Perovskites structural family

Naray-Szabo, I., The perovskite-structure family, Muegy. Kozl, Budapest, 1, (1947), 30. Cited onpages 385, 386,661, 662. 

Linking of the octahedra in different representatives of the structural family of the perovskites with different stacking sequences... 

These copper-oxide compounds crystallize in the perovskite structure and superconductivity is based on the (hole or electron) doping in the copper-oxide planes. This is the reason why these materials can be regarded as being 2D. The first compound of the family was La2 i Sr i Cu04 with Tc 38 K, which soon led to YBa2Cu307 5 with Tc — 92 K for 5 < 1 (Bums, 1993). The non-copper oxide electron-doped perovskite Bai-jcK cBiOa exhibits superconductivity near 30 K for 0.3 < X < 0.5 (Cavaeta/., 1988). 

The familiar cubic perovskite structure of ABO3 has of course just one structural parameter, the unit cell edge a. This requires the ratio of the A-O to the B-O bond lengths to be equal to y/2. When this condition cannot be met, the structme distorts in (one of) a number of well-documented ways . By far the largest of the families of derivative structures that arise when A is too small [/(A-0)//(B-0) < /2] is that of the orthorhombic perovskites (GdFeOa type) exemplified by the mineral perovskite (CaTi03) itself. 

The highest transition temperature for the "tungsten bronze" family was 7.7 K for an acid-etched (71) sample of composition Rb o g3W03. Certain researchers (62), after completing studies on cubic and tetragonai-II (semiconducting) bronzes, made the statement "It appears as though the (cubic) perovskite lattice is not favorable for superconductivity." This statement was made in 1965, prior to the major advances in copper oxides that are considered to have a related-perovskite structure. 

From a practical point of view, these compounds are models of crystalline matrices for nuclear waste disposal. One such storage material is SYNROC, a synthetic mineral whose major constituents are the complex oxides hollandite, zirconolite, and perovskite. We have chosen perovskite as a model structural family because of its efficient packing and its accommodation of a wide range of cations, both in size and oxidation state (5). 

An alternative structure that has also been widely investigated both for high temperature piezoelectric, as well as for ferroelectric memory applications is the bismuth layer structure family as shown in Figure 1.14 for SrBi2Ta209 (sbt), e.g. [8], The structure consists of perovskite layers of different thicknesses, separated by Bi20 + layers. It has been shown that when the perovskite block is an even number of octahedra thick, the symmetry imposes a restriction on the polarization direction, confining it to the a-b plane. In contrast, when the perovskite block is an odd number of octahedra thick, it is possible to develop a component of the polarization along the c axis (nearly perpendicular to the layers). This could be used in... 

For example, the family of perovskite minerals and high-temperature ceramic superconductors exhibits this descent of symmetry, from the cubic "ideal" perovskite structure (space group Pm3m, the real mineral perovskite is orthorhombic, space group Pnma, with a fourfold larger unit cell than the ideal cubic one) to orthorhombic structures for the highest-critical... 

From the structural standpoint it is preferable to write the formulae of compounds of the cryolite family in the form A2(B B")Xg rather than A3BX6 because in this structure one-third of the A atoms in the formula A3BX6 are, like the B atoms, in octahedral holes in a cubic close-packed A2X6 (AX3) assembly the other two-thirds, corresponding to the ions in the K2PtCl6 structure, are surrounded by twelve equidistant X ions. Accordingly the cryolite structure is very closely related also to the perovskite structure. In Fig. 10.7 the A and X atoms are... 

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