First-principles study of Ce-doped Y3Al5O12 with Si–N incorporation: electronic structures and optical properties

Abstract

Incorporation of Si–N into Ce-doped Y₃Al₅O₁₂ (YAG:Ce) has previously been shown to give distinct lower-energy emission but with stronger thermal quenching than the typical yellow YAG:Ce emission. Here, we investigate geometric and electronic structures of Ce and Si–N co-doped YAG with first-principles methods, to gain microscopic insight into effects of Si–N addition on electronic structures and optical properties of Ce³⁺. Hybrid density functional theory (DFT) calculations reveal that the Si–N prefers to be substituted for the tetrahedral Al(tet)–O sites with a random distribution, among which the nearest-neighbor (NN) Si_Al(tet)–N_O substitutions with the N_O coordinated to Ce³⁺ result in a slight upward shift of the 4f¹ ground-state level with respect to the host valence band. Wave function-based CASSCF/CASPT2 calculations at the spin–orbit level show that the NN Si_Al(tet)–N_O substitutions induce a redshift of the lowest energy Ce³⁺ 4f(1) → 5d(1) transition, in agreement with experimental observations. The redshift originates from an increase in the 5d crystal field splitting and a decrease in the 5d centroid energy of Ce³⁺ in comparable magnitude. Combining these results, we find that the energy separation between the lowest Ce³⁺ 5d(1) level and the host conduction band minimum (CBM) remains largely unchanged upon the NN Si_Al(tet)–N_O substitution, thus excluding the thermal ionization of the 5d electron as the underlying mechanism for the temperature quenching of the lower-energy Ce³⁺ emission. This finding also suggests that the thermally activated crossover from the 5d(1) to the 4f¹ states could be responsible for the luminescence quenching, which is also consistent with present calculated results.