Synthesis, single-crystal structure and magnetic properties of orthorhombic CuNb2O6

Abstract

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^IINb₂O₆ columbite system (orthorhombic, a= 14.019(11)Å, b= 5.623(7)Å, c= 5.107(7)Å, space group Pbcn) but the CuO₆ octahedra display a large tetragonal distortion [Cu–O, 1.976(6)× 2, 1.973(6)× 2, and 2.383(6)× 2 Å]. Above 225 K the compound is magnetically dilute and obeys the Curie law (µ_eff= 1.91µ_B), whereas Curie–Weiss behaviour was observed between 225 and 50 K (θ=–30 K) with a maximum susceptibility at 20 K. The magnetic data have been analysed in terms of a onedimensional 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.