INFLUENCE OF OCTAHEDRAL POLYMERIZATION ON NA-23 AND AL-27 MAS NMR IN ALKALI FLUOROALUMINATES

Solid state Na-23 and Al-27 MAS NMR spectra of NaF, alpha- and beta-AlF3, cryolite (Na3AlF6), chiolite (Na5Al3F14), and elpasolite (KNaAlF6) have been studied. Structures and chemical compositions have been checked by X-ray diffraction, DTA-TGA, and wet chemical analyses (ICP-AES, AAS, ISE). The Al-27 NMR isotropic chemical shifts range from - 13.2 to + 1.4 ppm, indicating octahedral coordination by F, which is in agreement with crystallographic data. Compared to AlO6 octahedra in aluminates, the AlF6 octahedra in fluoroaluminates are at least 20 ppm more shielded. The Al-27 results show that Al chemical shifts become more shielded as the degree of polymerization of the AlF6 octahedra increases. S0 structures with isolated AlF6 octahedra fall into a region around 0 ppm with a spread of only 2 ppm. Sheet structures with S2 and S4 octahedra have Al resonances at - 1 to - 3 ppm, and Al in network structures resonates at - 12.5 to - 13.2 ppm. This trend is explained by a decrease in the number of nonbridging F atoms per octahedron (NBF/O). Consequently, Na-23 NMR depends on the type of AlF6 polymerization. Na that is octahedrally coordinated stepwise becomes more shielded from 7.2 ppm for NaF, to 2.4 ppm for cryolite, to - 6 ppm for chiolite. Eightfold-coordinated Na resonates at more negative chemical shifts than sixfold-coordinated Na, - 9.3 ppm in cryolite and - 21 ppm in chiolite. The observation of the Al-27 and Na-23 signals of two closely related structures, cryolite and elpasolite, indicates that both Al-27 and Na-23 NMR are insensitive to cationic substitution within the lattice, probably because of the ionic character of the Al-F and Na-F bonds.