Synchrotron X-ray powder diffraction study of (Bi, Pb)(1.64)Sr1.43Ca1.57Mn2O9, Mn-analogue of BSCCO-2212 superconductor

The orthorhombic Mn-2212 phase, with nominal composition BiPbSr1.8CaMn2.1O9, was synthesized by solid state reaction. Synchrotron X-ray powder diffraction data (lambda = 0.9359 Angstrom) were collected at Brookhaven National Laboratory (U.S.A.) and used for a structure Rietveld refinement (a = 5.3314(3), b = 5.3697(2), c = 31.067(2) Angstrom). Convergence was obtained in space groups Fmmm, Amaa, and Abmm (Z= 4) with the same agreement indexes (wR(p) = 0.0216, R-p = 0.0158, R(/F/(2))= 0.0419). The two lower symmetry structure models correspond to different descriptions of partial ordering of oxygen atoms in the (Bi,Pb)-O layer. Refined site occupancies indicate a phase composition close to (Bi1-yPby)(1.64)(Sr0.33Ca0.67)(2)(Sr0.77Ca0.23)Mn2O9, with y between 0.4 and 0.5. This result, coupled with electroneutrality requirements, leads to an estimated Mn oxidation state of 3.8 /e/ Unlike in Cu-2212, an O atom is present in the (Sr,Ca) plane between adjacent Mn-O layers, producing a distorted octahedral (instead of pyramidal) coordination of Mn. (C) 1999 Academic Press.