Structural determination of high Tc superconductor (Bi,Pb)2Sr2Ca2Cu3Ox (2223) phase with powder diffraction data

Abstract

The (Bi,Pb)2Sr2Ca2Cu3Ox (2223) phase is regarded as the one of the most technologically significant high Tc phases in the Bi–Sr–Ca–Cu–O system. However, its structure has not been fully described so far. The definitive structure model of 2223 has been determined in this study by direct methods and Q-peak analysis of difference Fourier maps using synchrotron radiation data. Successive least squares refinement of the model with the preferred A 2aa space group was carried out by Rietveld procedures with neutron diffraction data. Anomalies were not observed for interatomic distances (bonded and non-bonded atoms), bond angles and isotropic temperature parameters. Two categories of copper atoms were clearly identified: (i) square coordinated to four oxygen atoms and (ii) pyramids with a fifth oxygen atom. The valence states of two Cu atoms have been estimated using bond valence calculations, as employed by Brown for YBCO. There is no direct bridging between the Cu squares and pyramids. The Bi3+ cations are coordinated by three oxygen atoms and form a single chain [BiO2]nn− along the a axis, which is the first report of this feature for the 2223 structure.