Does Cr Occupy Tetrahedral Sites and Luminesce in Oxides? A First-Principles Exploration.

The Cr 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 MgAlSiO:Cr elucidate that the activators are dominated by Cr at tetrahedral Al and octahedral Mg sites, while the experimentally reported near-infrared emission previously assigned to tetrahedral sites is actually produced by Cr at the octahedral site. Meanwhile, our results show that the emission energies of Cr activators at octahedral sites can be well predicted. Moreover, further calculations show that the quenching of the T → T transition of Cr at a tetrahedral site is general due to nonradiative relaxation pathways mediated by sublevels split off from the T 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 Cr and potentially more general activators in crystals, which benefit the design and optimization of luminescent materials.