Sensitivity of x-ray absorption at $5d$ edges of high-valent light actinides to crystal-field strength and covalency effects

TL;DR

This study investigates how crystal-field effects and covalency influence the $5f$ electronic states in high-valent light actinides using x-ray absorption spectroscopy and theoretical models.

Contribution

It provides a detailed analysis of $5f$ states in actinide oxides, highlighting the roles of crystal-field interactions and covalency through combined experimental and theoretical approaches.

Findings

Th(IV) $5f$ states mainly affected by crystal-field effects.

U(VI) and U(V) show significant $5f$-O $2p$ hybridization and covalency.

Hybridization influences $5f$ occupancy and electronic properties.

Abstract

The $5f$ states were probed in ThO$_2$ and U(VI) and U(V) oxides using x-ray absorption spectroscopy at the actinide $O_{4,5}$ edges. Measured data were analyzed by several approaches including atomic, crystal-field theory and the Anderson impurity model to take into account the hybridization of actinide valence states with oxygen $2p$ states. For Th(IV), the $5f$ states are mainly affected by the crystal-field interaction with the closest neighbors as can be seen from the corresponding spectrum of ThO$_2$. In turn, for U(VI) and U(V) oxides, the U $5f$-O $2p$ hybridization and high degree of covalency in the chemical bonding play a decisive role increasing the $5f$ occupancy and consequently governing the ground and excited state properties. That is additionally illustrated by results from the calculations of the U $5d$ x-ray photoemission spectra for U(V) in the lattice of La-doped UO$_2$.