Helium irradiation induced micro-swelling and phase separation in pyrochlore Lu<inf>2</inf>Ti<inf>2</inf>O<inf>7</inf>

Polycrystalline pyrochlore Lu2Ti2O7 was irradiated with 200 keV He+ ions at room temperature to fluences ranging from 2.0 × 1015 to 2.0 × 1017 He+/cm2. Irradiation-induced microstructural evolution was examined by grazing incidence X-ray diffraction and cross-sectional transmission electron microscopy. A micro-swelling (lattice expansion) induced by cation ion disordering was observed, which an obvious swelling was achieved at a fluence of 2.0 × 1016 He+/cm2, and saturated at a fluence of 1.0 × 1017 He+/cm2 with cation ions fully disordered. Furthermore, during the irradiation process, the pristine pyrochlore phase was found to form by a fluence of 5.0 × 1016 He+/cm2, corresponding to an average ballistic damage dose of ∼0.25 displacements per atom (dpa) for the near surface examined region. No amorphization was observed even to the highest fluence of 2.0 × 1017 He+/cm2. The experimental observations are further discussed in the context of antisite formation in the pyrochlore Lu2Ti2O7.