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

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.