Radiation-induced amorphization of rare-earth titanate pyrochlores

Single crystals of the entire series of A 2 Ti 2 O 7 (A =Sm to Lu, and Y) pyrochlore compounds were irradiated by 1-MeV Kr + ions at temperatures from 293 to 1073 K, and the microstructure evolution, as a function of increasing radiation fluence, was characterized using in situ transmission electron microscopy (TEM). The critical amorphization temperature, T c , generally increases from ∼480 to ∼1120 K with increasing A-site cation size (e.g., 0.977 A for Lu 3 + to 1.079 A for Sm 3 + ). An abnormally high susceptibility to ion beam damage was found for Gd 2 Ti 2 O 7 (with the highest T c of ∼1120 K). Factors influencing the response of titanate pyrochlores to ion irradiation-induced amorphization are discussed in terms of cation radius ratio, defect formation, and the tendency 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 configuration and the structural disorder. Pyrochlore compositions that have larger structural deviations from the ideal fluorite structure, as evidenced by the smaller 48f oxygen positional parameter, x, are more sensitive to ion beam-induced amorphization.

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