Does Cr³⁺ Occupy Tetrahedral Sites and Luminesce in Oxides? A First-Principles Exploration

The Cr³⁺ activators have been adopted to produce desired near-infrared broadband emission via ligand field engineering by choosing hosts with appropriate sites. First-principles calculations help to analyze the site, valence, and luminescent mechanism of the activators. Our calculations on Mg₂Al₄Si₅O₁₈:Cr elucidate that the activators are dominated by Cr³⁺ at tetrahedral Al and octahedral Mg sites, while the experimentally reported near-infrared emission previously assigned to tetrahedral sites is actually produced by Cr³⁺ at the octahedral site. Meanwhile, our results show that the emission energies of Cr³⁺ activators at octahedral sites can be well predicted. Moreover, further calculations show that the quenching of the ⁴T₂ → ⁴T₁ transition of Cr³⁺ at a tetrahedral site is general due to nonradiative relaxation pathways mediated by sublevels split off from the ⁴T₁ multiplet states by intrinsic or Jahn-Teller distortions. Our work shows that the sophisticated first-principles calculations put together here can be effective in exploring Cr³⁺ and potentially more general activators in crystals, which benefit the design and optimization of luminescent materials.