Mixed Alkaline-Earth Effect in the Metastable Anion Conductor Ba1-xCaxF2 (0 ≤ x ≤ 1): Correlating Long-Range Ion Transport with Local Structures Revealed by Ultrafast 19F MAS NMR

Fast ion conductors are urgently needed in many research areas of materials science. Advanced preparation strategies take advantage of an interplay of structural disorder, nanosize effects, and metastability. Getting access to detailed insights into the microstructure of such solids is crucial to identify the origins of fast ion conduction. High-resolution and high-sensitive spectroscopic techniques are well-suited to meet this challenge. Here, ion transport properties of a highly conducting, metastable fluoride with two isovalent cations were interrelated with the microscopic, atomic-scale structure probed by ultrafast F-19 magic angle spinning (MAS) nuclear magnetic resonance (NMR). Nanocrystalline samples of Ba1-xCaxF2 (0 <= x <= 1) were prepared according to a mechanochemical route from BaF2 and CaF2. The resulting DC ion conductivity, when plotted as a function of x, passes through a well-developed maximum, which is located at x(m) = 0.5, while the associated activation energy E-a shows a minimum at x(m). As revealed by F-19 MAS NMR, five magnetically inequivalent F sites are present in the cation-mixed fluorides. These sites are characterized by a distinct number of Ba and Ca cations in the first coordination shell: [Ba](n)[Ca](4-n) (0 <= n <= 4). The mixed sites with n = 1,2,3 dominate the NMR spectra at intermediate values of x. Presumably, the mixed cation sublattice, causing the metastability of the compounds, influences both the formation energy of, for example, F interstitials, as well as the migration energy leading to the fast ion conduction observed.