Orthorhombic superstructures within the rare earth strontium-doped cobaltate perovskites: Ln(1-x)Sr(x)CoO(3-delta) (Ln = Y3+, Dy3+-Yb3+; 0.750 <= x <= 0.875)

摘要

A combination of electron, synchrotron X-ray and neutron powder diffraction reveals a new orthorhombic structure type within the Sr-doped rare earth perovskite cobaltates Ln(1-x) SrxCoO3-delta (Ln = Y3+, Dy3+, Ho3+, Er3+, Tm3+ and Yb3+). Electron diffraction shows a C-centred cell based on a 2 root 2a(p) x 4a(p) x 4 root 2a(p) superstructure of the basic perovskite unit. Not all of these very weak satellite reflections are evident in the synchrotron X-ray and neutron powder diffraction data and the average structure of each member of this series could only be refined based on Cmma symmetry and a 2 root 2a(p) x 4a(p) x 2 root 2a(p) cell. The nature of structural and magnetic ordering in these phases relies on both oxygen vacancy and cation distribution. A small range of solid solution exists where this orthorhombic structure type is observed, centred roughly around the compositions Ln(0.2)Sr(0.8)CoO(3-delta). In the case of Yb3+ the pure orthorhombic phase was only observed for 0.850 <= x <= 0.875. Tetragonal (14/mmm; 2a(p) x 2a(p) x 4a(p)) superstructures were observed for compositions having higher or lower Sr-doping levels, or for compounds with rare earth ions larger than Dy3+. These orthorhombic phases show mixed valence (3+/4+) cobalt oxidation states between 3.2+ and 3.3+. DC magnetic susceptibility measurements show an additional magnetic transition for these orthorhombic phases compared to the associated tetragonal compounds with critical temperatures > 330 K. (C) 2007 Elsevier Inc. All rights reserved.