Crystal structure refinement of MgNb2O6 columbite from neutron powder diffraction data and study of the ternary system MgO-Nb2O5-NbO, with evidence of formation of new reduced pseudobrookite Mg5-xNb4+xO15-δ (1.14≤x≤1.60) phases

The crystal structure of the columbite-type phase MgNb2O6 has been refined from ponder neutron diffraction data, The compound is orthorhombic, space group Pbcn (60), Z = 4, with unit cell parameters a = 14.1875(1), b = 5.7001(1), c = 5.0331(1) Angstrom, The structure contains chains of NbO6 octahedra sharing edges along the c axis, which are arranged in double layers through common corners, The double layers, parallel to the br plane, are connected via MgO6, octahedra sharing corners, This compound contains Nb5+ with electronic configuration d(o). Attempts to obtain new materials with Nb in a mixed valence state were made in the ternary system MgO-Nb2O5-NbO. For molar ratios Mg:Nb = 1:2 new pseudobrookite-type phases have been identified, They have the general composition Mg5-x Nb4+xO15-delta with x values ranging from 1.14 to 1.60. The compounds Mg4Nb2O9-delta (corundum-type) and Mg3Nb6O11 were identified when the Mg:Mb ratio was 4:2. All the reduced materials were also characterized by thermogravimetric analysis. (C) 1997 Academic Press.