Abstract
Molybdenum (Mo) imposes strict loading limits in conventional borosilicate nuclear waste glasses due to the tendency of tetrahedral molybdate [MoO(4)](2-) species to phase-separate and crystallize as alkali molybdates. Here, we demonstrate an unprecedented 13.96 wt % (7.51 mol %) MoO(3) solubility in peraluminous sodium aluminoborosilicate glassesa ∼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 MoO(6) units, which polymerize into molybdite-like Mo-O clusters dispersed within the glass matrix. These Mo-rich clusters suppress the formation of depolymerized [MoO(4)](2-) environments typically responsible for Na(2)MoO(4) precipitation and instead promote the formation of Na(2)Mo(2)O(7) as the saturation phase. Concurrently, Mo solubility drives the conversion of AlO(4) (-) to higher-coordination AlO(5) 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.