LAUSR

We address here the chemical character and the mobility of hole polarons in BiVO4 (BVO). Two distinct structures for hole polarons in BVO have been reported in the literature so far. ‘Standard’ hybrid density functional theory (DFT) calculations have predicted hole polarons to be (BiO8) dodecahedron-centered polarons, while Hubbard U-DFT (DFT+U) calculations have predicted single oxygen atom-centered polarons. Resolving the contrasting findings is critical to validate theory in materials research. We investigated the structure of hole polarons in BVO using hybrid DFT with varying fractions (α) of the exact exchange interaction that enters hybrid functionals. For values of the fraction α, from α = 0.25 to α = 0.45, we obtained both structures, the h+(BiO8) hole structure and the h+(O) hole structure. For the smaller values of α, the h+(BiO8) structure is lower in energy, while for the larger values of α, the h+(O) structure is more stable. The piece-wise linearity of DFT energies E(q) with respect to the point defect charge q is maintained satisfactorily in all cases, although the deviation is smallest for hybrid DFT with small exact exchange fraction. For α=0.25, we determined the diffusivity of h+(BiO8) holes from the Marcus/Holstein two-state model. The activation barrier for Bi-to-Bi hops (ΔG ~ 76 meV) and mobility (μ ~ 0.029 cm2V-1s-1) are found to be in close agreement with the published Thz experimental data (ΔG ~ 90 meV and μexp ~ 0.02 cm2V-1 s-1). In h+(BiO8) structures, ~ 20 to 25 % of the hole charge and unpaired spin reside on the Bi atom and the rest is divided among the eight O atoms. The hole is best described as a Bi-centered hole, rather than as a Bi 6s hole. In contrast, in h+(O) structures, ~ 75% of the unpaired spin population resides on the O atom. In conclusion, in order to resolve the theoretical dichotomy, these findings suggest that there remains a need for further experimental characterization of hole polarons in BVO that would be able to identify the oxidation state of Bi and O atoms in operando conditions and help validate theoretical results of structure and transport.