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

The mechanosynthesis of highly pure nanocrystalline BaLiF3 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 BaF2 at ambient temperature. Compared to coarse-grained BaLiF3 with mm-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 BaLiF3 providing fast diffusion pathways for the charge carriers. Starting from mechanosynthesized nanocrystalline BaLiF3 it is possible to tune the transport parameters in a well defined way by subsequent annealing. Changes of the electrical response of mechanosynthesized BaLiF3 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.