Exchange-Induced Negative-U Charge Order in N-Doped WO3: A Spin-Peierls-Like System

An unconventional spin-Peierls-type distortion was found in a nonmagnetic atom N doped pseudo-one-dimensional WO3 system. The periodicity of the initial ferromagnetic WO3: N is doubled in one direction, and the band gap opens up due to this distortion. The magnetic moment at the N site is asymmetric in the distorted system, and the interaction between the localized spin is very weak. We show that the large exchange interaction of the nitrogen 2p atomic orbital and the pseudo-one-dimensional W-O-W chain in monoclinic WO3 structure is the origin of this spin-Peierls-like transition that leads to the stabilization of an unusual negative- U charge-ordered system.