The Effect of Potassium Superoxide (KO2) Surface Symmetry on Its Thermal Decomposition: Insights from First-Principles and Experimental Analyses

Potassium superoxide (KO2) can form during the oxidation of residual potassium in NaK-contaminated cold traps of sodium-cooled fast reactors. Its strong oxidizing nature, combined with limited thermal stability, raises safety concerns during shutdown and maintenance. Here, we integrate first-principles calculations with experiments to clarify the facet stability, temperature-driven surface evolution, and stepwise thermal decomposition of KO2. Guided by the tetragonal I4/mmm crystal symmetry of bulk KO2, symmetry-non-equivalent low-index facets and relevant surface terminations were systematically evaluated to identify physically meaningful exposed surfaces. Ab initio molecular dynamics (AIMD) simulations further show that heating induces progressive surface amorphization and enhanced oxygen mobility, accompanied by the emergence of shortened O-O bonds and outward migration of oxygen species. Kinetic analysis using the climbing-image nudged elastic band (CI-NEB) method indicates that oxygen evolution is preferentially mediated by O2 release rather than atomic oxygen escape. Differential scanning calorimetry (DSC) reveals two endothermic events consistent with sequential decomposition, while X-ray diffraction (XRD) confirms the transformation of KO2 into K2O. Collectively, these results provide an atomistic-to-macroscopic understanding of KO2 decomposition, offering practical guidance for defining safer preheating windows and handling strategies for NaK-contaminated components.