Substitutional Mechanisms and Structural Relaxations for Manganese in SrTiO₃: Bridging the Concentration Gap for Point-Defect Metrology

The methodology for experimentally verifying the point-defect chemistry (site of substitution, valence, and charge-compensation mechanisms) in manganese-doped SrTiO3 ceramics is presented for dilute and nondilute dopant concentrations. Experimental and theoretical techniques have strengths and weaknesses depending upon defect types and concentrations, so a combinatorial-characterization approach is required. Using electron-paramagnetic resonance and X-ray-absorption fine-structure measurements combined with density functional theory calculations, the charge-compensation mechanisms and local structural relaxations for five unique manganese defect centers are identified. Mn4+, as an isovalent dopant, occupies the octahedrally coordinated Ti sites without the need for charge compensation; its smaller ionic radius relative to Ti is accommodated by isotropic contraction of the [MnO6] octahedra. Mn3+ is an aliovalent dopant that also favors the Ti sites with the [MnO6] octahedra exhibiting Jahn–Teller distortions. The charge difference associated with the Mn3+Ti4+ substitution is compensated by formation of oxygen vacancies. A more complex behavior is observed for the Mn2+ species, which can occupy either the Sr or Ti sites depending on the Sr/Ti ratio. The effects of Mn concentration or high-temperature annealing on the site preference (i.e., Sr vs Ti) are negligible. The Mn2+ species on the Sr sites are strongly off-centered in the relatively large cuboctahedral cages within the oxygen framework. The dynamic nature of the Mn displacements in these configurations is confirmed using ab initio molecular dynamics simulations.