File Name : figure s1.tif

Caption : xanes spectra at the u-l3 edge (fluorescence mode) of reference solutions: uranyl solution in a hydrochloric medium measured at 250°c (orange) and u(iv) solution in a hydrochloric medium measured at room temperature (green). normalisation was applied at the intensity maximum.

File Name : figure s2.tif

Caption : (a) comparison of the initial spectrum (blue) with the uranium (vi) (orange) and uranium (iv) (green) references. (b) comparison of the final spectrum (red) with the uranium (vi) (orange) and uranium (iv) (green) references. normalisation was applied at the intensity maximum.

File Name : figure s3.tif

Caption : illustration of the linear combination fitting methodology used to qualitatively estimate the contribution of uranium (iv) and uranium (vi) during hydrothermal treatment. the experimental spectrum recorded during conversion is shown in black. the two references used for the linear combination are the spectrum recorded at the start of the experiment (in blue), reflecting the contribution of uranium (vi), and the spectrum recorded at the end of the experiment (in red), for uranium (iv). the fitted function is in green, while the purple curve represents the residual plot.

File Name : figure s4.tif

Caption : rietveld refinement plots of the xrd data collected on uo2+x samples prepared by hydrothermal conversion of a u(vi) – oxalate mixture (t = 250°c, r = 3, ph = 0.8) during a) 24 hours and b) 168 hours. the blue crosses represent the experimental data, the red line accounts for the substracted background, and the green line for the calculated pattern. the residue is plotted in black.