Network Structure and Dissolution Properties of Phosphate-Doped Borosilicate Glasses

The addition of phosphorus to conventional borosilicate glasses is explored as a way to increase the incorporation of high-field-strength cations such as Mo(VI) into chemically durable materials. We have studied the molecular-level structure of borosilicate glasses containing up to 4 mol % P2O5 using multinuclear magnetic resonance (NMR) spectroscopy, alongside their dissolution behavior in water using static tests. Phosphorus is predominantly present as phosphate dimers but tends to cluster into alkali-phosphate regions at higher loadings. In the presence of molybdenum, macroscale segregation into a water-soluble amorphous P- and Mo-rich phase is observed, reducing the chemical durability. However, the formation of a silicate-rich amorphous layer significantly retards ion release after a few days, suggesting that, despite the inferior durability of the phosphate phase, the overall durability of the glass monolith may prove acceptable for further consideration as a high-Mo nuclear wasteform.