RADIATION- AND PRESSURE-INDUCED TRANSFORMATIONS IN PYROCHLORE

Isometric pyrochlore compositions, A₂B₂O₇, have been proposed as materials for the immobilization of Pu due to their structural flexibility, wide range of composition, and chemical durability. However, energetic alpha-particles and recoil-nuclei from alpha-decay of incorporated Pu may lead to radiation-induced amorphization, microcracking, and a significant increase in leach rate. Furthermore, fission fragments with the energy ranging from several tens to 100 MeV create defect clusters inside fission tracks, as well as high pressures surrounding the track cores. Therefore, the phase stability and radiation-damage response of the pyrochlore structure must be understood under the extreme conditions of a high-radiation field and high pressures. We have systematically studied the structural transformation and damage processes in a wide range of pyrochlore compositions induced by either energetic ion beam irradiations or high pressures attained in a diamond anvil cell. The systematic ion beam irradiations on pyrochlore, A₂B₂O₇ (A = La ~ Lu, Y and Cd; B = Ti, Sn, Zr, and Nb), under in-situ transmission electron microscopy observation show that the radiation response of the pyrochlore structure-type depends strongly on composition and bond-type. Three different processes were observed: i) radiation-induced amorphization, ii) an order-disorder transformation and iii) phase decomposition. Additionally, in-situ synchrotron X-ray diffraction and Raman spectroscopy measurements were performed on a range of pyrochlore compositions at pressures up to 43 GPa. Similar structural transformations were observed in the pyrochlore structure-type at high pressures. These results demonstrate that there are parallel responses to energetic particle irradiations and high pressure, although the mechanisms of the structural transformations are quite different.