Atomic structure of uranium-incorporated sodium borosilicate glasses: An ab initio study

In this study, we aim to understand the atomic structure of uranium-incorporated sodium borosilicate glasses, with implications for a long-term storage of high-level nuclear waste. We simulated a series of glasses with a general composition of $70\phantom{\rule{0.28em}{0ex}}\text{wt.}%\phantom{\rule{4pt}{0ex}}[{({\text{SiO}}₂)}_{65\ensuremath{-}x}\ifmmode\cdot\else\textperiodcentered\fi{}{({\text{B}}₂{\text{O}}₃)}_x\ifmmode\cdot\else\textperiodcentered\fi{}{({\text{Na}}₂\text{O})}₂₅\ifmmode\cdot\else\textperiodcentered\fi{}{({\text{ZrO}}₂)}₅\ifmmode\cdot\else\textperiodcentered\fi{}{(\text{BaO})}₅]+30\phantom{\rule{0.28em}{0ex}}\text{wt.}%\phantom{\rule{4pt}{0ex}}{\text{UO}}₃$, where $x=5, 10, 15, 20$ mol%. We adopted force-enhanced atomic refinement (FEAR) and melt-quench (MQ) methods to simulate these glasses in cubic supercells consisting of 340--365 atoms. The structure factors obtained from the FEAR models reveal good agreement with diffraction experiments and closely match to those of the MQ models. We quantified the structural topologies of the glasses within local order using higher-order correlation functions such as partial pair distribution functions, bond-angle distribution functions, and coordination numbers. The splitting of uranium-oxygen correlations in the first coordination sphere is observed, and we discuss the structural origins of such splitting, along with the network connectivity of uranium in the glasses. We analyzed the effect of ${\mathrm{SiO}}₂/{\mathrm{B}}₂{\mathrm{O}}₃$ substitution on glass-network connectivity by quantifying bridging and nonbridging oxygen atoms, and ${Q}_n$ distributions. In addition, we investigated the vibrational and electronic properties of the MQ models with $x=10%$ and 20%. The vibrational signatures of the studied glasses are found to be similar to those of ${\mathrm{Na}}₂\mathrm{O}\text{\ensuremath{-}}{\mathrm{SiO}}₂\text{\ensuremath{-}}{\mathrm{B}}₂{\mathrm{O}}₃$ glasses. The electronic density of states near the valence and conduction edges are primarily attributed to oxygen and uranium orbitals, respectively, and the electronic eigenstates in those edges are highly localized.