Phase Relations Between a Fluorite and a Pyrochlore Structure in the System of Actinides and Zirconium Oxides

Abstract

Cubic stabilized zirconia (CSZ) exhibits the fluorite structure and zirconium pyrochlores form a special structure. Because of slight difference in crystallographic symmetry, local structures are formed in the pyrochlores: a distorted coordination of eight oxygen around a large dopant cation, and a symmetric coordination of six oxygen around Zr+. A similar local structure is confirmed in the Np-doped yttria stabilized zirconia (YSZ) solid solutions by means of 237Np Mössbauer spectroscopy. The Mössbauer study shows that small amounts of Np5+ are formed in the solid solution, and are distributed statistically on the available cation sites. Based o n these findings, a mean cation ionic radii model is developed to evaluate the lattice parameter change with composition in the Pu- and Np-doped YSZ solid solutions. An effect of the oxygen vacancy is treated as reducing the oxygen coordination number arouid Zr4+ from eight to seven in the model. The model represents successfully the observed lattice parameters. A phase transformation from the fluorite to the pyrochlore structure is considered to results in at average Zr coordination number of seven. Suggestions for further studies in this field are also provided.