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 Ma1−xMbxF2 (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 Ca1−xSrxF2 and Ba1−xSrxF2 samples. For the Ba/Ca-system this synthesis route is only partially successful. X-ray diffraction and 19F MAS NMR spectroscopy were used to characterize all samples concerning their crystal structure and local fluorine coordination. Calculations of 19F chemical shifts with the superposition model along with probability calculations for the intensity of the individual 19F 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.