Structural characterisation of glassy phases in the system Na 2O–Al 2O 3–P 2O 5 by MAS and solution NMR, EXAFS and vibrational spectroscopy

Abstract

The structure of sodium aluminophosphate glasses with a fixed O/P ratio of 4 and 3.5 has been investigated. For the monophosphate compositions, the phosphate network is completely depolymerised so that the glassy network is built through the aluminium atoms which are distributed on three different sites but lying mainly in tetrahedral coordination. 31P Magic angle spinning (MAS)-NMR spectra show the existence of three different cationic environments for the PO 4 3− ions. In the glasses with the diphosphate compositions, we observed the disproportionation of some diphosphate groups into monophosphate and triphosphate ions crosslinked by the aluminium atoms whose average coordination increases. The diphosphate groups give a typical vibrational spectrum but the detection of the other phosphate molecular groups is not obvious. The Ga–Al substitution allows better vibrational assignment. MAS-NMR spectra of these two cations show that these two conditional glass former does not behave exactly in the same way in the glassy state.