CRYSTAL-STRUCTURE OF THE CERAMIC SUPERCONDUCTOR BAPB0.75BI0.25O3

The ceramic superconductor BaPb0.75Bi0.25O3 is monoclinic at T < 430 K with space group I2/m. Lattice constants and the atomic coordinates result from 20 reflection groups (12 < 2-THETA < 120-degrees, Lamda = 1.1267 angstrom) collected with a high resolution powder diffractometer using a synchrotron radiation source and a full neutron powder pattern (6 < 2-THETA < 140, lamda = 1.909 angstrom). Synchroton and neutron data were simtultaneously refined with SIMREF, a new profile refinement program, based on the Rietveld method. Group theoretical analysis of the distortion field leading from the ideal perovskite structure to I2/m exhibits R4+ symmetry. A comparison is made with the corresponding distortions in nonsuperconducting BaBiO3 where three different symmetry types are identified. The space group I2/m found for BaPb0.75Bi0.25O3 allows only one type of averaged (Pb/Bi)O6 octahedron in the structure: this may support valence fluctuations and hence contribute to the mechanism of superconductivity in this particular compound.