Local ordering, defect dynamics, and phase stability in Sc–Hf oxides under ion irradiation

High-level radioactive waste requires long-term isolation, motivating the development of durable materials beyond conventional glass matrices, which are thermodynamically metastable. Crystalline ceramics, particularly fluorite-derived oxides, are promising due to their stability, radiation tolerance, and capacity to incorporate radionuclides. In this study, we performed swift heavy ion irradiation into β-Sc2Hf7O17, one of the fluorite structural derivatives in the Sc2O3–HfO2 pseudo-binary system, and investigated its order-to-disordered phase transformation by transmission electron microscopy (TEM). The rhombohedral ordered β-phase transformed into a cubic disordered phase due to damage accumulation. Electron diffraction experiments and dark-field TEM observations revealed the formation of an intermediate phase with an ordered structure distinct from both the β-phase and the other equilibrium phases present in the phase diagram. Irradiation-induced evolution in these materials is complex, involving not only disorder and amorphization but also the formation of intermediate ordered phases and nanoscale heterogeneity. Model systems such as β-Sc2Hf7O17 provide insight into these processes, revealing that local ordering can persist or re-emerge under irradiation, potentially enhancing defect accommodation and delaying structural degradation. This highlights the importance of understanding defect dynamics, local structure, and interfacial effects in designing radiation-resistant waste forms capable of maintaining integrity over geological timescales.