Local Rigid-Unit Design Modulates Interstitial Oxygen Coordination Geometry for Developing Langasite-Type Oxide-Ion Solid Electrolytes

The langasite family, featuring a three-dimensional open framework, offers exceptional structural and compositional flexibility for accommodating and transporting oxide ions. However, conventional langasite-type oxide-ion solid electrolytes (SEs) suffer from charge trapping of interstitial oxygens within the edge-sharing coordination geometry, severely limiting long-range migration. Here, we introduce a local rigid-unit design strategy to modulate interstitial oxygen coordination and unlock interstitial oxide-ion conduction in langasite La3Ga5–xGexSiO14+x/2. Incorporation of a rigid SiO4 unit and partial Ge4+ substitution stabilizes interstitial oxygen at terminal sites of GaO5 polyhedra, rather than at edge-sharing traps, enhancing migration mobility. 29Si solid-state NMR, supported by DFT calculations, and neutron PDF analysis with RMC modeling, reveal that SiO4 units preserve their intrinsic coordination geometry, while interstitial oxygens are preferentially accommodated at terminal oxygens of SiO4–GaO5 and GaO4–GaO5 linkages, and at relaxed bridging sites within edge-sharing GaO4 units, which effectively circumvent edge-sharing charge trapping and facilitate interstitial oxygen mobility. Complementary AIMD and classical MD simulations confirm the interconnected 2D long-range interstitial oxide-ion migration involving structurally distinct oxygen sites. These findings establish a design principle in which rigid structural units modulate interstitial oxygen coordination from bridging to terminal sites, providing mechanistic insight into enhancing both structural rigidity and ionic mobility in next-generation interstitial oxide-ion SEs.