RIXS/HERFD Study of Covalency in Pr4+ metal-ligand bonds as a function of symmetry and donor type

The electronic structure of lanthanide materials inherits on-site correlations and unquenched orbital degrees of freedom from atomic f-electron states. In the most stable trivalent oxidation state (Ln3+), the core-like 4f orbitals are usually only weakly perturbed by the crystal field (CF) yielding anisotropic magnetic moments. Although covalent metal-ligand interactions involving the 4f shell are generally weak in Ln3+ systems, this paradigm breaks down for Ln4+ ions as 4f orbitals hybridize with the valence orbitals of the ligands analogous to transition metals. The unusual nature of Ln4+ ions have significant effects on the electronic structure of bulk materials as evidenced in PrBa2Cu3O7 where in superconductivity is suppressed and has been attributed to 4f-2p hybridization in the Pr-O bond. Therefore, to understand the electronic structure of tetravalent lanthanides, we propose to use soft x-ray RIXS and HERFD at L3 edges to probe covalency in Pr-O bond.