ABO3 crystal structure

The crystal structure of the 1-2-3 superconductor, YBazCusOy- is depicted in Figure 10.8. Figure 10.8(a) depicts only the positions of the metal atoms. If we discuss it in terms of the perovskite structure ABO3, where B=Cu, the central section is now an A-type perovskite unit cell and above and below it are also A-type perovskite unit cells with their bottom and top layers missing. This gives copper atoms at the unit cell corners and on the unit cell edges at fractional coordinates A and Ys. The atom at the body-centre of the cell (i.e., in the centre of the middle section) is yttrium. The atoms in the centres of the top and bottom cubes are barium... 

Figure 15 Idealized crystal structures of the Ruddlesden-Popper phases, (A0)(AB03) , n = 00, ABO3 n = 1, A2BO4 n = 2, A3B2O7 n = 3, A4B3O10. The comer-sharing BO3 octahedrals are shown in polyhedral representation while the A atoms are shown as spheres...

FIG. 13.3. (a) The perovskite structure for compounds ABO3 (or ABX3). Large open circles represent 0 (or F) ions, the shaded circle A, and the small circles B ions, (b) The crystal structure of ReOs. 

The high-temperature superconductors have a crystal structure of so-called perovskite, whose primary structure is ABO3, where A is a di- or trivalent metal atom with a larger ion size and B is a tetra- or trivalent metal atom with a smaller ion size. In the preceding high-temperature superconductors, the A site is occupied by La3+, Y3+, Ba +, Sr2+, and Ca2+ and the B-site is occupied by Cu2+, Bi3+, and TP+. If the copper valency is 2, the cation valency is insufficient to form the nominal ABO3. This insufficiency is compensated for by... 

The superconductors in class II have the crystal structure A2BO4, which is conventionally called a K2NiF4 compound. A typical class II superconductor is (Lai- cBa c)2Cu04 with Tc - 30 K, discovered by Bednorz and Muller. As seen in Fig. 2, A2BO4 is composed of an ABO3 perovskite unit and an AO unit... 

Figure 7.3 Schematic iiiustration of (a) an antiphase domain boundary in a cubic perovskite structure for an ABO3 material, and (b) a ferro-electric domain boundary in a material with a distorted perovskite cell such as BaTi03, which is tetragonal below the Curie temperature. Each sketch shows the projected atomic positions along a viewing direction corresponding to the a-axis, or [100], of the crystal structure. A atoms are dark, B atoms are grey, and oxygen atoms are the large white circles.

Crystal Structure of Ideal and Distorted Ternary ABO3 Perovskites... 

The 100 face of a perovskite ABO3 crystal is the most stable surface, although it cannot be prepared by cleavage. There exist two non-equivalent terminations with respective stoichiometries AO and BO2 - for example, SrO and Ti02 for SrTiOs. In the surface layer, the transition metal cations B are five-fold coordinated, instead of six-fold in the bulk, and the cations A are surrounded by eight oxygens, instead of twelve in the bulk. The 100 surfaces of both SrTiOs and BaTiOs have been studied, but a quantitative surface structure determination has only been performed on the first compound a contraction of about 10% on the SrO face and a weak expansion of the order of 2%, at the limit of the error bar, on the Ti02 face, have been found by Bickel et al (1989 1990). Hikita et al (1993), on the other hand, deduce an outward relaxation on both faces. 

The most commonly used and extensively studied cathode material in ceramic fuel cell is based on LaMn03 which is basically a perovskite oxide (ABO3) with p-type conductivity. In order to achieve a high electronic conductivity it is doped heavily by acceptor which leads to an enhanced hole concentration and the resultant conductivity is due to hopping of an electron hole between Mn " and Mn. It is doped at A-site or both at A- and B-sites with other cations. Normally cations with larger ionic radii (such as Ca ", Sr " ) are preferred to substitute at A-site and cations with smaller ionic radii (Co, Fe, Ni, Mn, Cr) preferred to occupy B-site. The crystal structure is a function of the composition which is solely dependent on A- and B-site dopant and on oxygen nonstoichiometry which is influenced by temperature and oxygen partial... 

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