On the relation between lattice parameter and O/M ratio for uranium dioxide-trivalent rare earth oxide solid solution

The lattice parameters and the O/M ratios of the solid solutions between UO2 and trivalent rare earth oxides (ROsub(1.5), R = Y, Gd, Eu or Nd) were investigated. The lattice parameter behavior for the types of solid solutions of Usub(1-y)Rsub(y)Osub(2.00) and Usub(1-y)Rsub(2-y/2) was discussed using the radii of ions composing the fluorite lattice. It was found that, in the former type of solid solution, the oxidation state of uranium was pentavalent. The analysis of the lattice parameter for the latter type of solid solution showed that the radius of the oxygen vacancies, rsub(ov), was approximately 10% larger than that of the O2- ion. Aside from rsub(ov), an apparent hypothetical radius of the oxygen vacancies, rsup(*)sub(ov), which was smaller than that of the O2- ion than that of the O2- ion was defined for the hypostoichiometric solid solution. With this rsup(*)sub(ov), an explanation for the lattice parameter change with x for Usub(1-y)Pusub(y)Osub(2-x)x was made. (orig.)