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.
Highlights
LiUO2 BO3 (LUBO) crystallizes in the monoclinique P21 /c space group (Table 1) in agreement with the structure published by Gasperin [7]
NaUO2 BO3 (NUBO) crystallizes in the orthorhombic Pcam space group (Table 1) in agreement with the structure published by Gasperin [6]
The equatorial coordination decrease observed when changing from UO2 B2 O4 (UBO) to LUBO, can be seen as analogous with the behavior of divalent transition metal ions in alkali borate glasses [38,39] and uranyl in lithium borate glasses [15]: the addition of alkalis to the borate glass induces the decrease of the coordination number of 3d ions and uranyl
Summary
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