Origin of the green persistent luminescence of Eu-doped SrAl₂O₄ from a multiconfigurational Ab initio study of 4f⁷ → 4f⁶5d¹ transitions

The green persistent luminescence of Eu²⁺ in SrAl₂O₄ has been widely applied in self-sustained night vision due to its long duration and ability to be excited by sunlight. However, the question of which substitutional site in the lattice is responsible for the green emission remains unresolved. Herein, we perform multiconfigurational ab initio calculations at the CASSCF/CASPT2 level with the spin-orbit effect on 4f⁷ → 4f⁶5d¹ (4f → 5d) transitions of Eu²⁺ located at the Sr1 and Sr2 sites of SrAl₂O₄. By comparison of the calculated transition energies with data from experimental low-temperature excitation spectra, the green luminescence is unambiguously assigned to Eu²⁺ at the Sr2 site. In addition, by using the present results as a calibration, the accuracies of the literature-reported DFT-based approaches to the Eu²⁺ 4f → 5d transition energies are discussed. This investigation is expected to motivate more exploration of multiconfigurational ab initio methodology to elucidate luminescence properties of Eu²⁺-activated complex phosphors.