Alkali Uranyl Borates: Bond Length, Equatorial Coordination and 5f States

Abstract

Three uranyl borates, UO2B2O4, LiUO2BO3 and NaUO2BO3, have been prepared by solid state syntheses. The influence of the crystallographic structure on the splitting of the empty 5f and 6d states have been probed using High Energy Resolved Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS) at the uranium M4-edge and L3-edge respectively. We demonstrate that the 5f splitting is increased by the decrease of the uranyl U-Oax distance, which in turn correlates with an increased bond covalency. This is correlated to the equatorial coordination change of the uranium. The role of the alkalis as charge compensating the axial oxygen of the uranyl is discussed.

Probing the Local Coordination of Hexavalent Uranium and the Splitting of 5f Orbitals Induced by Chemical Bonding

“…Distance between peaks A and C of U M 4 edge HERFD versus U–O yl bond length from refs. ,,, and the present data. The dashed line is the linear fit obtained considering uranyl samples, that is, with a U–O yl bond length < 1.8 Å.…”

“…36 This assignment was then assessed by polarization-dependent HERFD XANES at U M 4 edge supported by molecular orbital calculations on Cs 2 UO 2 Cl 4 . 3 Many subsequent works using U M 4 HERFD XANES and EXAFS data confirmed the correlation between the length of the uranyl bond and the position of peak C. 8,23,37 A shift of C to lower energy was observed upon dehydration of studtite and associated to the elongation of the U−O yl bonds and DFT-based calculations supported the results. 23 Uranyl compounds have been frequently studied with M 4,5 HERFD XANES, and uranyl spectral shape, characterized by three wellseparated peaks, is well-known and exploited to detect its presence.…”

“…Separation between Peaks A and C. Recent investigations on uranyl systems with M 4,5 HERFD XANES reveal a tight correlation between the separation of peaks A and C and the length and covalency of the U−O yl bond. 4,37 Our results show that peaks A and C are present under several U 6+ local geometries and not only in the case of uranyl. For all symmetries investigated, peaks A and C correspond to the nonbonding and antibonding f-orbitals, respectively.…”

High-energy resolution X-ray spectroscopy at actinide M4,5 and ligand K edges: what we know, what we want to know, and what we can know

“…The spectrum contains three primary peaks separated by 2 and 4 eV between each other on the incident energy scale. These peaks reflect the splitting of the U 5f unoccupied states and were assigned to transitions to the nonbonding 5fδ u and 5fφ u , antibonding 5fπ u *, and antibonding 5fσ u * molecular orbitals8,[91][92][93][94] . The corresponding transitions are shown in Fig.7, where more accessible information can be found.…”

Oxygen K-Edge X-ray Absorption Spectra of ThO2and CeO2: Experiment, Interpretation, and Structural Effects

“…Few techniques can access this information, and a prominent role is played by X-ray absorption near edge structure (XANES). In recent years, XANES at the actinide M 4,5 edges − and at the ligand K-edge − ,− has been largely used. While M 4,5 edges probe directly the 5f orbitals, the ligand K-edge is sensitive to actinide 5f and 6d orbitals only through their mixing with ligand p orbitals.…”