Role of la 5p in Bulk and Quantum-Confined Solids Probed by the la 5p⁵4f¹³D₁Excitonic Final State of Resonant Inelastic X-ray Scattering

The varied electronic localization of rare earth elements is essential to functional materials and a key to tailoring their properties. We establish with unprecedented spectral resolution the excitonic nature of the lanthanum 5p⁵4f¹³D₁and ³D₂final states of resonant inelastic X-ray scattering (RIXS) at the La N_4,5edges. We extract the intrinsic lifetime, energy distance, and relative intensity ratio from single crystal LaAlO₃and construct an empirical model. With help of the model, we precisely determine the RIXS ³D₁final state position and identify La 5p as a descriptor of covalency with the host material. For metallic lanthanum, La³⁺ions in mixed-covalent-ionic simple oxides and phosphates, and ionic salts alike, we find a sizable chemical shift, indicating band-like and free-ion-like La. The different electronic relaxation of the La 5p⁵hole and the La 4f¹electron is discussed with local and nonlocal screening contributions. In addition, the energetics of the excitonic La 5p⁵4f¹Coulomb attraction is quantified in its variation from lanthanum metal to mixed-covalent-ionic La₂O₃and the ionic LaF₃salt.