Combined Experimental and ab Initio Study of Site Preference of Ce3+ in SrAl2O4

Low-temperature photoluminescence properties of Sr1-2xCexNaxAl2O4 (x = 0.001) synthesized by a solid-state reaction method are measured with excitation energies in the vacuum ultraviolet (VUV) to ultraviolet (UV) range. Two distinct activator centers with different emission and excitation intensities are observed and attributed to CO occupying the Sri and ST2 sites of SrAl2O4 with different probabilities. Hybrid density functional theory (DFT) calculations within the supercell model are then carried out to optimize the local structures of Ce3+ located at the two Sr sites of SrAl2O4, on which wave function-based CASSCF/CASPT2 embedded cluster calculations with the spin orbit effect are performed to derive the Ce3+ 4f(1) and 5d(1) energy levels. On the basis of the observed relative spectral intensities, the calculated DFT total energies, and the comparison between experimental and calculated 4f -> 5d transition energies, we conclude that, in SrAl2O4:Ce3+, the dopant Ce3+ prefers to occupy the slightly smaller Sr2 site, rather than the larger Sri site as proposed earlier. Furthermore, by using an established linear relationship between the lowest 4f -> Sd transition energies of Ce3+ and Eu2+ located at the same site of a given compound, we find that, in SrAl2O4:Eu2+, the dominant green emission observed at room temperature arises from Eu2+ located at the ST2 site of SrAl2O4.