Phosphate speciation in sodium borosilicate glasses studied by nuclear magnetic resonance

The structure of RNa2O · B2O3 · KSiO2 · xP2O5 (0.5 < R < 2; 0.86 < K < 3) borosilicate glasses has been studied by nuclear magnetic resonance (NMR). 31P magic angle spinning (MAS), double quantum-magic angle spinning (DQ-MAS) and 31P–11B transfer of populations under double resonance magic angle spinning (TRAPDOR MAS) NMR were used to determine the phosphate speciation in the glasses and their connectivity with the borosilicate network. The structure of the glass network was characterized with 11B, 29Si and 23Na MAS NMR. Ab initio calculations of the 31P chemical shielding were carried out in order to confirm the connectivity between phosphorus and the structural units of the borosilicate glass network. Na3PO4 (monophosphate), Na4P2O7 (diphosphate) and P–O–B species (mono- and diphosphate groups with borate units as the next nearest neighbors) are found all along the compositional range studied. The proportion of the P–O–B groups increases as the glass optical basicity decreases, while the proportions of mono- and diphosphate species decrease. The change in the glass transition temperature of the phospho-borosilicate glasses with respect to that of the borosilicate ones is discussed in terms of the structural characterization. The formation of phosphate species gives rise to the increase in Tg, which is attributed to the re-polymerization of the silicate network, while the formation of P–O–B bonds weakens the glass network and produces a decrease in the glass transition temperature.