U(v) in metal uranates: a combined experimental and theoretical study of MgUO₄, CrUO₄, and FeUO₄

Abstract

Although pentavalent uranium can exist in aqueous solution, its presence in the solid state is uncommon. Metal monouranates, MgUO4, CrUO4 and FeUO4 were synthesized for detailed structural and energetic investigations. Structural characteristics of these uranates used powder X-ray diffraction, synchrotron X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and (57)Fe-Mössbauer spectroscopy. Enthalpies of formation were measured by high temperature oxide melt solution calorimetry. Density functional theory (DFT) calculations provided both structural and energetic information. The measured structural and thermodynamic properties show good consistency with those predicted from DFT. The presence of U(5+) has been solidly confirmed in CrUO4 and FeUO4, which are thermodynamically stable compounds, and the origin and stability of U(5+) in the system was elaborated by DFT. The structural and thermodynamic behaviour of U(5+) elucidated in this work is relevant to fundamental actinide redox chemistry and to applications in the nuclear industry and radioactive waste disposal.

Structure and thermodynamics of uranium-containing iron garnets

“…Figure 5 shows U L III XANES spectra of the samples with x = 0.5, 0.6 and 0.7, which give normalized absorption edge energies (E 0 ) of 17170.5, 17170.3 and 17170.3 eV, respectively. Although the differences are small, the decrease in E 0 from 17170.5 eV for x = 0.5 to 17170.3 eV for x = 0.6 and 0.7 is consistent with the trend that E 0 generally becomes smaller when U is reduced from U 6+ to U 5+ (Guo et al, 2016). The XANES edge energy decrease is in line with the correlation between the decreased formal charge and a lower core electron binding energy.…”

“…Energy calibration was obtained using a Y metal foil (first derivative peak defined to be 17037 eV). Two synthetic valence standards (Guo et al, 2016) were measured: CrUO 4 and MgUO 4 . The samples and standards were prepared as thin powder layers each mounted between two Scotch tapes.…”

Atomistic scale investigation of cation ordering and phase stability in Cs-substituted Ba1.33Zn1.33Ti6.67O16, Ba1.33Ga2.66Ti5.67O16 and Ba1.33Al2.66Ti5.33O16 hollandite

“…Similar differences have been observed between calculated and measured oxides in literature. For example, the calculated and measured enthalpy of formation for metal uranates differ by about 10–40%, where limitations due to the computational theory are noted only for the system with 40% deviation from measurements 11 , 28 . Further, the calculated enthalpy of formation for the Ba-Al-hollandite end member ranged from −200.75 kJ/mol to −239.55 kJ/mol depending on the A-site and B-site configuration.…”

Thermodynamic studies of studtite thermal decomposition pathways via amorphous intermediates UO3, U2O7, and UO4

“…Flushing oxygen gas at ~ 50 mL/min through the calorimeter chamber assisted in maintaining a constant gas environment above the solvent [25]. Dissolution of uranium oxides and other uranium-containing compounds as U 6+ species has been demonstrated in this solvent, and their drop solution enthalpy data were obtained previously [10,11,22,[26][27][28]. Upon rapid and complete dissolution of the sample, the enthalpy of drop solution, ΔH ds , was obtained.…”