Preparation, Crystal and Magnetic Structure of the Double Perovskites Ba2CoBO6 (B = Mo, W)

Ba2CoBO6 (B = Mo, W) perovskites have been prepared in polycrystalline form by thermal treatment, in air, of previously decomposed citrate precursors. These materials have been characterized by X-ray (XRD) and neutron powder diffraction (NPD) data and magnetization measurements. At room temperature, the crystal structure is cubic, space group Fm m, with a = 8.08623(3) and 8.10799(3) Å for the Mo and W compounds, respectively. The crystal contains alternating CoO6 and Mo(W)O6 octahedra, with an almost negligible anti-site disordering. The low temperature antiferromagnetic ordering has been followed from sequential neutron diffraction data. Peaks of magnetic origin appear at the NPD patterns below temperatures of TN = 27 K and TN = 19 K for the Mo and W compounds, respectively. The magnetic structures are both defined by a propagation vector k = (1/2, 1/2, 1/2). They can be described as an array of ferromagnetic layers of Co moments, perpendicular to the [111] directions, coupled antiferromagnetically. The refined magnitude of the Co2+ magnetic moments suggests a high-spin electronic configuration (S = 3/2); covalence effects account for the slight reduction in the low-temperature ordered moments, although this effect is minimized for the W compound. Structural and magnetic features are discussed in relation to the chemical nature of the species present in the solids. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)