ANTIFERROMAGNETISM AND METALLIC CONDUCTIVITY IN NB12O29

AT the heart of the controversy about the microscopic origin of superconductivity in high-transition-temperature copper oxide superconductors is the question of whether or not the antiferromagnetism associated with the single-hole Cu2+ 3d9 state is of fundamental importance. To test whether this unconventional spin-mediated superconductivity might be electron/hole symmetric, oxides of the elements with single d-orbital electrons, such as Ti3+ (3d1), Nb4+ (4d1) and W5+ (5d1), are of particular interest. Localized magnetic spin states and magnetic ordering have never been observed previously in transition-metal oxides with one or two electrons in the 4d or 5d states, because of the preference for conventional d-band metallic conductivity or for metal-metal bonding. In exploring the possibility of a d1-d9 and ferroelectricity-superconductivity relationship in oxides (see, for instance, ref. 1), we have found that Nb12O29, a material with a 'crystallographic shear' structure, displays simultaneously both metallic conductivity, a signature of delocalized electrons, and local-moment magnetism with an antiferromagnetic ordering temperature of 12 K. This suggests that the bonding to oxygen of the 4d levels of early-transition-metal elements may not be sufficiently covalent to yield the kind of exotic conductivity (and thus exotic superconductivity) observed for copper oxides.