Multinuclear NMR studies on the sol-gel preparation of sodium aluminophosphate glasses

Sodium aluminophosphate gels and glasses in the system NaPO(3)-Al(2)O(3) with P/Al ratios ranging from 9:1 to 1:1 have been synthesized by a novel sol-gel route based on the reaction of aluminum lactate with sodium polyphosphate in aqueous solution. The route from the solution to the gel and the final glass was monitored in situ by liquid and solid state NMR techniques, characterizing the influence of composition and pH on the hydrolysis, polymerization, and vitrification processes. The site distribution in the xerogels is strongly influenced by the gel-processing temperature. At temperatures near 150 degrees Celsius ligation with lactate groups can be nearly suppressed, resulting in maximum Al/P connectivity in the gel. Annealing the gels at temperatures near 400 degrees Celsius produces significant structural rearrangements, resulting in a glassy network that has close structural similarity to the glasses derived from the usual melt-cooling procedure at 1100-1450 degrees Celsius. This has been confirmed by extensive (27)Al, (31)P and (23)Na MAS NMR as well as (27)Al[(31)P] and (27)Al [(1)H] double resonance experiments. Compared to melt-cooling, the sol-gel process permits a significant extension of the glass-forming region towards higher alumina contents.