Luminescence of iron ions in crystals: Site occupancy, valence states, and excited-state properties

Iron ions play significant roles as recombination centers and activators in insulators. However, their identification and mechanisms through which they operate are often controversial. In this study, we utilized first-principles calculations to investigate the site occupancy, valence states, excited states, and optical properties of iron ions acting as luminescent centers. We also explored potential quenching processes when these ions serve as recombination centers in various crystals. We found that iron ions can occupy sites with tetrahedral, octahedral and dodecahedral coordinations. Specifically, tetrahedrally coordinated Fe3+ in oxide insulators exhibit nearinfrared emission in the range of 670-830 nm, with the specific wavelength dependent on the degree of structure distortion. In contrast, octahedrally coordinated Fe3+ ions in spinels and garnets exhibit considerably lower transition energies due to stronger ligand fields and significant excited-state structural relaxation. This makes them more susceptible to nonradiative decay and quenching. We also successfully elucidated the luminescence of those six-coordinated Fe3+ in perovskites, where the weaker ligand fields and smaller excited-state relaxation than those in spinels and garnets contribute to the observed emissions. Furthermore, we have confirmed the existence of dodecahedrally coordinated Fe3+ in hosts with a zircon structure, which results in a large transition energy due to the combined effect of the small nephelauxetic effect in phosphates and the small ligand field strengths associated with a subspherical coordination distribution. Additionally, we revisited and reinterpreted some experimental results based on our calculations. This study offers consistent and reliable interpretations of optical transitions of iron impurities in solids, which can be beneficial for the design and optimization of luminescent materials.