THERMAL-BEHAVIOR OF TRANSITION-METAL NIOBATES .3. SYNTHESIS AND STRUCTURE OF ORTHORHOMBIC CUNB2O6

CuNb2O6 can be prepared in either a monoclinic (T = 680-740-degrees-C) or an orthorhombic (T > 740-degrees-C) modification by heating equimolar amounts of CuO and Nb2O5 in air. From the products of this solid state reaction, which are always powdered, single crystals of orthorhombic CuNb2O6 could be prepared in the presence of iodine as a mineralizing agent. Thus its similarity to the columbite structure of NiNb2O6, previously observed in powder photographs, could be confirmed by a single-crystal study. Crystal structure refinement (space group Pbcn; a = 14.1027(15) angstrom; b = 5.6093(5) angstrom; c = 5.1223(5) angstrom; Z = 4), starting from known parameters for NiNb2O6, gave R = 5.93% (R(w) = 3.65%). The cations have a distorted octahedral coordination in which the elongation of the Cu-O polyhedra (Jahn-Teller effect) is especially pronounced. As in all compounds of the type AB2O6 with the columbite structure the metal cations are situated in layers parallel to the b-c plane. These layers form sequences of the type ABBABB... in the a direction (A, copper; B, niobium).