Sol-gel synthesis of sodium-modified AlPO4–SiO2 glasses and structural characterization by solid state NMR

Abstract

Sodium aluminophosphosilicate gels and glasses along the composition line (Na₂O)_x–[(AlPO₄)_0.5(SiO₂)_0.5]_1−x were synthesized via the sol-gel process using sodium acetate, aluminium lactate, phosphoric acid and tetraethyl orthosilicate (TEOS) as precursors. The structural evolution from solution to gel and to the final glass was monitored by ²⁷Al, ³¹P, ²⁹Si and ²³Na magic-angle spinning (MAS) NMR. In the solution state aluminium forms a mixture of octahedral coordinated complexes coordinated to lactate, water and phosphate units with varying degrees of polymerization. With increasing temperature up to about 150 °C the lactate ligands are successively replaced by phosphate, resulting in an increased extent of Al–O–P linking. Annealing the xerogel at 400 °C results in a glassy network containing mostly four-coordinated Al species (Al(IV)), in addition to small concentrations of Al(V) and Al(VI). While the distribution of Al coordination numbers remains constant, the compositional trend of the ²⁷Al chemical shift indicates a gradual replacement of Al–O–P linkages by Al–O–Si linkages with increasing sodium content. This conclusion is confirmed by ²⁷Al{³¹P} Rotational Echo Double Resonance (REDOR) experiments, which reveal a reduction in the number of Al–O–P linkages. Concomitantly, the interaction of Na with the phosphate species is successively increased as indicated by ³¹P chemical shift trends and ³¹P{²³Na} REDOR results. The results illustrate that the network segregation found in pure AlPO₄–SiO₂ sol-gel glasses can be overcome, at least partially, by introduction of the network modifier Na₂O.