Structure Resolution of Ba5Al3F19 and Investigation of Fluorine Ion Dynamics by Synchrotron Powder Diffraction, Variable-Temperature Solid-State NMR, and Quantum Computations

The room temperature structure of Ba5Al3F19 has been solved using electron microscopy and synchrotron powder diffraction data. One-dimensional (1D) 27Al and ultrafast magic-angle-spinning (MAS) 19F NMR spectra have been recorded and are in agreement with the proposed structural model for Ba5Al3F19. The 19F isotropic chemical shift and 27Al quadrupolar parameters have been calculated using the CASTEP code from the experimental and density functional theory geometry-optimized structures. After optimization, the calculated NMR parameters of both the 19F and 27Al nuclei show improved consistency with the experimental values, demonstrating that the geometry optimization step is necessary to obtain more accurate and reliable structural data. This also enables a complete and unambiguous assignment of the 19F MAS NMR spectrum of Ba5Al3F19. Variable-temperature 1D MAS 19F NMR experiments have been carried out, showing the occurrence of fluorine ion mobility. Complementary insights were obtained from both two-dimensional (2D) exchange and 2D double-quantum dipolar recoupling NMR experiments, and a detailed analysis of the anionic motion in Ba5Al3F19 is proposed, including the distinction between reorientational processes and chemical exchange involving bond breaking and re-formation.