PNNL

Abstract

Iron phosphate glasses, known for their exceptional chemical durability and applicability in nuclear waste management, have gained significant attention over the years. The complex nature of these glasses is linked to the coexistence of Fe3+ and Fe2+, which plays a crucial role in determining their structures and properties. This work used molecular dynamics simulations to study the structural alterations in Na2O-Fe2O3-FeO-P2O5 glasses with varying oxide content, elucidating the influence of the Fe2+/Fe3+ redox ratio. The redox ratio and modifier contents significantly affected short-range and medium-range order in the glasses. Significant changes in the local environments around P5+ and Fe3+ were observed, as reflected by the bond distances and coordination numbers. Preference ratios supported the association of Na+ cations with Fe3+, while the strong preferential association of Fe2+ ions toward P5+ confirmed its primary contribution as a glass modifier. The disruptions in P–O–P bonds upon changing FeO suggest that glass depolymerization is caused by FeO. These glasses achieved higher connectivity with increasing Fe3+/?Fe ratios compensating phosphorous Q2 and Q3 units and iron Q5 units to form Q4 units. The elimination of these non-bridging oxygens with increasing Fe3+/?Fe ratios, thus creating P–O–Fe, is recognized as one of the possible causes of enhanced connectivity. Quantitative structure-property relationship (QSPR) analyses with different structural descriptors were used to correlate with measured properties. The analyses provided valuable insights into structure-property relationships, emphasizing the importance of choosing relevant energy parameters and defining glass network connectivity, particularly in Fnet descriptors. It was found the Fe–O–P linkage density exhibits strong correlations to measured dissolution rates supporting the importance of these linkages on improving the chemical durability in iron phosphate glasses.