Role of La 5p in Bulk and Quantum-Confined Solids Probed by the La 5p54f1 3D1 Excitonic Final State of Resonant Inelastic X-ray Scattering

The varied electronic localization of rare earth elementsis essentialto functional materials and a key to tailoring their properties. Weestablish with unprecedented spectral resolution the excitonic natureof the lanthanum 5p(5)4f(1) D-3(1) and D-3(2) final states of resonant inelasticX-ray scattering (RIXS) at the La N-4,N-5 edges. We extractthe intrinsic lifetime, energy distance, and relative intensity ratiofrom single crystal LaAlO3 and construct an empirical model.With help of the model, we precisely determine the RIXS D-3(1) final state position and identify La 5p as a descriptorof 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, indicatingband-like and free-ion-like La. The different electronic relaxationof the La 5p(5) hole and the La 4f(1) electron isdiscussed with local and nonlocal screening contributions. In addition,the energetics of the excitonic La 5p(5)4f(1) Coulombattraction is quantified in its variation from lanthanum metal tomixed-covalent-ionic La2O3 and the ionic LaF3 salt. The power of the approach and analysis is applied tomap the influence of geometric quantum confinement to La 5p sharingwithin quantum dots and quantum wires in comparison to bulk-like microrodsof monoclinic LaPO4.