Secondary Uranium Phases of Spent Nuclear Fuel – Coffinite, USiO4, and Studtite, UO4 . 4H2O – Synthesis, Characterization, and Investigations Regarding Phase Stability
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Beschreibung
The miscibility behavior of the USiO4 – ThSiO4 system was investigated. The end members and
ten solid solutions UxTh(1x)SiO4 with x = 0.12 – 0.92 were successfully synthesized, without
formation of other secondary uranium or thorium phases. Lattice parameters of the solid solutions
evidently follow Vegard’s Law. Investigation of the local structure with EXAFS reveals
small diUerences between U and Th environment attributed to diUerent atomic radii of the metal
atoms but no implications for a miscibility gap. The data provided conVrms complete miscibility
for the system USiO4 – ThSiO4. The structure of the end members was studied in detail with
XRD and discussed with special regard to the oxygen positions and the often neglected Si-O
bond length. USiO4 could be obtained without UO2 impurities and the lattice parameters derived
from Rietveld reVnement as c = 6.2606(3) Å and a = 6.9841(3) Å. The Si-O distance in USiO4
appears to be 1.64 Å, which is more reasonable than earlier reported values.
Synchrotron X-ray powder diUraction pattern and Raman spectra of synthetic coXnite, USiO4,
were obtained for pressures up to 35 GPa and 18 GPa, respectively. From the changes in the
diUraction pattern it can be concluded that USiO4 undergoes a Vrst order phase transition from
zircon-type (space group I 41/amd) to scheelite-type structure (space group I 41/a) at 15 GPa
and room-temperature. Contrary to earlier reports, the data indicates that this transition is
completely reversible upon pressure release. Pressure dependencies of the Raman modes for the
zircon structured phase are larger than those reported for hafnon, HfSiO4, and zircon, ZrSiO4,
indicating that coXnite, USiO4, is more compressible than these orthosilicates. Bulk moduli
Vtted from the p-V data for the zircon-type and scheelite-type USiO4 phase are compared to
those known to literature for other MSiO4 (M = U, Hf, Zr) compounds. The bulk modulus for
zircon-type USiO4 is 180(7) GPa and hence lower than those of ZrSiO4 (205 GPa1) as expected
from the larger unit cell. The pressure dependence of the Raman modes of USiO4 was studied up
to 18 GPa, yet no abrupt changes of peaks or in the peak shifts appear. Furthermore it could be
established, that the B1g- and the A1g-modes of the SiO4
4 -tetrahedron in the Raman spectrum
are very close and overlap at ambient conditions
Autor*in
Sabrina Labs