Ba ₇ Zn ₅ Ga ₆ O ₂₁ : Terminal Oxygens Enable High Oxide-Ion and Proton Conductivity in a Condensed Three-Dimensional Tetrahedral Framework

Functional oxides exhibiting high oxide-ion and proton conductivity are crucial for advancing solid oxide fuel cells and related electrochemical technologies. Here, we demonstrate that terminal oxygens can endow the condensed three-dimensional (3D) tetrahedral framework with additional local structural flexibility and thus enable significant dual oxide-ion and proton conduction in Ba 7 Zn 5 Ga 6 O 21 . The polar P 31 c -structure of Ba 7 Zn 5 Ga 6 O 21 was solved by combined powder X-ray diffraction (PXRD), 3D electron diffraction (ED), and neutron powder diffraction (NPD), and its noncentrosymmetry was confirmed by second-harmonic generation (SHG). The framework structure comprises highly strained tridymite-type layers linked by (Zn/Ga) 3 O 10 units and contains ∼9.5% terminal oxygens. Interestingly, NPD and theoretical calculations further revealed that, in Zn-doped Ba 7 Zn 5+ x Ga 6– x O 21–0.5 x, oxygen vacancies preferentially occupy terminal oxygen sites, enabling moderate oxide-ion conduction at elevated temperatures and substantial proton conductivity (∼0.1 mS/cm) under humidified air below 400 °C. Ab initio molecular dynamics (AIMD) simulations deciphered a predominantly 2D vacancy-mediated oxide-ion conduction mechanism within the tridymite layers, where migration is enabled by cooperative opening of six-membered rings and concurrent formation of ten-membered rings. These findings establish Ba 7 Zn 5+ x Ga 6– x O 21–0.5 x as a vacancy-mediated 3D tetrahedral framework dual-ion conductor.