27Al and 23Na NMR spectroscopy and structural modeling of aluminofluoride minerals

Simulations of high-resolution 19 F-decoupled 27 Al and 23 Na magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of the aluminofluoride minerals, cryolite, cryolithionite, thomsenolite, weberite, chiolite, prosopite, and ralstonite combined with theoretical modeling have given accurate values of chemical shift (δ iso ), and quadrupolar interaction parameters (C q and η), thereby eliminating ambiguities incurred by the complex nuclear interactions. These NMR data have been correlated with local electronic environments in the minerals, which were calculated using Full Potential Linearized Augmented Plane Wave (FP LAPW) modeling based on the structures from X-ray diffraction (XRD) data. This combination of NMR, XRD, and modeling techniques allowed the analysis and optimization of the crystal structures. The electronegativities and distances of neighboring ions, represented here by an environmental parameter χ, are shown to control δ iso of both 23 Na and 27 Al. The calculations using χ, also show that the ions beyond the nearest neighbor play an important role in determining δ iso of 27 Al and 23 Na in these aluminofluoride minerals, and the substitution of OH for F significantly affects the shielding around 27 Al in prosopite and ralstonite. There is a positive correlation between the site distortion at the Na and Al sites and the values of C q in these aluminofluoride minerals.