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

Abstract

Low-temperature photoluminescence properties of Sr_{1–2<i>x</i>}Ce_<i>x</i>Na_<i>x</i>Al₂O₄ (<i>x</i> = 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 Ce³⁺ occupying the Sr1 and Sr2 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⁺ 4f¹ and 5d¹ 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 Sr1 site as proposed earlier. Furthermore, by using an established linear relationship between the lowest 4f → 5d 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 Sr2 site of SrAl₂O₄