Why Hole Polaron Formation on Oxygen Is Limiting the Fermi Level in Fe Acceptor-Doped BaTiO3 under Oxidizing Conditions.

Oxidizing Fe-doped BaTiO3 is commonly expected to convert substitutional Fe3+ acceptors into formal Fe4+ centers. However, the experimentally accessible picture based on electron paramagnetic resonance (EPR) is dominated by Fe3+-related signatures, while Fe4+ is not a straightforward observable. Here, we show that this apparent discrepancy reflects the preferred location of the oxidizing hole: not on Fe but on oxygen. Using density functional theory with occupation matrix control and a piecewise-linearity-based Hubbard correction (DFT + U) for O 2p states, we find that an oxygen-centered hole polaron is forming a Fe3+-O- complex that is lower in energy than the formal Fe4+ configuration. Our results identify ligand hole formation as a favorable charge-compensation mechanism in oxidized Fe-doped BaTiO3 and provide an explanation for the predominance of Fe3+-based centers in spectroscopy. More broadly, they show how oxygen polarons can limit Fermi level shifts and control the electronic response of acceptor-doped ferroelectric perovskites.