Why hole polaron formation on oxygen is limiting the Fermi level in Fe acceptor doped BaTiO₃ under oxidizing conditions

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Oxidizing Fe-doped BaTiO₃ is commonly expected to convert substitutional Fe³⁺ acceptors into formal Fe⁴⁺ centers. Yet, 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 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.