Synthesis and Characterization of the Reduced Single-Layer Manganite Sr₂MnO_3.5+x

The nuclear and magnetic structures of polycrystalline Sr₂MnO_3.5 have been determined by the Rietveld analysis of neutron powder diffraction data and electron diffraction techniques. The pure Mn³⁺ single-layered phase crystallizes in the primitive monoclinic space-group P2₁/c with lattice constants a = 6.8524(3) Å; b = 10.8131(4) Å; c = 10.8068(4) Å; β = 113.247(4)°. The oxygen defects form an ordered superstructure within the perovskite block layers consisting of interconnected MnO₅ square pyramids, slightly different from those observed for the defect perovskites SrMnO_2.5 and Ca₂MnO_3.5. Magnetic susceptibility studies show a broad transition at ∼280K, which is attributed to an overall antiferromagnetic ordering of spins, which leads to doubling of the unit cell along [100]. The magnetic unit cell comprises ferromagnetic clusters of four corner-sharing MnO₅ pyramids, which are antiferromagnetically aligned to other similar clusters within the perovskite block layers.