Unveiling anionic migration mechanisms governed by mixed-anion chemistry in melilite oxyfluorides

Abstract

Mixed-anion compounds provide a chemically flexible platform for tuning the anion sublattice configurations and defect landscapes, making them promising platforms for the design of novel anionic conductors. However, understanding how specific anionic configurations influence ionic migration remains a critical challenge. In this study, we report La_1+xSr_1−xGa₃O_{7+0.5(x−δ)}F_δ melilite oxyfluoride synthesis via topotactic fluorination of oxide precursors. Combined diffraction, impedance spectroscopy, solid-state nuclear magnetic resonance, and density functional theory calculations indicate that defect-level fluorine preferentially occupies interstitial voids within pentagonal rings rather than framework oxygen sites. However, this site-specific incorporation of fluorine appeared immobile, which disrupts the intrinsic oxide-ion migration pathway in melilite structures through impeding the site exchange between interstitial and framework oxygen atoms, thereby suppressing the oxide-ion transport and highlighting the unfavorable hetero-anionic site exchange and the importance of homoanionic site exchange for anionic conduction in the mixed-anion compounds. Our results highlight that anion site selectivity and configuration play a dominant role in governing anionic migration in mixed-anion compounds, offering valuable guidance for the future rational design of advanced mixed-anion ionic conductors.