Local environments of three forms of Ba2YCu3O6+x

The Wigner-Seitz local atomic cell construction procedure, provides a topol ogical means for comparing series of related structures in terms of the num ber and type of nearest neighbors. We used this construction to further our understanding of the chemical bonding environment (and hence, the supercon ducting properties) in three structurally related materials, Ba2YCu3O6+x (x = 1, 1/2, and 0). These are high-T-c superconductors of T-c approximate to 92 K, 56 K, and a nonsuperconductor, respectively. The local environment a round Y is found to be similar in all three compounds, and the topology aro und Ba is generally similar to that in the ideal perovskite. The Cu sites i n the Cu-O plane in all three materials show similar anisotropic bonding be havior. Significant results are: (i) that Ba contributes to the bonding wit h Cu sites, and (ii) the most noticeable difference in the local environmen t in these compounds is around the Cu sites in the Cu-O "chains". We propos e a modified charge-transfer model, involving the Cu-O "chains", "planes" a nd Ba as an electronic unit, to correlate structure with the observed super conducting properties. The bonding behavior of these oxides also appears to be different from the conventional metal alloy superconductors. (C) 1999 E lsevier Science B.V, All rights reserved.