Self-irradiation induced structural changes in the transplutonium pyrochlores An2Zr2O7 (An=Am, Cf)

We have pursued the fundamental chemistry of actinide pyrochlore oxides, An2Zr2O7 (An=Am, Cm, Bk, and Cf), using X-ray diffraction as well as optical spectroscopy. One recent facet of our studies has been to observe the structural changes of these materials under self-irradiation as a function of time. It has been reported that both titanate and silicate materials transform from a crystalline to an amorphous state under irradiation. With the Zr-based actinide pyrochlores studied here, we have observed a phase change from a pyrochlore structure to a fluorite-type structure with the retention of crystallinity. We focus here on the impact of α-radiation (243Am and 249Cf), rather than that from neutrons (248Cm) or β-radiation (249Bk), on the An2Zr2O7 pyrochlore structures. As a result of this phase change, the local coordination environments of both the actinide and zirconium atoms are altered. We consider a defect/ion deficiency driven mechanism and also address the occurrence of oxidation of the trivalent actinides during the self-irradiation process as being potential mechanisms responsible for the observed phase change.