Mixed Alkaiine-Earth Effect in the Metastable Anion Conductor Ba_(1-x)Ca_xF2 (0 ≤ x ≤ 1): Correlating Long-Range Ion Transport with Local Structures Revealed by Ultrafast ~(19)F MAS NMR

摘要

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 metastabiiity. 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 magic angle spinning (MAS) nuclear magnetic resonance (NMR). Nanocrystal-line samples of Ba_(1-x)Ca_xF2 (0 ≤ x ≤ 1) were prepared according to a mechanocheniical route from BaF2 and CaF2. The resulting DC ion conductivity, when plotted as a function of x, passes through a well-developed maximum, wliich is located at x_m = 0.5, while the associated activation energy E_a shows aminimum at x_m. As revealed by ~(19)F MAS NMR, five magnetically inequivalent F sites are present in the cation-mixed fluorides. These sites are characterized by a disrinct 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 interstitiais, as well as the migration energy leading to the fast ion conduction observed.