19F high-resolution NMR studies on cation distribution and F− dynamics in highly conductive BaF2–CaF2 composite prepared by thermal plasma processing

Mixing of immiscible BaF₂ and CaF₂ by mechanical milling or thermal plasma processing leads to a solid composite, whose ionic conductivity is much higher than those of BaF₂ and CaF₂. Distribution of Ba and Ca cations in the BaF₂–CaF₂ composite prepared by thermal plasma processing is examined by ¹⁹F NMR. Analysis of signal intensities of ¹⁹F high-resolution solid-state NMR shows that occupation ratios for the cations at four vertexes of a tetrahedron unit in a fluorite-type structure with one fluoride ion at its center are roughly given by binomial distribution. Further, ¹⁹F–¹⁹F dipolar correlation experiment shows that spatial distribution of both cations is random in a length scale of ca. 2 nm or less. These show that homogeneous mixing of vaporized BaF₂ and CaF₂ in thermal plasma is maintained in solids obtained by rapid cooling and leads to random distribution of both cations. Rearrangement of local configuration of both cations in a length scale less than a few nm occurs well below the decomposition temperature, which relaxes local strain associated with homogeneous mixing of different cations and lowers ionic conductivity. It is further shown that the fluoride ions in Ba-rich local environment are less mobile as compared to those in Ca-rich. The ionic conductivity evaluated using the fluoride-ion exchange rate estimated from ¹⁹F NMR spectra is consistent with the observed one, thus showing that exchange among the fluoride ions in different local environments is the fundamental step of the bulk ion conduction.