Abstract
CuNb 2O 6 can be prepared in either a monoclinic ( T=680–740 °C) or an orthorhombic ( T>740 °C) modification by heating equimolar amounts of CuO and Nb 2O 5 in air. From the products of this solid state reaction, which are always powdered, single crystals of orthorhombic CuNb 2O 6 could be prepared in the presence of iodine as a mineralizing agent. Thus its similarity to the columbite structure of NiNb 2O 6, previously observed in powder photographs, could be confirmed by a single-crystal study. Crystal structure refinement (space group Pbcn; a=14.1027(15) A ̊ ; b=5.6093(5) A ̊ ; c=5.1223(5) A ̊ ; Z=4), starting from known parameters for NiNb 2O 6, 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 AB 2O 6 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).
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