Unexpected origin of magnetism in monoclinic Nb12O29 from first-principles calculations

Abstract

Nb₁₂O₂₉ is a 4d transition metal oxide that occurs in two forms with different symmetries, monoclinic (m) and orthorhombic (o). The monoclinic form has unusual magnetic properties; below a temperature of 12 K, it exhibits both metallic conductivity and antiferromagnetic ordering. Here, first-principles density-functional theory calculations are used to study the structure, relative stability and electronic properties of Nb₁₂O₂₉. The optimized crystal structures are in good agreement with experimental observations and total energy calculations show similar stability of the two phases, while a magnetic electronic state is slightly favoured for m-Nb₁₂O₂₉. The unusual magnetism of the monoclinic phase originates from a Stoner instability that can be attributed to the Nb atoms with valence states close to Nb⁵⁺, i.e., the atoms with an electronic configuration of ∼d⁰. This is in clear contradiction to current models in which the magnetism is attributed to the presence of localized Nb⁴⁺ ions with a formal d¹ configuration. Our study demonstrates that in complex structures, magnetic properties are best not inferred from ionic models, but require a full quantum mechanical calculation over the whole unit cell.