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 La 3 Ga 5– x Ge x SiO 14+ x /2 . Incorporation of a rigid SiO 4 unit and partial Ge 4+ substitution stabilizes interstitial oxygen at terminal sites of GaO 5 polyhedra, rather than at edge-sharing traps, enhancing migration mobility. 29 Si solid-state NMR, supported by DFT calculations, and neutron PDF analysis with RMC modeling, reveal that SiO 4 units preserve their intrinsic coordination geometry, while interstitial oxygens are preferentially accommodated at terminal oxygens of SiO 4 –GaO 5 and GaO 4 –GaO 5 linkages, and at relaxed bridging sites within edge-sharing GaO 4 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.