Perovskites sheets

Several examples are reported in Table IV. The bismuth compounds consist of single sheets of BiO square pyramids separated regularly by one or more perovskite sheets (Figure 9) (12). Elements of the pervoskite structure may also intergrow with NaCl-type sheets in the phases (ATi03).A0, and ordered phases are now known in a composition region once thought to contain a berthollide. 

A2B2Ti3xNbxOio K, Rb, Cs La Partial substitution of Ti with Nb leads to a decrease in the negative charge density of the perovskite sheets. 461... 

Halides Containing (llO)-Oriented Perovskite Sheets Science 1995, 267, 1473. 

Mitzi, D.B. Wang, S. Feild, C.A. Chess, C.A. Guloy, A.M. Conducting layered organic-inorganic halides containing (llO)-oriented perovskite sheets. Science 1995, 267, 1473. 

The other superconducting cuprate structures are constructed from perovskite-like layers separated by a variety of different linking regions. The constmction principle can be illustrated with reference to compounds in which the perovskite sheets are linked by Bip or P2 The formula of the Bip series is Bi Srpa jCup ... 

Figure 4.72 Oxygen vacancy ordering (slightly idealised) in each perovskite sheet in )3, oc...

From symmetry considerations, the ferroelectric layered perovskites, such as the Ruddlesden-Popper, Dion-Jacobson and Aurivillius phases in which the perovskite layers are an even number of octahedra in thickness, tend to have the spontaneous polarisation vector lying parallel to the perovskite sheets. In phases with an odd number of octahedra in thickness, the spontaneous polarisation can lie perpendicular to the sheets or at least have a component in this direction. Thus the n=2 Dion-Jacobson phase RbBiNbjO has the spontaneous polarisation vector lying parallel to the perovskite layers. This polarisation arises from the tilted NbOg octahedra and, more importantly, from the displaced lone pair Bi -" cations that are found between the perovskite layers. 

If the pair of A atoms at the boundaries of the perovskite-like sheets in the Ruddleston-Popper phases are replaced with just one A atom, the series of phases takes the formula A (A iB 03 +i), where A and A are large ions, typically a (+1/+3) pairing, and B is a medium-sized transition-metal ion, typically Nb5+. These materials are called Dion-Jacobson phases. The majority of examples synthesized to date are of the n = 2 phase, typified by KLaNb207, CsBiNb207, and so on (Fig. 4.29). A few examples of the n = 3 phase are also known, including CsCa2Nb3Oio-... 

Figure 4.34 Brownmillerite structure (a) ideal perovskite structure (circles indicate oxygen atoms that need to be removed to convert octahedra to tetrahedra in brownmillerite) and (b) idealized brownmillerite structure consisting of sheets of octahedra and tetrahedra.

Figure 1 Relationship between the perovskite structure ABOs (left) and the defect-perovskite superconductor YBajCugOy (right). Metal atoms are shaded. Note the missing oxygen atoms in the latter drawing that result in formation of copper-oxygen sheets (above and below the Y atoms), and copper-oxygen chains (between the Ba atoms).

Another closely related family of superconductors is represented by the formula TlBa2Can 1Cun02n+3 (n = 1, 2, 3, 4, 5). They contain single layers of T1 and O atoms that separate the perovskite-like Ba-Cu-Ca-O slabs (24)(25)(26)(27) (Figure 6). Distortions in the Tl-O sheets are also found in these compounds (26)(27). Note that if these phases were stoichiometric, copper would always have a formal oxidation state of greater than two. Therefore, the chemical composition of this homologous series allows the existence of holes in the copper-oxygen sheet. 

PtL4X+2 chains (X = halide, L = NHS, NEtH2 etc.) and its undistorted nickel analog (11). (b) The perovskite structure, (c) The structure of the two dimensional Cu02 sheets found in all copper-containing high-Tc superconductors. 

Another group of materials that has displayed high oxide ion conductivity is based upon a layered bismuth perovskite-based structure, first reported by Aurivillius in 1949 [90-92], The so-called Aurivillius phases are chemically expressed normally as Bi2A B 03 +3 [82], where A is a large 12-coordinated cation and B a small 6-coordinated cation. The structure is formed by n perovskite-like layers, (A 1B 03n+1)2, sandwiched between bismuth-oxygen fluorite-type sheets, (Bi/) 2 [93,94],... 

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