Synthesis and characterization of coffinite

Coffinite, USiO4, has been produced by hydrothermal synthesis. The synthesis products, coffinite nanoparticles (50 nm in size) with UO2 nanoparticles (a few nanometers), are always associated even if they are not always detected by XRD measurements. The formation of coffinite was shown to be very sensitive to several experimental parameters. The most important of these parameters are the pH, which must be in the range 8–9.5, the pressure, which must be below 50 bars, and the reaction conditions, which must be oxygen-free to maintain uranium in its tetravalent oxidation state. XRD and TEM reveal that tetragonal coffinite accounts for more than 90% of the final products while the by-products UO2 and a Si-rich amorphous phase are also present. The structural formula of the obtained coffinite is close to USiO4 as determined from EMPA (U0.99±0.06Si0.97±0.07O4). XPS measurements show a peak chemical shift of the U-4f core levels by 1 eV toward higher binding energies in coffinite compared with stoichiometric UO2. The U-4f7/2 and U-4f5/2 positions in coffinite are found to be near 380.8 ± 0.3 eV and 391.7 ± 0.3 eV, respectively.