Interstitial Oxide Ion Migration Mechanism in Aluminate Melilite La_1+xCa_1–xAl₃O_7+0.5x Ceramics Synthesized by Glass Crystallization

Gallate melilite materials have attracted considerable interest as new interstitial oxide ion conducting electrolytes for solid oxide fuel cells for more than a decade. However, the preparation of aluminate melilite materials as interstitial oxide ion conductors remains a challenge. Here, we show that interstitial oxide ion conducting aluminate melilite materials La1+xCa1–xAl3O7+0.5x (x = 0–0.5) can be prepared via a full crystallization from bulk glass process. Rietveld refinements performed from combined neutron and synchrotron X-ray powder diffraction (NPD and SPD) data reveal multiple interstitial defect positions within the pentagonal ring, demonstrating the diversity of local structures around the oxygen interstitial defects in La1+xCa1–xAl3O7+0.5x. Variable temperature solid-state 27Al nuclear magnetic resonance (NMR) spectroscopy measurements demonstrate the existence of 5-coordinated AlO5 polyhedra and dynamic exchange processes between these 5-coordinated Al sites, representing the first example of evidence for the migration mechanism of interstitial oxide ions in melilites by NMR. This latter involves framework and interstitial oxide ions, and is assisted by rotation and deformation of tetrahedra. These calculations reveal reduced mobility of interstitial oxide ions in aluminate tetrahedral network owing to its rigidity.