SYNTHESIS, SINGLE-CRYSTAL STRUCTURE AND MAGNETIC-PROPERTIES OF ORTHORHOMBIC CUNB2O6

The conditions necessary for the synthesis of the two reported polymorphs of CuNb2O6 from the binary oxides have been investigated. The black, orthorhombic phase has been prepared in a pure crystalline form and its single-crystal structure determined. It is isomorphous with the M(II)Nb2O6 columbite system (orthorhombic, a = 14.019(11) angstrom, b = 5.623(7) angstrom, c = 5.107(7) angstrom, space group Pbcn) but the CuO6 octahedra display a large tetragonal distortion [Cu-O, 1.976(6) x 2, 1.973(6) x 2, and 2.383(6) x 2 angstrom]. Above 225 K the compound is magnetically dilute and obeys the Curie law (mu(eff) = 1.91 mu(B)), whereas Curie-Weiss behaviour was observed between 225 and 50 K (theta = -30 K) with a maximum susceptibility at 20 K. The magnetic data have been analysed in terms of a one-dimensional antiferromagnetic chain model. The yellowish-green, monoclinic phase is more difficult to prepare in a pure form than the black, orthorhombic phase. Some provisional details for its preparation are reported.