Understanding the M4 edge HERFD XANES of U6+

Hexavalent uranium is of considerable importance in the actinide field, and its characterization, consequently, of fundamental concern. The application of High-Energy-Resolution Fluorescence-Detected X-ray Absorption Near-Edge Structure (HERFD XANES) spectroscopy at the M4 edge of actinides probes the 5f electronic structure with augmented resolution compared to conventional XANES and is gaining great popularity. If, from one side, the extreme sensitivity of M4 HERFD XANES to actinides’ oxidation state is well-established [1], its sensitivity to the local environment has been less extensively explored [2,3]. The partially filled 5f subshell makes intra-atomic electron interactions the primary force shaping the spectrum, and simulations treating multi-electronic effects can only approximately account for the local environment.
In these regards, U6+, with its empty 5f shell, is a fortunate exceptional case. Simulation approaches based on the one-electron approximation can then be applied, and the dependence on the local environment be carefully investigated. We recently reported a detailed investigation where experimental and simulated M4 HERFD XANES are obtained on a set of U6+ compounds with different local environments [4]. The coordination of U6+ represented by the set of samples comprises an almost perfect UO6 octahedral bipyramid (Sr3UO6), two UO6 featuring the uranyl ion (Cs2UO2Cl4 and SrUO4), and a UO8 distorted hexagonal bipyramid (CaUO4).
Experimental M4 HERFD are shown in Figure 1. The spectral shape has a similar structure, made of three or four peaks of decreasing intensity. At the same time, it presents significant differences indicating the substantial impact on the spectral shape of the local environment. We simulated the set of U6+ compounds with the DFT-based code FDMNES [5]. The good agreement between theory and experiment, see Fig. 1, makes it reasonable to have a closer look at the underlying f-density of states and to assign spectral features to specific f-orbitals.
Simulations allow to rationalize how the local coordination of U6+ affects the M4 HERFD XANES and demonstrate its high sensitivity to the local environment. Simulations based on crystal field theory were also performed. Their comparison to FDMNES results will be discussed.