Mechanosynthesized nanocrystalline BaLiF3: The impact of grain boundaries and structural disorder on ionic transport

The mechanosynthesis of highly pure nanocrystalline BaLiF 3 is reported. The product with mean crystallite diameter of about 30 nm was prepared by joint high-energy ball-milling of the two binary fluorides LiF and BaF 2 at ambient temperature. Compared to coarse-grained BaLiF 3 with μm-sized crystallites, which is available via conventional solid-state synthesis at much higher temperatures, the mechanosynthesized product exhibits a drastic increase of ionic conductivity by several orders of magnitude. This is presumably due to structural disorder introduced during milling and to the presence of a large volume fraction of interfacial regions in the nanocrystalline form of BaLiF 3 providing fast diffusion pathways for the charge carriers. Starting from mechanosynthesized nanocrystalline BaLiF 3 it is possible to tune the transport parameters in a well defined way by subsequent annealing. Changes of the electrical response of mechanosynthesized BaLiF 3 during annealing are studied in situ by impedance spectroscopy. The results are compared with those of a structurally well-ordered single crystal which clearly shows extrinsic and intrinsic regions of ionic conduction. Enhanced ion conductivity is observed for nanocrystalline BaLiF 3 when compared with the coarse-grained and single-crystalline counterparts.