The conditions necessary for the synthesis of the two reported polymorphs of CuNb,O, 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"Nb,O, columbite system (orthorhombic, a = 14.019(11) A, b = 5.623(7) A, c = 5.107(7) A, space group Pbcn) but the CuO, octahedra display a large tetragonal distortion [Cu-0, 1.976(6) x 2, 1.973(6) x 2, and 2.383(6) x 2 A]. Above 225 K the compound is magnetically dilute and obeys the Curie law (peff= l.91pB), whereas Curie-Weiss behaviour was observed between 225 and 50 K (8= -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.

The conditions necessary for the synthesis of the monoclinic polymorph of CuNb,O, from the binary oxides are reported. The yellow-green monoclinic phase has been prepared in a pure microcrystalline form and its structure determined by means of a Rietveld profile-refinement of powder X-ray data. The structure is a monoclinic distortion of the M"Nb20, columbite type [a =4.9991(1) A, b = 14.1 566(4) A, c = 5.7540(1) A, /?= 91.71 8(1)", space group P2,/c] with a small tetragonal distortion of the CuO, octahedra [Cu-0, 2.038(16), 2.079(19), 1.968(16), 2.01 6(16), 2.1 50(20) and 2.1 68(18) A]. Above 90 K the compound is magnetically dilute and obeys the Curie law (peff= 1.96 pB). Below 90 K there is a fairly abrupt departure from simple Curie law behaviour and a Nee1 point occurs at 25 K. The magnetic data have been analysed in terms of a dimeric model. The electronic spectrum of the black orthorhombic form of CUNb20, is consistent with a band structure whereas that of the yellow-green monoclinic form is characteristic of isolated copper(ii) ions. The results of the study suggest a likely explanation for the variability in colour of the CuNb,O, polyrnorphs.