Comment on "Charge Transfer-Triggered Bi3+ Near-Infrared Emission in Y2Ti2O7 for Dual-Mode Temperature Sensing".

Undoped Y2Ti2O7 exhibits impurity emission bands at low temperatures due to Mn4+ and Cr3+, as established by codoping with these ions. Contrary to a recent report by Wang et al., ACS Appl. Mater. Interfaces 2022, 14, 36834-36844, we do not observe Bi3+ emission in this codoped host, as also is the case for Fe3+. The emission reported in that paper as being due to Bi3+ in fact corresponds to Cr3+ emission. The Cr3+ and Mn4+ emissions are quenched with increasing temperature, so that Mn4+ emission is scarcely observed above 80 K. We present variable temperature optical data for Y2Ti2O7 and this host codoped with Mn, Cr, Fe, and Bi, as well as a theoretical justification of our results.

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.

Elucidating the Multisite and Multivalence Nature of Mn Ions in Solids and Predicting Their Optical Transition Properties: A Case Study on a Series of Garnet Hosts.

The abundant site occupancy and optical transitions of multivalence Mn dopants in luminescent materials have attracted much attention. Here, detailed first-principles calculations based on density functional theory have been carried out to clarify the multisite and multivalence nature of Mn ions in solids and predict their optical transition properties by using garnets as prototype systems. The formation energies of dodecahedral, octahedral, and tetrahedral coordinated Mn dopants are evaluated with chemical potential environments, and the preferable site occupancy and valence state of Mn ions in three garnet systems are clarified. The results show that even in a fixed atmosphere, taking Ca3Al2Ge3O12 in air as an example, not only can the preference of Mn ions switch between dodecahedral and octahedral sites, but also can the valence state change from Mn2+ to Mn3+ and Mn4+. Furthermore, for all of the three garnet systems, the calculation results of the energy-level structure and photoluminescence of Mn ions at different sites in the different valence states provide a reliable interpretation of the available spectroscopic data. The proposed first-principles scheme, with general applicability and encouraging predictive power, provides an effective approach for elucidating and characterizing the site occupancy, valence state, and optical transition of Mn activators in insulators, and will greatly benefit the design and optimization of related materials.

Angular Jahn-Teller Effect and Photoluminescence of the Tetrahedral Coordinated Mn2+ Activators in Solids-A First-Principles Study.

First-principles calculations based on density functional theory have been performed to investigate the electronic structure, excited-state Jahn-Teller distortion, and photoluminescence of the multielectron d5 system of the strongly covalent tetrahedral coordinated Mn2+ activator in solids. The electronic structure of the 4T1 and 4A1/4E excited states is analyzed, and Slater's transition-state method and occupation matrix control methodology are applied to deal with the spin contamination in the lower-spin excited states, which is due to the mixing of the ground state of the same spin projection number. In a series of covalent tetrahedral coordinations, the 6A1 → 4T1 and 4A1/4E excitations and the 4T1 → 6A1 emission energies are obtained and compared to the reported experimental results. The nephelauxetic effect follows O2- < S2- ≈ Se2- < N3-, and the larger nephelauxetic effect and crystal field strength lead to the red-shifted emission of nitride phosphors. The Jahn-Teller distortion of the 4T1 states is dominated by the e-type angular distortion of the [MnL4] moiety (L being the ligand), which accounts for the small Stokes shift of tetrahedral coordinated Mn2+. The results show that the ground- and excited-state electronic and geometric structures and the luminescent property of tetrahedral coordinated Mn2+ can be reliably predicted. The method can be further explored to interpret and discriminate the luminescent properties of materials containing a variety of different Mn2+ sites and complexes and even other transition metals.