 array 

Besides PTBs, A-site defective perovskite oxides are known to be formed when B = Ti. Nb.Ta and soon 13. Such compounds exhibit metallic properties and perovskite structures when the B atom occurs in a low oxidation stale. Compositions such as A0 5Nb03 (A = Ba. Pb etc.) where niobium is in the highest oxidation state adopt non-perovskite network structures. An interesting example20-21 of a A-site defective perovskite is Cu 5Ta03 which crystallizes in a pseudocubic perovskite structure. The unit cell is orthorhombic with a = 7.523, / = 7.525 and c = 7.520 A and eight formula units per cell. Tantalum atoms form 

B-site vacancies. B-site vacancies in perovskite ReOe octahedra share corners, and the vacancy occurs oxides are energetically not favoured because of the at octahedral sites between h-h layers (Fig. 4b). large formal charge and the small size of B-site cations. Somewhat exceptional to this general trend27,28,31 are 

Plate 1—Lattice image of a highly ordered CsxW03 ITB (from Ref. 22). The narrow HTB strip can be readily distinguished from the W03 slabs. The ITB phase corresponds to the (1,7) system containing two tunnel-wide HTB strips separated by 7 W06 octahedra. 

Plate 2—Lattice image of a disordered Cs.WO, ITB showing different W03 slab widths (from ref. 22). 

3 Tungsten bronze structures (a) tetragonal tungsten bronze (TTB). (b) hexagonal tungsten bronze (HTB) and (c) perovskite 

and ReO, may be regarded as the limiting cases of A-site vacancy perovskites. Both the oxides possess corner-linked framework of the octahedra, but unlike ReO, WO3 is never cubic. It shows several polymorphic transitions starting from the low temperature triclinic structure to more symmetric forms with increasing temperature. The transitions arise from temperature-dependent displacements of the tungsten atom from the centre of the WOg octahedron,  

B-site vacancies B-site vacancies in perovskite oxides are energetically not favoured because of the large formal charge and the small size of B-site cations. 

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Rao, C.N.R., Gopalakrishnan, J., and Vidyasagar, K. Superstructures, ordered defects and nonstoichiometry in metal-oxides of perovskite and related structure. Indian J. Chem. 1984, 23A, 265-284. 

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