A novel sol−gel route based on aluminum lactate and sodium polyphosphate aqueous
solutions has been developed to produce sodium aluminophosphate gels and glasses with
the composition 43.8:12.5:43.8 Na2O/Al2O3/P2O5. Preparations were optimized on the basis
of detailed 27Al solution- state NMR data, revealing the nature of the dominant precursor
species as a function of pH and gelation temperature. A pH range of 1.8 to 3.6 was found to
provide optimum conditions for the formation of homogeneous and transparent glasses. This
pH range corresponds to the maximum concentration of mixed Al(lact)1(H2O)4
2+ and the
Al(lact)2(H2O)2
+ precursor complexes, which appear to have the highest reactivity toward
phosphate. The further replacement of lactate by phosphate ligands is favored by increasing
the gelation temperatures above ambient. Upon heating to 400 °C the gels are converted to
bulk glasses with thermal and structural characteristics identical to those of glasses prepared
via traditional melt-cooling from 1350 °C. Solid-state 27Al and 31P NMR data and 27Al{31P}
rotational echo double resonance (REDOR) spectra in particular confirm that the degree of
Al/P connectivity is virtually identical in the glassy materials prepared by both routes. Thin
films of this material have been prepared successfully on silica substrates and subsequently
characterized by atomic force microscopy (AFM) and secondary neutral mass spectrometry
(SNMS). These data indicate that the amount of residual carbon can be removed oxidatively
upon heating by using HClO4 rather than HCl as the proton source.
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