The response of the pyrochlore structure-type to ion-beam irradiation

Pyrochlore with the general formula of $A3+2B4+2O7$ (Fd3m; Z = 8) has been proposed as the candidate waste form for the immobilization of actinides, particularly plutonium from dismantled nuclear weapons. Because actinides decay by α-decay events, radiation effects on the waste form are a concern. The effects of radiation on different pyrochlore compositions, A₂B₂O₇ (A = La ∼ Lu, and Y; B = Ti, Sn, and Zr), have been investigated by 50 KeV He+, 600 KeV Ar+, 1.0 MeV Kr+, and 1.5 MeV Xe+ ion irradiations.; Titanate pyrochlores are generally sensitive to ion beam damage and can be amorphized at a low damage level (∼0.2 dpa). The critical amorphization temperature, T_c, increases from ∼480 to ∼1120 K with increasing A-site cation size. A dramatically increasing radiation “resistance” to ion beam induced-amorphization has been observed with increasing Zr-content in the Gd₂Ti_2−xZr_xO₇ system. The pure end-member, Gd₂Zr₂O₇, cannot be amorphized, even at doses as high as ∼100 dpa. Although zirconate pyrochlores are generally considered to be radiation “resistant”, ion beam-induced amorphization occurs for La₂Zr₂O₇ at a dose of ∼5.5 dpa at room temperature. Stannate pyrochlores A₂Sn ₂O₇ (A = La, Nd, Gd) are readily amorphized by ion beam damage at a relatively low dose (∼1 dpa) at room temperature; while no evidence of amorphization has been observed in A₂Sn₂O₇ (A = Er, Y, Lu) irradiated with 1 MeV Kr+ ions at a dose of ∼6 dpa at 25 K.; The factors that influence the response of different pyrochlore compositions to ion irradiation-induced amorphization are discussed in terms of cation radius ratio, defect formation energies, and the tendency of the pyrochlore structure-type to undergo an order-disorder transition to the defect-fluorite structure. The “resistance” of the pyrochlore structure to ion beam-induced amorphization is not only affected by the relative sizes of the A- and B-site cations, but also the cation electronic configurations. Pyrochlore compositions that have larger structural deviations from the ideal fluorite structure are more sensitive to ion beam-induced amorphization. These fundamental results provide insight into the structural and compositional controls on radiation-induced amorphization of pyrochlores. This understanding can be used for the design and selection of materials used for the immobilization of actinides.