A‐Site Doping Enables Stabilization and Migration of Oxygen Vacancies in Langasite Frameworks

Open‐framework oxides are promising oxide‐ion conducting candidates for next‐generation solid oxide fuel cells (SOFCs) owing to their ability to accommodate and transport oxygen defects. Here, we demonstrate that strategic A‐site substitution with Sr 2+ in the langasite framework La 3‐ x Sr x Ga 5 GeO 14‐ x /2 effectively introduces oxygen‐vacancy migration, with enhanced oxide‐ion conductivity of 3.08 × 10 −4 S/cm at 1000°C. Neutron powder diffraction (NPD), pair distribution function (PDF) analysis combined with reverse Monte Carlo (RMC) modeling, and static lattice calculations, reveal that oxygen vacancies induced by Sr doping were stabilized by local structural relaxation involving only the surrounding GaO 4 units transforming into GaO 5 coordination environments through edge‐sharing, whereas GaO 6 units and GeO 4 units largely preserve their original geometries. Complementary bond valence site energy calculations and molecular dynamics simulations further uncover the migration mechanism, revealing that oxide‐ion transport proceeds predominantly through two‐dimensional long‐range diffusion within the ab plane, with dynamic participation of all the crystallographic oxygen sites. This work establishes A‐site doping in langasites as a viable route for stabilizing oxygen defects and promoting their migration, offering fundamental insights into the design of high‐performance oxide‐ion conductors.