Synchrotron X-ray and TOF neutron powder diffraction study of a lyonsite-type oxide Co3.6Fe3.6(VO4)6

Co3.6Fe3.6(VO4)(6) was synthesized by solid-state reaction at 900 degreesC, and its structure was determined by X-ray and neutron powder diffraction. Co3.6Fe3.6(VO4)(6) is isotypic with lyonsite (space group: Pnma; Z = 2), having lattice parameters of a = 4.96480(9) Angstrom, b = 10.2245(2) Angstrom, and c = 17.2100(3) Angstrom. Its structure parameters were refined by the Rietveld method with two sets of synchrotron X-ray (wavelengths: 0.85001 and 1.74500 Angstrom) and time-of-flight neutron powder diffraction data. Co2+ and Fe3+ ions are distributed among three different metal sites M1-M3. Not Co2+ ions but Fe3+ ions are mainly contained in face-sharing M3O(6) octahedra, forming single chains parallel to the [100] direction. Edge- and corner-sharing octahedra M2O(6) build up layers perpendicular to the [001] direction. Edge-sharing trigonal prisms M1O(6) form zig-zag single chains along the [100] direction. The M1 and M3 sites are about 6.0(2)% and 34.0(2)% deficient, respectively. The tendency for Co2+ and Fe3+ ions to distribute among the three metal sites is discussed on the basis of the electrostatic valence rule. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.