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Investigating spectroscopic properties of luminescent ions in multiple sites of host materials is not only of scientific interest, but also technological importance. The present investigation focuses on this problem and reports the analysis results of the optical properties of 3d3ions in the multiple octahedron sites. The phosphor materials considered are double-perovskite La4Ti3O12:Mn4+and Ca3−xLuxHf2Al2+xSi1−xO12:Cr3+(x= 0−1.0) phosphors. An analysis method is demonstrated for obtaining reliable and site-dependent crystal-field (Dq) and Racah parameters (Band C) based on a ligand field theory with paying an attention to difficulty in the exact estimation of such important crystal-field-related parameters. For example, the difference in the crystal-field splitting energies between the slightly and highly-distorted octahedron sites determined are ΔDq∼ 1615 and 405 cm−1(x= 0) for the [MnO6]8−and [CrO6]9−octahedrons, respectively. All results are in satisfactory agreement with the Tanabe−Sugano energy-level diagram plots, promising good understanding of such 3d3-ion-activated phosphors with multiple octahedron sites.

Thermal stability of the phosphor materials is of crucial importance and scientific interest. It is well known that various Mn4+-activated phosphors, especially Mn4+-activated fluoride and oxyfluoride phosphors, show an anomalous thermal quenching behavior, i.e., an increase in the integrated photoluminescence intensity IPLwith increasing temperature T, known as the negative thermal quenching (TQ) behavior. The negative TQ has been understood to be due to the electric dipole (parity) forbidden transitions of 2Eg→ 4A2ggained by coupling with the odd-parity lattice vibrations, ν3, ν4, and ν6. This article discusses the effects of the 4A2g→ 4T2gexcitation transitions on a negative TQ phenomenon. Our previous study suggested that the 4A2g→ 4T2gexcitation transitions in Mn4+-activated fluoride phosphors are strongly connected with the certain mode phonons, namely the gerade-mode ν2phonons, with an energy of ∼65 meV. Here, our analysis model considers this effect and is found to show good agreement with the experimental data. Discussion is also given of the temperature dependence of decay time τ(T) and quantum efficiency η(T), in comparison with IPL(T), demonstrating a strong correlation among such important phosphor properties except for an occurrence of negative TQ only in IPL(T).

Yan argued that an anomalous negative thermal quenching (TQ) of the A2XF6:Mn4+fluoride phosphors was a pitfall caused by the diminishment in optical path lengths of the spectrofluorometer originating from the increasing volume of the phosphor (namely, thermal expansion) at elevated temperatures [S. Yan, J. Solid State Sci. Technol., 9, 106004 (2020)], but not from any intrinsic effects that can be rigidly modeled based on the solid-state physics. We disagree with this comment and explain in more detail that anomalous negative TQ is surely explained by the intrinsic properties of the Mn4+-activated fluoride phosphors with denying his/her previous work.