Reversible Control of the Mn Oxidation State in SrTiO₃ Bulk Powders

We demonstrate a low-temperature reduction method for exhibiting fine control over the oxidation state of substitutional Mn ions in strontium titanate (SrTiO₃) bulk powder. We employ NaBH₄ as the chemical reductant that causes significant changes in the oxidation state and oxygen vacancy complexation with Mn²⁺ dopants at temperatures <350°C where lattice reduction is negligible. At higher reduction temperatures, we also observe the formation of Ti³⁺ in the lattice by diffuse-reflectance and low-temperature electron paramagnetic resonance (EPR) spectroscopy. In addition to Mn²⁺, Mn⁴⁺, and the Mn²⁺ complex with an oxygen vacancy, we also observe a sharp resonance in the EPR spectrum of heavily reduced Mn-doped SrTiO₃. This sharp signal is tentatively assigned to surface superoxide ion that is formed by the surface electron transfer reaction between Ti³⁺ and O₂. The ability to control the relative amounts of various paramagnetic defects in SrTiO₃ provides many possibilities to study in a model system the impact of tunable dopant-defect interactions for spin-based electronic applications or visible-light photocatalysis.