Thermal stability of a lanthanum uranyl oxyhydroxide polycrystalline film

The purpose of the study was to investigate the thermal stability of uranyl mineral phases that should occur as a result of corrosion of spent nuclear fuel in a deep geological waste repository. Crystallographic textures of two crystalline uranium oxyhydroxide films, metaschoepite and a lanthanum(III)-intercalated phase, precipitated on silicon, by grazing-incidence wide angle X-ray scattering (GIWAXS) in vacuo up to 200 °C were investigated. The X-ray data are supported by SEM-EDS measurements before and after thermal evacuation, as well as thermogravimetric analysis of powders in air up to 800 °C. The lanthanum-intercalated phase exhibited higher water desorption temperature, and a greater thermal stability compared with metaschoepite. These results are consistent with interactions between La(III) and water interactions, the formation of La(III)-hydrate layers that kept apart the uranyl polyhedral sheets. Grazing-incidence X-ray diffraction (GIXRD) revealed a progressive inability of aqua cations of early (Pr), mid-series (Sm, Gd, Tb, Dy) and late series (Ho, Er, Tm, Yb, Lu plus Y) rare earths to interact with metashoepite at 70 °C.