Correlation between 19F environment and isotropic chemical shift in barium and calcium fluoroaluminates

High-speed MAS F-19 NMR spectra are recorded and reconstructed for 10 compounds from BaF2-AlF3 and CaF2-AlF3 binary systems which leads to the determination of 77 isotropic F-19 chemical shifts in various environments. A first attribution of NMR lines is performed for 8 compounds using a superposition model as initially proposed by B. Bureau et al. The phenomenological parameters of this model are then refined to improve the NMR line assignment. A satisfactory reliability is reached with a root-mean-square (RMS) deviation between calculated and measured values equal to 6 ppm. The refined parameters are then successfully tested on alpha-BaCaAlF7 whose structure was recently determined. Finally, the isotropic chemical shift ranges are defined for shared, unshared, and "free" fluorine atoms encountered in the investigated binary systems. So, the fluorine surroundings can be deduced from the NMR line positions in compounds whose structure is unknown. Such an approach can also be applied to fluoride glasses.