Structural Responses of Aluminoborosilicate Glasses to Pressure and Iodine Incorporation: An Advanced NMR Investigation

Abstract

Iodine-129 is a critical challenge for nuclear waste immobilization due to its high mobility and extremely long half-life (1.57 × 10⁷ years). Although high-pressure glass synthesis has been shown to significantly enhance iodine solubility, the structural mechanisms governing its incorporation are only partially understood. Here, we combine advanced multinuclear solid-state NMR spectroscopy (¹¹B, ¹⁷O, ²³Na, ²⁷Al, and ²⁹Si) with molecular dynamics and DFT calculations of NMR interactions to disentangle the respective roles of pressure and iodine incorporation in doubly isotopically enriched (¹⁷O and ²⁹Si) aluminoborosilicate glasses synthesized at 1 and 2 GPa. NMR results show that iodine incorporation increases network polymerization primarily through the consumption of Si non-bridging oxygens and the formation of additional Si–O–Si and Si–O–B linkages, whereas pressure independently promotes densification via enhanced BO₄ formation and stabilization of higher-coordinated aluminum species. Notably, ¹⁷O MAS, 3QMAS, and heteronuclear correlation experiments provide novel atomic-scale insights into oxygen speciation in high-pressure aluminoborosilicate glasses, a regime poorly documented. MD-derived structural models reproduce pressure-induced trends and enable quantitative interpretation of NMR data. These results establish a mechanistic framework for iodine incorporation under high-pressure vitrification and help of designing advanced glass matrices for long-term ¹²⁹I immobilization.