Detailed structure of a Pb-doped Bi2Sr2CuO6 superconductor

The crystal structure of (Bi1.82Pb0.18) (ST1.84Pb0.16) CuO6 was determined from single-crystal intensity data measured on a four-circle diffractometer. The structure is orthorhombic, space group Cccm (No. 66); the lattice parameters: a=5.392(3) Angstrom, b=24.603(5) Angstrom, c=5.300(3) Angstrom, V=703.2 (5) Angstrom(3) (the space group can be denoted as Amaa by a cyclic transformation of the cell constants). The R value was 0.048 for 418 unique reflections with I>3.00 sigma(I). One of the O atoms bonded to Bi ions is disordered at two sites with a half occupancy and B-eq=3.6(6) Angstrom(2); although the thermal parameter is not so large as that reported by Torardi et al. [Phys. Rev. B 38, 225 (1988)], the disorder suggests that the crystal has a lower symmetry. The valence state of the Cu ion in the present compound was evaluated to be trivalent based on the Cu-O distances [Asbrink and Norrby, Acta Crystallogr. B 26, 8 (1970)] and the valence of Bi was estimated in comparison with that in other compounds. The O(3) sublattice dynamics in these types of superconductors is discussed based on the structural investigations.