Electronic Effects of Anion Substitution in Titanate and Vanadate Pyrochlores from First Principles

Pyrochlore oxides with chemical formula A₂B₂O₇ exhibit rich magnetic and electronic behavior linked to the geometric frustration of spins on the pyrochlore lattice. Although cation doping in Y₂Ti₂O₇ and Y₂V₂O₇ has been explored to tune transport and magnetic properties, the role of anion substitution is significantly less studied. Using density functional theory (DFT), we investigate the impact of nitrogen and fluorine substitution at the two crystallographically distinct oxygen sites in these pyrochlores. Our results show that in Y₂Ti₂O₇, both N^3- and F^- dopants introduce shallow defect levels within the band gap, suggesting a pathway to enhance electronic conductivity. In contrast, for Y₂V₂O₇, F^- substitution yields shallow donor-like states, whereas N^3- incorporation produces deeper localized defect states associated with strong hybridization between V-3d and N-2p orbitals. These findings indicate the sensitivity of defect energetics to B-site chemistry and highlight anion doping as a promising route to manipulate the electronic properties in frustrated oxide systems.