Mechanochemical synthesis, structure, and properties of solid solutions of alkaline earth metal fluorides: Ma1−xMbxF2 (M: Ca, Sr, Ba)

Abstract The capability of mechanochemical synthesis for the formation of solid solutions of alkaline earth metal fluorides M a 1− x M b x F 2 (M: Ca, Sr, Ba) was tested by fluorination of metal acetates and metal hydroxides with ammonium fluoride directly at milling. Evidence was found for a mutual substitution of cations on their lattice positions in Ca 1− x Sr x F 2 and Ba 1− x Sr x F 2 samples. For the Ba/Ca-system this synthesis route is only partially successful. X-ray diffraction and 19 F MAS NMR spectroscopy were used to characterize all samples concerning their crystal structure and local fluorine coordination. Calculations of 19 F chemical shifts with the superposition model along with probability calculations for the intensity of the individual 19 F lines, performed in dependence on the molar composition of the samples, perfectly agree with the experimental findings. The fluoride ion conductivity of as-prepared samples, determined by temperature dependent DC conductivity measurements, is significantly higher than those of crystalline binary fluorides. Moreover, a higher F − ion conductivity is observed for samples with higher mixing grade in the Ca/Sr-and the Ba/Sr-systems.