Formation and Elimination of Anti-site Defects during Crystallization in Perovskite Ba1–xSrxLiF3

The defect density of a material is decisive for its physical, chemical and mechanical properties. Accordingly, defect tuning is desirable for applications spanning, e.g., batteries, fuel cells, electronics, optics, catalysis and mechanical strength and resilience. Here we simulate the mechanochemical synthesis of the perovskite Ba1-xSrxLiF3 by compressing a BaLiF3 nanoparticle with a SrLiF3 nanoparticle under conditions likely to occur during high-energy ball milling. We investigate the crystallization process and the ionic mobility of the system and compare with experiment. Animations of the crystallization, simulated under high pressure, revealed that cations, within the crystallization front, would commonly condense onto ‘incorrect’ lattice sites, in some cases eventually leading to the formation of anti-site defects. However, most of these cations would then re-amorphise and the ‘correct’ cation would take its place – rectifying the defect. Crucially, it is the amorphous/crystalline interface that enables such repair because the ions are mobile in this region. The simulations reveal high ion mobility close to the anti-site defects and other defective regions, but no ion mobility in the defect free regions of BaLiF3. The MD simulations indicate that high-energy ball milling might reduce the anti-site defect density in a material by exposing these defects to the surface or creating amorphous regions within the crystallite which then would allow localized recrystallization, enabling defect repair. This assumption is a possible explanation for the reduced ion mobility, revealed by NMR spectroscopy, and, thus, most likely smaller defect density in BaLiF3 prepared by high-energy ball milling compared to thermally synthesized BaLiF3¬ samples.