Perovskites slabs

The general formulation of these oxides, (ACuOg.x)m(AO)n, reflects for each of them the number m of copper layers which form each perovskite slab, and the number n of AO layers which form each rock salt-type slab (the AO layers which lie at the boundary of the perovskite slabs and rock salt type slabs can only be counted as for 1/2). Thus all these oxides (2-34) can be represented by the symbol [m,n] in which m,n will be integral numbers. In most of these oxides one observes for one compound only one m and n value, corresponding to single intergrowths. 

Figure 16 (a) High density of intergrowths with various perovskite slab widths (arrowed), with the SADP (inset), (b) High resolution lattice image from FEG STEM showing layers with different perovskite slabs A and B. (c) and (d) EDX nanometer analysis from A and B, showing increased Cu content in A with more Cu-O layers. 

Most of the dislocations found have been of this type. In principle, dislocations introducing only a single perovskite slab are possible. In these, the antiphase displacement across the Bi202 layers, i.e. 2ass must still be accommodated, and the smallest Burgers vector for such dislocations would appear to be [a0c]ss. 

Bi2W209, Bi2W06 and Bi3TiNb09 3.0, 2.8 and 3.1 Structure consists of perovskite slabs interleaved with Bi202 layers. 469... 

Microdomains within the perovskite-like slabs of layered perovskite phases in the Ba-Bi-O system illnstrate these ideas.The structures are layered perovskites with K2Nip4 related structures and gross changes in the Ba to Bi ratio are accommodated by the formation of a homologous series Ba +iBi 03 +i, made up of slabs of BaBi03 perovskite structure (see Section 8.4). In addition, subtle changes in the Ba to Bi ratio are accommodated by microdomains of ordered structure within the perovskite slabs themselves. These microdomains are ordered fragments of perovskite structure, and cation variation occurs at the interface between the microdomains in such a way that local excess or deficit of one cation over the other is accomplished. 

A third series of perovskite related intergrowths has a general formula A B 03 +2, where A is a large cation, and B a medium sized cation, typified by the (Na,Ca) Nb 03 +2 series. The oxides are intergrowths of slabs of perovskite stracture with the halite structure. They differ from the Ruddleston-Popper series in that the (idealized) perovskite slabs are sliced along 110 rather than 100. ... 

In the K2NiF4 series of phases described above, composition variation can take place by disorder in the width of the perovskite slabs, and the solid contains planar defects. In cases where the components of the mtergrowth contain cations with a variable valence, additional defect... 

Representative structures of two-dimensional layered perovskite can be described as a combination of a perovskite slab and an intervening layer. Three series of perovskites are well known, based on the intervening layer Ruddlesden-Popper,... 

The Bismuth Titanates. This family of materials was one of the first complex perovskite groups to be discovered the early studies being made by Aurivillius and reported in 1949. The structures are well known and described in standard texts such as Wells. The general architectural principle of these materials is a repetition of perovskite layers n octahedra thick cut parallel to (110) as in the Sr +iTi 03 +i oxides described above. Instead of being held together by a sequence of SrO layers, the perovskite layers in the bismuth titanates are joined by (Bi202) layers. The perovskite slabs have an overall formula where /I is a large cation that can readily... 

I. Zvereva et al., Complex aluminates RE2SrA1207 (RE = La, Nd, Sm-Ho) Cation ordering and stability of the double perovskite slab-rocksalt layer P-2/RS intergrowth. Solid State Sci. 5(2), 343-349 (2003). 

Figure 8.6 Schematic representation of the structures of the Srn(Nb,Ti) 03 +2 phases (a) Sr4Nb4Oi4 (b), Sr5(Ti,Nb)50i7 (c) Sr9(Ti,Nb)9029. The squares represent (Nb,Ti)06 octahedra and the circles represent Sr atoms. In (a) the perovskite slabs are four octahedra wide, and in (b) they are five octaheda wide. In (c) slabs of four and five octahedra alternate...

K2NiF4-related structures, also called Ruddleston-Popper phases, are built of slabs of perovskite structure cut parallel to the (idealized) perovskite 100 planes united by way of lamellae of halite (NaCl) structure. The series formula for these compounds is A +iB 03 +i where A is a large cation, typically an alkali metal, alkaline earth or rare earth and B is a medium sized cation, typically a 3d transition metal cation. The first examples characterized were Sr2Ti04, Sr3Ti207 and Sr4Ti30g, in which the perovskite slabs are derived from SrTiOs and the halite slabs from SrO. 

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