Radiation-induced amorphization of rare-earth titanate pyrochlores

Single crystals of the entire series of $A_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ $(A=\mathrm{Sm}$ to Lu, and Y) pyrochlore compounds were irradiated by 1-MeV ${\mathrm{Kr}}^{+}$ 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 Å for ${\mathrm{Lu}}^{3+}$ to 1.079 Å for ${\mathrm{Sm}}^{3+}).\mathrm{}$ An abnormally high susceptibility to ion beam damage was found for ${\mathrm{Gd}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ (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.