Effect of Nitrogen Substitution in V₂ O₃ on the Metal-Insulator Transition

The effect of N-doping on the paramagnetic-antiferromagnetic transition associated with the metal-insulator (M-I) transition of V₂ O₃ at 150 K has been studied in bulk samples as well as in nanosheets. The magnetic transition temperature of V₂ O₃ is lowered to ∼120 K in the N-doped samples. Electrical resistivity data also indicate a similar lowering of the M-I transition temperature. First-principles DFT calculations reveal that anionic (N) substitution and the accompanying oxygen vacancies reduce the energy of the high-temperature metallic corundum phase relative to the monoclinic one leading to the observed reduction in Nèel temperature. In the electronic structure of N-substituted V₂ O₃ , a sub-band of 2p states of trivalent anion (N) associated with its strong bond with the vanadium cation appears at the top of the band of O(2p) states, the 3d-states of V being slightly higher in energy. Its band gap is thus due to crystal field splitting of the degenerate d-orbitals of vanadium and superexchange interaction, which reduces notably (ΔE_g =-0.4 eV) due to their hybridization with the 2p states of nitrogen. A weak magnetic moment arises in the monoclinic phase of N-substituted V₂ O₃ with O-vacancies, with a moment of -1 μ_B /N localized on vanadium atoms in the vicinity of oxygen vacancies.