Review—Negative Thermal Quenching of Mn <sup>4+</sup> Luminescence in Fluoride Phosphors: Effects of the <sup>4</sup> A <sub>2g</sub> → <sup>4</sup> T <sub>2g</sub> Excitation Transitions and Normal Thermal Quenching
Sadao Adachi
Abstract
Thermal stability of the phosphor materials is of crucial importance and scientific interest. It is well known that various Mn 4+ -activated phosphors, especially Mn 4+ -activated fluoride and oxyfluoride phosphors, show an anomalous thermal quenching behavior, i.e., an increase in the integrated photoluminescence intensity I PL with 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 2 E g → 4 A 2 g gained by coupling with the odd-parity lattice vibrations, ν 3 , ν 4 , and ν 6 . This article discusses the effects of the 4 A 2 g → 4 T 2 g excitation transitions on a negative TQ phenomenon. Our previous study suggested that the 4 A 2 g → 4 T 2 g excitation transitions in Mn 4+ -activated fluoride phosphors are strongly connected with the certain mode phonons, namely the gerade-mode ν 2 phonons, 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 I PL ( T ), demonstrating a strong correlation among such important phosphor properties except for an occurrence of negative TQ only in I PL ( T ).