Synthesis, structure and magnetic properties of monoclinic CuNb2O6 and the electronic spectra of both polymorphs of CuNb2O6

Abstract

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^IINb₂O₆ columbite type [a= 4.9991(1)Å, b= 14.1566(4)Å, c= 5.7540(1)Å, β= 91.718(1)°, space group P2₁/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)Å]. Above 90 K the compound is magnetically dilute and obeys the Curie law (µ_eff= 1.96 µ_B). Below 90 K there is a fairly abrupt departure from simple Curie law behaviour and a Néel 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(II) ions. The results of the study suggest a likely explanation for the variability in colour of the CuNb₂O₆ polymorphs.