Fig. 1: High-temperature in situ setup and basics of U M4 HERFD XAS methodology.

a High-temperature cell for in situ HERFD XAS measurements at the U M4 edge and b the U M4 HERFD XAS spectra of uranium oxides UO2, U4O9, U3O7, U3O8, and UO355. The inset in the upper right corner shows a simplified single-electron scheme of the U 3d–5f transitions at the M4 edge of U for the different oxidation states.