Antiferromagnetism and metallic conductivity in Nb12O29

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, it was 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 observed for copper oxides. (C.D.)