PNNL

Abstract

Metal monouranates, MgUO4 and CrUO4, were synthesized by a coprecipitation method, and FeUO4 was prepared from high temperature sintering of U3O8 and Fe3O4 in a sealed silica tube. Structural characteristics of these uranates were studied using powder X-ray diffraction, Synchrotron X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, 57Fe-Mössbauer spectroscopy, and density functional theory (DFT) calculations. Presence of U5+ has been confirmed in CrUO4 and FeUO4, but not in MgUO4. Standard enthalpies of formation were measured by high temperature oxide melt solution calorimetry in molten sodium molybdate (3Na2O·4MoO3) at 702 °C or in lead borate (2PbO·B2O3) at 802 °C. The obtained formation enthalpies from binary oxides (MgO, Cr2O3, Fe2O3, UO2, and ?-UO3) at 25 °C are -33.8 ? 1.7 kJ/mol for MgUO4, -33.4 ? 1.3 kJ/mol for CrUO4, and -32.2 ? 2.8 kJ/mol for FeUO4. The measured structural and thermodynamic properties show good consistency with those predicted from DFT calculations.