CRYSTAL-STRUCTURE OF (BI, PB)2SR2CAN-1CUNO4+2N+DELTA HIGH-TC SUPERCONDUCTORS

The crystal structure of (Bi, Pb)2Sr2Can-1CunO4+2n+δ high-temperature superconducting phases was studied by transmission electron microscopy. The displacive modulation is largest for the n=1 material and yields displacement amplitudes of 70 pm with respect to the average structure. In all phases the largest distortions are found in the Bi-O double layers. The structural distortions were determined explicitly for the cations and amplitudes for the atomic modulation functions are given for the n=1 and n=2 phases. For Bi2Sr2CaCu2O8+δ(Tc=85 K) the space group Pbnb was determined for the average structure and accounts for reflections which cannot be indexed by Bbmb. In material doped with Pb the periodicity of the modulation changes significantly, from 4.7 times the lattice parameter b for the undoped phases to 8b for the doped n=2 phase and 8.8b for the doped n=3 phase, respectively. For the doped phases systematic extinction conditions are compatible with the space group PBb1m1̄b1. Due to the modulation, shifts of atoms may lead to distortions of CuO polyhedra and should effect the electronic structure of the various phases. © 1990.