Niobium doping in BiVO4: an interplay between effective mass, stability and pressure.

We have applied density functional theory to study the electronic structure changes caused by Nb incorporation in BiVO4 and the application of external pressure. The overall solubility of Nb in BiVO4 is usually high, and the presence of oxygen vacancies affect the dopability of Nb in BiVO4. Through the analyses of the chemical-potential landscape, we have determined the single-phase stability zone of BiVO4 with the Nb doping. The most favorable Nb doping is simultaneous substitutions at both V and Bi sites. Nb substitution at only V-site even though is next favorable, the band gap change is not very significant which agrees with an earlier experiment. However, it does change the electron effective mass by 20% due to the presence of Nb 4d bands in the conduction bands, which explains better catalytic activity by Nb-doped BiVO4. In addition, application of external pressure improved the single-phase stability zone in the chemical-potential landscape. We have also focused on the local structural distortions near the Nb doping site, especially on the BiO8 octahedra. We have shown here that pressure-induced symmetrization of BiO8 dodecahedron lower the electron's effective mass further and therefore can help to improve the photo-conduction property of BiVO4.