Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Ceramics intended for use as nuclear fuels, transmutation targets or actinide immobilization matrices have to endure severe conditions including internal radiation. While zirconia-based materials with defect-fluorite structure have shown high tolerance against external ion-beam irradiation, few experimental studies have demonstrated that these structures also resist under more realistic conditions, i.e. with homogeneous internal radiation from α-emitting actinides within the structure. Here, we provide for the first time experimental insight into the radiation-resistance mechanisms of americium pyrochlore ²⁴¹Am₂Zr₂O₇. We combined X-ray diffraction and X-ray absorption spectroscopy to probe changes of both the long-range and the short-range structure. The phase transition from the pyrochlore to the defect-fluorite structure was accompanied with an unusual negative lattice expansion. Once the fluorite structure was reached, neither volume changes nor amorphization were observed over a time course of 4 years corresponding to 0.8 dpa. The disorder relaxation proceeds through the simultaneous formation of cation antisites and oxygen Frenkel pairs, in line with former molecular dynamics studies. Moreover, EXAFS analysis revealed a disruption in the long-range order and a markedly different development in the local environments of zirconium and americium: while Am-O polyhedra show an increasing disorder, the Zr-O polyhedral units remain unchanged. However, they rotate along edges and corners, thereby reducing the structural strain imposed by the growing disorder around americium. We believe it is this particular property of the compound that provides the remarkable resistance to radiation, making it attractive for a wide range of nuclear applications.