Mechanisms of cesium incorporation and thermal stability in sodalite

The immobilization of radioactive cesium (Cs) within sodalite frameworks is a critical challenge in nuclear waste management. This study employs density functional theory and ab initio molecular dynamics simulations to investigate the structural evolution, thermodynamic stability, and bonding mechanisms of Cs-incorporated sodalite (Na_8-xCs_xAl₆Si₆O₂₄Cl₂, 0 ≤ x ≤ 8). Formation energy calculations reveal a concentration-dependent preference for Cs substitution, with a minimum of 0.33 eV per Cs atom at x = 4-5, signifying optimal stability at intermediate loadings. Symmetric Cs distributions across adjacent Na/Cs-Cl tetrahedrons suppress lattice strain, whereas clustering triggers pronounced distortion and elevates formation energy by up to 0.7 eV. Moreover, non-equivalent Cs substitution sites in symmetric configurations are energetically favored over equivalent ones. Cs-Cl bonds exhibit predominantly ionic character (Bader charge: Cs +0.9, Cl -0.7) with subtle covalent contributions, as evidenced by electron localization function (ELF ≈ 0.1) and projected density of states overlap between Cs 5p and Cl 3p orbitals. In contrast, Na-Cl bonds remain purely ionic. Ab initio molecular dynamics further establish x = 6 as the thermal stability threshold: Cs migration accelerates beyond this concentration (MSD > 0.3 Å), while Na atoms remain immobile across all compositions. These atomic-scale insights yield quantitative design criteria for durable sodalite-based waste forms through optimized Cs concentration and distribution symmetry.