Incorporation of Uranium in Zirconolite (CaZrTi ₂ O ₇ )

Abstract

Up to 0.15 formula unit (f.u.) of U 4+ incorporated in the Zr site of zirconolite by firing CaU x Zr (1− x ) Ti 2 O 7 compositions in argon at 1400°C allows retention of the 2M polytype. Further U 4+ substitution for Zr, up to 0.4 f.u., produces the 4M polytype (containing ∼0.4 f.u. of U) plus the 2M polytype containing 0.15 f.u. of U. The pyrochlore structure (containing 0.6 f.u. of U) forms in conjunction with the 4M polytype at U contents of 0.4 f.u. up to 0.7 f.u. Higher U contents give the pyrochlore structure, but the solid‐state reactivity of even alkoxide‐based preparations becomes increasingly poor for x > ∼0.7 and hot pressing in graphite dies at ∼1250°C is necessary to achieve near single‐phase pyrochlore structures for x = 1. When samples of CaU x Zr 1− x Ti 2 O 7 stoichiometry ( x = 0.1 and 0.2) are oxidized at 1400°C in air, diffuse reflectance spectroscopy (DRS) shows evidence for U 5+ formation at the expense of U 4+ via enhanced absorption bands, ∼50 nm in half‐width, near 970 and 1500 nm and correspondingly weakened absorption at 1150 and 1660 nm. Weight gains consistent with complete oxidation of U 4+ to U 5+ are observed when finely powdered argon‐fired samples with x = 0.1 and 0.4 are heated in air to 1200°C. Evidence for U valence states higher than +4 in both argon‐ and air‐heated materials containing charge compensators to encourage U 5+ or U 6+ formation was also derived from DRS (showing U 5+ in particular), and XANES. DRS shows weak absorption bands attributable to U 4+ in zirconolites containing ∼0.2 f.u. of U incorporated in the Ca site via Mg or Al substitutions in the Ti sites, with the spectrum being closely but not exactly similar to that attributed to U 4+ in the Zr site. Zirconolite and pyrochlore compositions which are melted at 1500° or 1550°C in argon and furnace‐cooled yield broadly similar phase assemblages to the corresponding sintered materials, but there is evidence of incongruent melting in all materials.