A new sol-gel route to aluminum fluoride phosphate glasses: Mechanistic investigations by NMR spectroscopy

Homogeneous and transparent sodium aluminum fluoride phosphate gels and glasses were prepared by a novel single-step sol-gel route Using aqueous solutions of di-sodium fluorophosphate (Na2PO3F) and aluminum lactate (Al(laCt)(3)) as precursors. The structural evolution from the solution to the gel and the final glass is monitored by multinuclear high-resolution liquid and solid-state NMR techniques, providing detailed insight into the reaction mechanism. The F-19 NMR results indicate that the covalent F-P bond is stable in these solutions and remains unaffected by the formation of Al-O-P linkages during the polycondensation process from the sol to the gel state. The subsequent annealing at T > 100 degrees C of the xerogel promotes intramolecular fluorine transfer from phosphorus to aluminum, concomitant with the removal of lactate ligands upon heating, resulting in sodium aluminum fluoride phosphate glasses as the final product. In these glasses, all of the fluorine is coordinated to octahedral aluminum (Al(F,OP)(6) units), thereby resulting in a significant increase in the Al(VI) concentration compared with the corresponding fluoride-free glasses.