How to Figure Out Local Order Against Average Order in UO2?
Résumé
Recent results, using different experimental methods, evidenced a shortening of UO distance in UO2 PDF, while the average unit cell is expanding with increasing temperature. This shortened UO distance induces a change in the uranium coordination polyhedron, going from cubic in the regular Fm-3m to octahedral with shorter UO distances. Different interpretations were given for this change of coordination polyhedron, amongst which static multi-domains, dynamic domains or coexisting phases. Whatever its origin, this change in uranium coordination polyhedron might have consequences on UO2 behavior at low and high temperature. At high temperature, the existence of a local octahedral environment could be a reason why UO2 can incorporate so many fission products, because this environment is more favorable for fission products. At low temperature, the PDF uranium coordination polyhedron is very close to the one in Pa-3 symmetry, which is the symmetry of UO2 under Neel transition.