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) SiAl(tet)–NO substitutions with the NO 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 SiAl(tet)–NO 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 SiAl(tet)–NO 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. Copyright © 2016 The Royal Society of Chemistry.