Molybdenum (Mo) imposes strict loading limits in conventional borosilicate nuclear waste glasses due to the tendency of tetrahedral molybdate MoO42– 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 MoO42– 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.
Joseph et al. (Wed,) studied this question.