Ion irradiation effects for two pyrochlore compositions: Gd2Ti2O7 and Gd2Zr2O7

Pyrochlore is an important nuclear waste form phase for actinide immobilization. Two synthetic pyrochlores, Gd2Ti2O7 and Gd2Zr2O7 were irradiated at various temperatures (25 K to 1073 K) by different ion species (1.5 MeV Xe+, 1.0 MeV Kr+, and 0.6 MeV Ar+). The titanate pyrochlore amorphized at relatively low doses (0.5 ∼ 0.6 dpa). Temperature dependence of the amorphization dose for titanate pyrochlore was measured, and the critical temperatures for amorphization were 1300 K, 1100 K and 950 K by 1.5 MeV Xe+, 1.0 MeV Kr+ and 0.6 MeV Ar+, respectively. The higher critical temperature for the heavier ion irradiation is consistent with an amorphization mechanism by which the heavier ion produces a larger cascade. The zirconate pyrochlore, Gd2Zr2O7, showed a strong amorphization resistance. Gd2Zr2O7 did not become amorphous under 1.0 MeV Kr+ and 1.5 MeV Xe+ irradiation. After prolonged irradiation (up to 7 dpa) even at a temperature of 25 K, no amorphization was observed. The irradiated zirconate pyrochlore showed abundant dislocations as observed by TEM. The pyrochlore structure of Gd2Zr2O7 transformed to the fluorite structure after irradiation. The diffraction patterns of irradiated Gd2Zr2O7 showed the existence of short-range ordering of cations. The large difference between these two pyrochlores emphasizes the strong effect of chemical composition on radiation-induced amorphization