New insights into oxygen environments generated during phosphate glass alteration: a combined 17O MAS and MQMAS NMR and first principles calculations study

Abstract

In the present study, we used a combination of ¹⁷O NMR methods at a high magnetic field with first-principles calculations in order to characterize the oxygen sites in a series of hydroxylated sodium phosphate compounds, namely the hydrogen pyrophosphate Na₂H₂P₂O₇ and the hydrogen orthophosphates NaH₂PO₄, NaH₂PO₄·H₂O and NaH₂PO₄·2H₂O. The chemical shifts and quadrupolar parameters of these compounds were interpreted in terms of local and semi-local environment, i.e., the chemical composition of the immediate surroundings and the nature of the bonds, e.g. hydrogen bonding. The magnitude of the quadrupolar interaction and its asymmetry were revealed to be a precise indicator of the local structure in sodium hydrogen phosphates. Our ¹⁷O NMR experimental and computing approach allowed for identification and quantification of the different crystalline phases involved in the weathering mechanism of a sodium phosphate glass, even in small amount.