Sol–gel synthesis of Al2O3–P2O5 glasses: mechanistic studies by solution and solid state NMR
Colorless and transparent aluminium phosphate gels and glasses with P/Al ratios ranging from 0.5 to 3.0 are prepared via the sol–gel process using aluminium lactate and phosphoric acid as precursors. This glass-forming range is significantly wider than accessible via the melt cooling route. Optimum pH ranges and thermal treatment conditions have been established on the basis of extensive solution- and solid state NMR spectroscopies. The NMR spectra help identify the various types of local environments present in the sols, gels and glasses, thereby providing a mechanistic understanding of the molecular transformation processes involved. Furthermore, they provide a basis for discussing the structural evolution as a function of composition. The formation of transparent gels and glasses is closely connected to high concentrations of the mixed [Al(lact)1(H2O)4]2+ and [Al(lact)2(H2O)2]+ complexes in the precursor solution. Addition of H3PO4 to such solutions results in multiple equilibria involving octahedrally coordinated aluminium complexes having variable lactate, phosphate and water ligands. The structure of the dried xerogels is dominated by the four octahedrally coordinated aluminium environments [Al(lact)3], [Al(lact)2(PO3)2]−, [Al(lact)1(PO3)4]2−, and [Al(PO3)6]3−, involved in various degrees of polymerization. Heat treatment of the dried xerogels at temperatures up to 400 °C results in further polymerization, producing AlO4, AlO5, and AlO6 coordination states. 27Al–31P double resonance experiments confirm that the extent of Al/P connectivity in the glasses is generally maximized. For those glasses that can be prepared by either sol–gel or melt-cooling procedures, all of the NMR experiments indicate virtually identical network structures for both preparation routes.