Ion beam irradiation in La 2Zr 2O 7–Ce 2Zr 2O 7 pyrochlore

Abstract

Generally, zirconate pyrochlores do not experience a radiation-induced transformation from the crystalline-to-amorphous state, but rather disorder to a defect fluorite structure-type. Thus Gd 2Zr 2O 7 has been proposed as a nuclear waste form for the immobilization of plutonium because of its radiation “stability”. In contrast, La 2Zr 2O 7 can be amorphized by a 1.5 MeV Xe + ion irradiation (∼5.5 dpa at room temperature), and the critical amorphization temperature is low (∼310 K). In this study we present data on ion beam irradiations of compositions in the solid solution: (La 1− x Ce x ) 2Zr 2O 7 ( x=0, 0.1, 0.2, 1). Ce is used as an analogue element for Pu because of similarities in charge and size. La 2Zr 2O 7 can be amorphized by a 1.0 MeV Kr + ion irradiation at 25 and 293 K at doses of ∼1.19 and ∼3.42 dpa, respectively, confirming that La 2Zr 2O 7 is susceptible to ion irradiation-induced amorphization. With the addition of 10 mol% Ce in lanthanum–zirconate pyrochlore structure, no ion irradiation-induced amorphization has been observed at room temperature. The critical amorphization dose for (La 0.9Ce 0.1) 2Zr 2O 7 at 25 K is ∼3.55 dpa, and with increasing Ce-content, a higher dose (∼5.20 dpa) is required to fully amorphize (La 0.8Ce 0.2) 2Zr 2O 7 at 25 K. No amorphization occurred for Ce 2Zr 2O 7 at 25 K at a dose of ∼7 dpa. These results suggest that the addition of Ce into the La 2Zr 2O 7 structure increases the stability of the La 2Zr 2O 7 waste form in a radiation environment, which may be attributed to the decreasing average radius of cations in the A-site, resulting from the smaller ionic radii of Ce 3+ (0.114 nm) and Ce 4+ (0.097 nm) as compared to La 3+ (0.116 nm). An ion beam-induced anion-disordered pyrochlore was observed prior to the final transformation to a disordered fluorite structure. The local structural evolution upon ion irradiation was also investigated for Ce-doped La 2Zr 2O 7 pyrochlores with different average A-site cation sizes and valence states using electron energy-loss spectrometry (EELS).