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

Abstract

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.