Origin of the green persistent luminescence of Eu-doped SrAl2O4 from a multiconfigurational ab initio study of 4f(7) -> 4f(6)5d(1) 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 4f(7) -> 4f(6)5d(1) (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.