Maarit Karppinen

Square Coordinated MnO2-Units in BiMn7O12

Room temperature high-resolution synchrotron X-ray diffraction (HRSXRD) measurements were carried out on polycrystalline samples of BiMn(7)O(12) and LaMn(7)O(12). Rietveld refinements of HRSXRD data describe the crystal structures of both compounds in the monoclinic space group I2/m. The maximum entropy method-based pattern fitting (MPF) method inversion of HRSXRD data was utilized to obtain the spatial electron density (ED) distribution. Obtained results show clear differences in ED distributions around Mn-O bonds between isomorphic BiMn(7)O(12) and LaMn(7)O(12). The scheme of ED distributions in BiMn(7)O(12) indicates the presence of orbital ordering in the MnO(2) planar arrangement that appears as prominent features in very distorted MnO(12) and MnO(6) polyhedrons in the perovskite related structure of [BiMn(3)](A-site)[Mn(4)](B-site)O(12).

Oxygen-content dependence of the flux pinning in Pb-substituted Bi-2212 superconductors

The Pb-substituted Bi-2212 superconductors with different oxygen contents were synthesized by post-annealing the samples in air or varying the sintering temperature in N2. With decreasing oxygen content, Tc increased and the structure changed in the following series: tetragonal → orthorhombic → “monoclinic”. The same tendency was observed ever when the Pb content increased. It was confirmed that there existed two kinds of pinning mechanisms in this system. One that was effective at low magnetic fields was enhanced by the inhomogeneity in the oxygen distribution. The other that was effective at high magnetic fields increased near the boundary between orthorhombic and “monoclinic” phases.