Lattice Parameter Behavior with Different Nd and O Concentrations in (U_1−yNd_y)O_2±x Solid Solution

The solid solution of (U_1−yFP_y)O_2±x has the same fluorite structure as UO_2±x, and the lattice parameter is affected by dissolved fission product and oxygen concentrations. The relation between the lattice parameter and the concentrations of neodymium and oxygen in the fluorite structure of (U_1−yNd_y)O_2±x was investigated using X-ray diffraction. The lattice parameter behavior in the (U_1−yNd_y)O_2±x solid solution shows a linear change as a function of the oxygen-to-metal ratio and solubility of neodymium. The lattice parameter depends on the radii of ions forming the fluorite structure and also can be expressed by a particular rule (modified Vegard’s law). The numerical analyses of the lattice parameters for the stoichiometric and nonstoichiometric solid solutions were conducted, and the lattice parameter model for the (U_1−yNd_y)O_2±x solid solution was assessed. A very linear relationship between the lattice parameter and the Nd and O concentration for the stoichiometry and nonstoichiometry of the (U_1−yNd_y)O_2±x solid solution was verified.