Structural Origins of High MoO₃ Solubility in Peraluminous Borosilicate Glasses

Molybdenum (Mo) imposes strict loading limits in conventional borosilicate nuclear waste glasses due to the tendency of tetrahedral molybdate [MoO₄]^2- species to phase-separate and crystallize as alkali molybdates. Here, we demonstrate an unprecedented 13.96 wt % (7.51 mol %) MoO₃ solubility in peraluminous sodium aluminoborosilicate glassesa ∼15× increase over their peralkaline counterparts. Using Raman spectroscopy, multinuclear and dipolar-correlation magic angle spinning nuclear magnetic resonance (MAS NMR), electron paramagnetic resonance (EPR), and scanning transmission electron microscopy (STEM)-energy dispersive spectroscopy (EDS), we reveal that Na-deficient, low optical basicity conditions stabilize octahedral MoO₆ units, which polymerize into molybdite-like Mo-O clusters dispersed within the glass matrix. These Mo-rich clusters suppress the formation of depolymerized [MoO₄]^2- environments typically responsible for Na₂MoO₄ precipitation and instead promote the formation of Na₂Mo₂O₇ as the saturation phase. Concurrently, Mo solubility drives the conversion of AlO₄ ^- to higher-coordination AlO₅ species, liberating Na⁺ that is subsequently sequestered in molybdate-rich domains. The combined evolution of Mo coordination, modifier redistribution, and network depolymerization provides a mechanistic basis for the markedly enhanced Mo solubility in peraluminous compositions. These findings establish new structural guidelines for designing aluminoborosilicate waste forms with substantially greater capacity to incorporate Mo-rich nuclear waste streams.