Donor and Acceptor Characteristics of Group IV and VII Doped BiNbO₄: A Hybrid Density Functional Investigation

This work systematically investigates the intrinsic defect behavior and corresponding conductivity type of BiNbO₄ under different representative thermodynamic equilibrium growth conditions using hybrid density functional theory calculations. The modulation effects of group IVB (Ti, Zr, and Hf) and group VIIA (F, Cl, Br, and I) element doping on its conductivity and electronic and optical properties are also explored. It is revealed that, under Bi-rich, relatively Nb-rich, and O-poor conditions, the easy ionization of the main native defects V_O1 (two O vacancy types), Nb_Bi, and unintentional H_i as shallow donors promotes BiNbO₄ to exhibit an unintentional <i>n</i>-type conductivity character. Still, under O-rich, Bi-poor, and Nb-poor conditions, the ionization of the dominant defects V_Bi as excellent acceptors makes it present an intrinsic <i>p</i>-type behavior. Therefore, this affirms the experimental observation of the <i>n</i>-type character in BiNbO₄ and predicts its <i>p</i>-type behavior. For extrinsic doping, the substitution of Ti on Nb (Ti_Nb^1-) significantly enhances the <i>p</i>-type conductivity under the presence of O-rich conditions. Ti becomes the best <i>p</i>-type doping candidate for BiNbO₄ among the group IVB elements. In contrast, the substitution of F on O (F_O¹⁺) effectively boosts the superior <i>n</i>-type conductivity under the O-poor conditions, and F is the best <i>n</i>-type doping candidate among group VIIA elements. Furthermore, the electronic structure and optical absorption analyses indicate that the major intrinsic defects V_O1¹⁺, V_O1²⁺, and V_Bi^3- and the optimal extrinsic doping Ti_Nb^1- and F_O¹⁺ do not lead to deep-level recombination centers but instead serve as active sites for photocatalytic reactions, synergistically improving visible-light absorption and charge carrier concentration. In addition, it is found that interstitial Nb_i and an O2 vacancy defects respectively induce significant visible light absorption, especially for Nb_i. Thus, via the control of growth conditions and the optimization of doping elements, this study provides theoretical guidance on the intrinsic and extrinsic doping strategies for tuning the conductivity and other properties, further enhancing the photocatalytic performance of BiNbO₄.