Structural evolution of Lu2-xCexTi2O7 pyrochlores under 400 keV Ne irradiation

Lu_2-xCe_xTi₂O₇ (LCTO) pyrochlores were irradiated by 400 keV Ne²⁺ with fluences (dose) of up to 5 × 10¹⁵ ions/cm² (1.875 dpa). The detailed damage process was investigated by combining grazing incident angle X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). Subsequent to the 2% volume swelling at a fluence of 1 × 10¹⁴ ions/cm² (0.037 dpa), the initially swollen LCTO pyrochlore formed both a disordered fluorite phase and a nanocrystalline pyrochlore phase at a fluence of 5 × 10¹⁴ ions/cm² (0.185 dpa). At higher fluences, the fluorite phase diminished as amorphous domains increased in volume when the dose reached a fluence of 1 × 10¹⁵ ions/cm² (0.371 dpa), while the nanocrystalline pyrochlore phase persisted. At the highest fluence of 5 × 10¹⁵ ions/cm² (1.854 dpa), the amorphous fraction decreased, meanwhile the degree of crystallinity of nanocrystalline pyrochlore phase was enhanced, as evidenced by the increased intensity of superlattice diffraction maxima. The phase transformation and recrystallization can be explained by the release of strain in irradiation-induced swollen pyrochlore crystallites. The evolution of the damage process is mainly driven by the differences in the Gibb's free energies of fluorite phase as compared with the pyrochlore phase as a function of grain size. We have demonstrated that ion beam techniques can be used to manipulate the phase stability and crystallite size of pyrochlore. These results provide the basis for tailoring the mechanical strength and response of pyrochlores to extreme radiation environments.