Immobilization of radiotoxic elements with Y-stabilized zirconia: the Thorium case

Y-stabilized zirconia (YSZ) phases were found to incorporate Th atoms and the corresponding solubility ranges depend on the relative Y content. For the tetragonal phases with lower Y concentration of 14 at.% a maximal possible Th intake on the Zr/Y metal site reached ca. 10.3 at.%. Cubic phases with higher Y content could dissolve 11 at.% Th in equilibrium state and up to ca. 12.3 at.% Th under non-equilibrium conditions. Larger Th-Zr/Y solubility range for phases with higher Y concentration was found to be related to the associated symmetry increase, as concluded from synchrotron radiation powder diffraction data. Specifically, introduction of Th into tetragonal YSZ induces evolution towards higher cubic symmetry via flattening of the Zr/YO8 polyhedra. In addition, tetragonal YSZ crystal lattice exhibits strongly anisotropic expansion with concomitant decrease in tetragonality upon the intake of Th. This results in easier accommodation of bigger Th atoms via structural stabilization of longer Zr/Y-O bonding distances which yields more symmetrical coordination of central Zr/Y metal ions by surrounding O atoms. Cubic symmetry is, therefore, more favorable to incorporation of large tetravalent Actinide elements.