RADIATION DAMAGE IN LA- AND TH-DOPED YTTRIUM TITANATES

Pyrochlore, A₂B₂O₇, is a phase that has received considerable attention for the immobilization of plutonium and the “minor” actinides, Np, Cm, Am. Y₂Ti₂O₂ pyrochlore, doped with La, have been sintered at 1373 K for 12 h with designed compositions of (La_xY_1-x)₂Ti₂O₇ (x = 0, 0.08, 0.5, and 1), and the phase compositions were analyzed by X-ray diffraction. Limited amounts of La were incorporated into yttrium titanate pyrochlore structure for La-doped samples; while, the end member composition of La₂Ti₂O₇ formed a layered perovskite structure. Ion beam-induced amorphization occurred for all compositions in the (La_xY_1-x)₂Ti₂O₇ binary under 1 MeV Kr²⁺ irradiation at room temperature, and the critical amorphization dose decreased with increasing amounts of La³⁺. The critical amorphization temperatures for Y₂Ti₂O₇, (La_0.162Y_0.838)₂Ti₂O₇ and La₂Ti₂O₇ were determined to be ~780, 890 and 920 K, respectively. Th⁴⁺ and Fe³⁺-doped yttrium titanate pyrochlores were synthesized at 1373 K by sintering Y₂Ti₂O₇ with (ThO₂+Fe₂O₃). Pyrochlore structures and the chemical composition were primarily identified by the X-ray diffraction and energy dispersive X-ray (EDX) measurements. The unit cell parameter and the critical amorphization dose (1 MeV Kr²⁺ at room temperature) increase for yttrium titanate pyrochlores with the addition of Th. The increasing “resistance” to amorphization with less La and greater Th and Fe contents for (Y_1-xLa_x)₂Ti₂O₇ and Y₂Ti₂O₇-Fe₂O₃-ThO₂ systems, respectively, are consistent with the changes in the average ionic radius ratio at the A-sites and B-sites. These results suggest that the addition of lanthanides and actinides (e.g., Th, U, or Pu) will affect the structural stability, as well as the radiation response behavior of the pyrochlore structure-type.