O/N Mixed-Anion Defect Engineering Mediates Structural Evolution and Li-ion Transport in La24Li23Ti5O53N3.

Mixed-anion defect engineering is an effective strategy for tuning local structures and ion transport in complex oxides. Herein, we report the Li3N-assisted synthesis of the oxynitride La24Li23Ti5O53N3 (LLTON) and clarify the role of O/N incorporation in defect reconstruction and Li-ion migration. Structure analyses reveal that nitrogen preferentially substitutes apical oxygen sites in Li/Ti octahedra, inducing Li-site redistribution and oxygen-vacancy formation. These coupled defects reconstruct the local coordination environment and disrupt the ordered defect arrangement of the parent oxide while retaining the tetragonal framework. Variable-temperature diffraction further reveals a thermally driven structural evolution involving Li-site reorganization, enhanced positional disorder in the fluorite-related lithium sublattice, and a halving of the crystallographic repeat along the c direction. Ab initio molecular dynamics (AIMD) and bond valence site energy (BVSE) calculations show that these defect-induced and thermally activated rearrangements lower the Li-ion migration barrier and promote preferential one-dimensional migration along the c direction. Consequently, LLTON delivers an ionic conductivity of 3.78 mS cm-1 at 400 °C, with the activation energy reduced from 0.88 to 0.67 eV. This work establishes a structure-defect-evolution-transport correlation in mixed-anion oxides and highlights O/N defect engineering as a useful approach for regulating ion migration in complex oxide materials.