 To whom correspondence should be addressed. JOURNAL OF SOLID STATE CHEMISTRY 134, 76 — 84 (1997) ARTICLE NO. SC977538 Crystal Structure Refinement of MgNb 2 O 6 Columbite from Neutron Powder Diffraction Data and Study of the Ternary System MgO–Nb 2 O 5 –NbO, with Evidence of Formation of New Reduced Pseudobrookite Mg 5 –x Nb 4x O 15 – (1.144x41.60) Phases S. Pagola and R. E. Carbonio INFIQC, Departamento de Fisicoquı ´ mica, Facultad de Ciencias Quı´ micas, Universidad Nacional de Co ´ rdoba, Agencia Postal 4, CC 61, 5000 Co ´ rdoba, Argentina J. A. Alonso Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientı´ ficas, Cantoblanco, E-28049 Madrid, Spain and M. T. Ferna´ndez-Dı´az Institut Laue-Langevin, 156X, F-38042 Grenoble Cedex, France Received November 18, 1996; in revised form July 8, 1997; accepted July 8, 1997 The crystal structure of the columbite-type phase MgNb 2 O 6 has been refined from powder 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) A  . The structure contains chains of NbO 6 octahedra sharing edges along the c axis, which are arranged in double layers through common corners. The double layers, parallel to the bc plane, are connected via MgO 6 octahedra sharing corners. This compound contains Nb 5 with electronic configuration d 0 . Attempts to obtain new materials with Nb in a mixed valence state were made in the ternary system MgO–Nb 2 O 5 –NbO. For molar ratios Mg : Nb  1 : 2 new pseudobrookite-type phases have been identified. They have the general composition Mg 5x Nb 4x O 15– , with x values ranging from 1.14 to 1.60. The com- pounds Mg 4 Nb 2 O 9 (corundum-type) and Mg 3 Nb 6 O 11 were identified when the Mg : Nb ratio was 4 : 2. All the reduced materials were also characterized by thermogravimetric analy- sis.  1997 Academic Press Key Words: magnesium niobium oxides; columbite structure; neutron powder diffraction. INTRODUCTION Recently there has been increased interest in Nb mixed oxides due to the appearance of some superconducting oxides, such as Li  NbO  (1) and Sr  R  Nb  O  (R"rare earths) (2), containing Nb in a mixed-valence state, although superconductivity of the latter compound has not been reproduced by other groups. In these superconducting phases the formal oxidation state of niobium is lower than # 5. Also, mixed oxides in which Nb presents an oxidation state between # 4 and # 5, such as Sr  NbO  (0.75(x( 0.90) and BaNbO  , have also been investigated, although no superconducting behavior was reported (3, 4). During the study of perovskite-related phases of com- position Ba  Nb  O  (5) we found interesting elec- trical properties, making these oxides good candidates to search for new superconductors. Aiming to expand our research to the Mg—Nb—O system we first became involved in the study of some phases containing Nb and sub- sequently explored the possibilities of preparation of new reduced compounds. The phase diagram of the binary system MgO—Nb  O  was first reported by Tilloca and Pe´rez y Jorba (6) and later confirmed by Abbattista et al. (7) and Bru¨ck et al. (8). Four compounds were reported: Mg  Nb  O  , Mg  Nb   O  , Mg  Nb  O  , and MgNb  O  . 76 0022-4596/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.