Seeing actinide covalency from the inside out with ADF and RIXS

Actinide chemistry is controlled by 5f orbitals, but directly measuring how these orbitals participate in bonding remains difficult. This is especially important for transuranium systems, where covalency affects reactivity, separation chemistry, spectroscopy, and the interpretation of electronic structure.

In a recent study, M₄-edge 3d4f resonant inelastic X-ray scattering was combined with ligand field DFT calculations in ADF to probe the 5f radial wavefunction in [AnCl₆]²⁻ complexes, with An = U, Np, and Pu. The experiments revealed fine structure in the HERFD and RXES spectra, while the ADF-based LFDFT model supplied the Slater integrals, spin-orbit coupling constants, and ligand field parameters needed to assign the spectral features.

The modeling was essential because different RIXS features respond to different parts of the 5f wavefunction. The 5f inter-electron repulsion terms, Fᵏ5f5f, are most sensitive to the outer component of the 5f radial wavefunction. In contrast, the 4f–5f spin-exchange term, G⁰4f5f, probes the inner component close to the nucleus. By connecting these quantities to the spectra, the authors could separate how bonding affects the 5f orbital inside and outside its radial node.

This decomposition revealed a non-uniform response across the early actinides. From U to Np to Pu, An–Cl bonding increasingly expands the outer 5f density, supporting stronger covalent participation. At the same time, the inner 5f component remains increasingly constrained by the rising nuclear charge. The result is a distorted 5f radial wavefunction, where the outer region becomes more available for bonding while the inner region reflects actinide contraction.

For actinide R&D, this provides a rare experimental and computational route to quantify 5f covalency independently of ligand element, nuclear spin, oxidation state, or coordination symmetry. More broadly, it shows how ADF-based ligand field modeling can turn high-resolution spectroscopy into chemically interpretable information about orbital radial structure, helping validate electronic structure models for complex radioactive materials.