Additive Lattice Response From Doping and Oxygen Vacancies in Perovskite Oxides: Connecting Experiment and Theory Beyond Dilute‐Limit Models

SrTiO 3 and La‐doped SrTiO 3 (La fraction 0.0625–0.2188) were synthesized and thermally treated in oxidizing and reducing atmospheres. X‐ray diffraction reveals that La doping contracts the lattice, whereas reduction induces oxygen‐vacancy‐driven chemical expansion, with oxygen nonstoichiometry quantified by thermogravimetric analysis (TGA). Complementary density functional theory (DFT) calculations on the same compositions as in the experiments employ 4 × 4 × 4 supercells that explicitly include A‐site vacancies for charge compensation and up to four oxygen vacancies. These models capture defect–defect and defect‐dopant interactions beyond isolated‐defect supercell approximations. Hybrid DFT (PBE0) reveals that La–– associations reduce oxygen‐vacancy formation energies by up to 0.75 eV and collapse the band gap from 3.7–3.9 eV to 1.2–1.6 eV, accompanied by Ti 3+ polaron localization. Structural analysis establishes an additive lattice response, quantified by a nearly constant per‐vacancy expansion coefficient (α ≈ 0.0028 Å/). Combining elastic response with vacancy energetics explains the 32‐fold increase in oxygen deficiency derived from experiment. These insights provide a predictive framework for tuning reducibility and mechanical stability in aliovalent‐doped perovskites for electrochemical energy conversion.