Microstructural evolution in irradiated uranium-bearing delta-phase oxides A₆U₁O₁₂ (A = Y, Gd, Ho, Yb, and Lu)

Irradiation-induced microstructural evolution in uranium-bearing delta-phase oxides of A₆U₁O₁₂ (A = rare earth cations) were characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Polycrystalline Y₆U ₁O₁₂, Gd₆U₁O₁₂, Ho ₆U₁O₁₂, Yb₆U₁O ₁₂, and Lu₆U₁O₁₂ samples were irradiated with 300 keV Kr⁺⁺ to a fluence of 2 × 10 ²⁰ ions/m² at cryogenic temperature (∼100 K). The crystal structure of these compounds was determined to be an ordered, fluorite derivative structure, known as the delta-phase, a rhombohedral symmetry belonging to space group R3̄. Experimental results indicate that all these compounds are resistant to amorphization to a displacement damage dose of ∼60 displacements per atom. In these experiments, we sometimes observed an irradiation-induced order-to-disorder phase transformation, from an ordered rhombohedral to a disordered fluorite structure.