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 Synroc, a polyphase ceramic proposed for the immobilization of high level nuclear waste. Structural damage due to alpha-decay events can significantly affect the chemical and physical stability of the nuclear waste form. Pyrochlore can effectively incorporate a variety of actinides into its structure. Four titanate pyrochlores were synthesized with compositions of Gd(2)Ti(2)O(7), Sm(2)Ti(2)O(7), Eu(2)Ti(2)O(7) and Y(2)Ti(2)O(2). These samples were irradiated with 1 MeV Kr(+) in order to simulate alpha-decay damage and were observed by in situ electron microscopy. Irradiations were conducted from 25 K to 1023 K. At room temperature, 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 higher at elevated temperatures with different rates of increase for each composition. The critical temperatures for amorphization are similar to 1100 K for Gd-, Sm-, Eu-pyrochlore and similar to 780 K for Y-pyrochlore. The rare-earth-pyrochlores are more susceptible to amorphization and have higher critical temperatures than Y-pyrochlore, The difference in amorphization dose and critical temperature is attributed to the different cascade sizes caused by the different cation masses of the target. Based on a model of cascade quenching, the larger cascade is related to a lower amorphization dose and higher critical temperature. The irradiated materials were studied by electron diffraction and high-resolution electron microscopy. All the pyrochlores transformed to a fluorite substructure prior to the completion of amorphization of the observed regions. This transformation was caused by the disordering between cations and between oxygen and oxygen vacancies. The concurrence of cation disordering with amorphization suggests the partial recrystallization of the displacement cascades. Isolated cascade damage regions were observed by high-resolution electron microscopy, and the cation disordering was associated with the damaged regions. (C) 2000 Elsevier Science B.V. All rights reserved.