SrTiOj perovskite structure

Fig. 7.33 ta) Unit cell of the 1-2-3 superconductor, orthorhombic, space group Pmmm. Qne-dimensional CuOi chain.s run along the h axis, and two-dimensional CuO layers lie in the ah plane, (bl The cubic structure of perovskite. SrTiOj. Three unit cells are shown slacked vertically, (c) The unit cell of the 1-2-3 superconductor in the context of the surrounding crystal. Copper atoms arc surrounded either by five oxygen atoms in a square pyramid or four oxygen atoms in a square plane. [From Holland, G. F. Stacy, A. M. Acc. Cliem. Res. 1988. 2J, 8-15. Reproduced with permission.)... 

Many perovskites are known with deficiency in the A-site populations. In the extreme case the A-site can be empty if the B-site cation has a charge of 6-h. The simplest of these perovskite-related structures is that of rhenium trioxide, ReOj, a metallic conductor, with an apex-sharing array of ReO octahedra identical in disposition to the TiO octahedra in SrTiOj (Figure 2.5). 

Viewed as a layer stacking, the Ba and O " positions in the BaNiO, structure are identical to that of the Au and Cu positions in the alloy CUjAu and the S1O3 layers in SrTiOj (Figure 1.2a and b). In BaNiOj these layers are stacked in a two-layer hexagonal close-packed sequence. ..ABAB..., also designated as h (Sections 3.3 and 3.4). As with the cubic perovskite phase, the charge balance needed to maintain neutrality in this skeleton structure is provided by an ordered distribution of the Ni ions in the available octahedral interstices that are bounded by ions only (Figure 1.3). 

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