SYNTHESIS, STRUCTURE AND MAGNETIC-PROPERTIES OF MONOCLINIC CUNB2O6 AND THE ELECTRONIC-SPECTRA OF BOTH POLYMORPHS OF CUNB2O6

The conditions necessary for the synthesis of the monoclinic polymorph of CuNb2O6 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(II)Nb(2)O(6) columbite type [a=4.9991 (1) Angstrom, b=14.1566(4) Angstrom, c=5.7540(1) Angstrom, beta=91.718(1)degrees, space group P2(1)/c] with a small tetragonal distortion of the CuO6 octahedra [Cu-O, 2.038(16), 2.079(19), 1.968(16), 2.016(16), 2.150(20) and 2.168(18) Angstrom]. Above 90 K the compound is magnetically dilute and obeys the Curie law (mu(eff)=1.96 mu(B)). Below 90 K there is a fairly abrupt departure from simple Curie law behaviour and a Neel 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 CuNb2O6 is consistent with a band structure whereas that of the yellow-green monoclinic form is characteristic of isolated copper(ll) ions, The results of the study suggest a likely explanation for the variability in colour of the CuNb2O6 polymorphs.