2, University of Tennessee, Knoxville, Knoxville, Tennessee, United States
3, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
4, GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, , Germany
An ideal nuclear waste form will incorporate and immobilize a wide variety of large cations, including actinides, lanthanides, and other fission products. To accommodate this compositional diversity, complex oxides such as the pyrochlore structured A2B2O7 compounds have received a great deal of attention. However, the complex compositions and structures of these materials yield complex, multiscale disordering mechanisms in response to waste disposal conditions, which include self-irradiation. In this work, the disordering of stannate pyrochlores (A2Sn2O7) under irradiation with 2.2 GeV Au ions was characterized. X-ray diffraction characterization demonstrated irradiation-induced transformations of the fluorite-derivative pyrochlore structures to either disordered fluorite or amorphous structures, as has been commonly reported in the literature. In contrast, Raman spectroscopy provided evidence of a more complex reordering of the local structure, with both the disordered fluorite and amorphous irradiation-induced structures exhibiting weberite-type local order. These two phases differ only in that the disordered phase exhibits a long-range, modulated arrangement of weberite-type structural units into an average fluorite structure, while the amorphous phase remains fully aperiodic. Comparison with the behavior of titanate and zirconate pyrochlores showed minimal influence of the high covalency of the Sn-O bond on this phase behavior. An analytical model of damage accumulation was developed to account for simultaneous amorphization and recrystallization of the disordered phase during irradiation.