Order-to-disorder phase transformation in ion irradiated uranium-bearing delta-phase oxides RE6U1O12 (RE=Y, Gd, Ho, Yb, and Lu)

Polycrystalline uranium-bearing compounds Y6U1O12, Gd6U1O12, Ho6U1O12, Yb6U1O12, and Lu6U1O12 samples were irradiated with various ions species (300 keV Kr++, 400 keV Ne++, and 100 keV He+) at cryogenic temperature (∼100 K), and the microstructures were examined following irradiation using grazing incidence X-ray diffraction and transmission electron microscopy. The pristine samples are characterized by an ordered, fluorite derivative structure, known as the delta phase. This structure possesses rhombohedral symmetry. Amorphization was not observed in any of the irradiated samples, even at the highest dose ∼65 dpa (displacement per atom). On the other hand, some of these compounds experienced an order-to-disorder (O–D) phase transformation, from an ordered rhombohedral to a disordered fluorite structure, at ion doses between 2.5 and 65 dpa, depending on ion irradiation species. Factors influencing the irradiation-induced O–D transformation tendencies of these compounds are discussed in terms of density functional theory calculations of the O–D transformation energies.