High anion conductivity in a ternary non-equilibrium phase of BaF2 and CaF2 with mixed cations

Abstract

A highly conductive ternary fluoride with mixed cations is prepared by joint high-energy ball milling of cubic BaF₂ and CaF₂ in the ratio 0.4 : 0.6. The sample produced at room temperature consists of a nanocrystalline, defect-rich mixed (Ba,Ca)F₂ phase with retained cubic symmetry as well as of single-phase CaF₂ particles. The anion conductivity of the mixed phase, which decomposes at higher temperature (770 K) into BaF₂ and CaF₂, exceeds that of single-phase nanocrystalline BaF₂ by two and that of CaF₂ by four orders of magnitude. In turn, these conductivities are each greater by about two orders than those of the respective microcrystalline counterparts. Structural features of the samples are characterized by X-ray diffraction, TEM and ¹⁹F MAS NMR spectroscopy. Static ¹⁹F NMR spectra confirm the unexpectedly high anion conductivity probed by impedance spectroscopy.