Ion irradiation of rare-earth- and yttrium-titanate-pyrochlores

Pyrochlore, A(1-2)B(2)O(6)(O,OH,F)(0-1), is an actinide-bearing phase in Sy nroc, a polyphase ceramic proposed for the immobilization of high level nuc lear waste. Structural damage due to alpha-decay events can significantly a ffect the chemical and physical stability of the nuclear waste form. Pyroch lore can effectively incorporate a variety of actinides into its structure. Four titanate pyrochlores were synthesized with compositions of Gd2Ti2O7, Sm2Ti2O7, Eu2Ti2O7 and Y2Ti2O2. These samples were irradiated with 1 MeV Kr + in order to simulate alpha-decay damage and were observed by in situ elec tron microscopy. Irradiations were conducted from 25 K to 1023 K. At room t emperature, Gd-, Sm- and Eu-pyrochlores amorphized at a dose of similar to 2 x 10(14) ions/cm(2) (similar to 0.5 dpa) and Y-pyrochlore amorphized at 4 x 10(14) ions/cm(2) (similar to 0.8 dpa). The amorphization dose became hi gher at elevated temperatures with different rates of increase for each com position. The critical temperatures for amorphization are similar to 1100 K for Gd-, Sm-, Eu-pyrochlore and similar to 780 K for Y-pyrochlore. The rar e-earth-pyrochlores are more susceptible to amorphization and have higher c ritical temperatures than Y-pyrochlore, The difference in amorphization dos e and critical temperature is attributed to the different cascade sizes cau sed by the different cation masses of the target. Based on a model of casca de quenching, the larger cascade is related to a lower amorphization dose a nd higher critical temperature. The irradiated materials were studied by el ectron diffraction and high-resolution electron microscopy. All the pyrochl ores transformed to a fluorite substructure prior to the completion of amor phization of the observed regions. This transformation was caused by the di sordering between cations and between oxygen and oxygen vacancies. The conc urrence of cation disordering with amorphization suggests the partial recry stallization of the displacement cascades. Isolated cascade damage regions were observed by high-resolution electron microscopy, and the cation disord ering was associated with the damaged regions. (C) 2000 Elsevier Science B. V. All rights reserved.