Thermal Quenching Mechanism of Mn in NaSiF, NaKSiF, and KSiF Phosphors: Insights from the First-Principles Analysis.

This study aims to identify the key factors governing the thermal quenching of Mn ion luminescence in fluoride-based phosphor materials used as red emitters in modern-day phosphor-converted LED devices. Here, we employ first-principles calculations for Mn-doped NaSiF, NaKSiF, and KSiF hosts to explore how host properties and local coordination environments influence thermal quenching behavior. The ΔSCF method was used to model the geometric structures of the MnA (ground) and E, T (excited) states and the energies of the optical transitions between these states. Our results reveal that thermal quenching in NaSiF and KSiF phosphors occurs through thermally activated E → T → A crossover. In contrast, thermal quenching in NaKSiF is due to other nonradiative decay pathways. Investigations of the mechanical stability of these fluorides show that NaKSiF is mechanically unstable. We suggest that this property of the host limits the luminescence efficiency of the embedded Mn ions. We also determined the reason for the difference in the intensity of the E → A emission transition (ZPL) in the systems. These findings advance our fundamental understanding of the thermal quenching mechanism of Mn ion luminescence in fluorides, and the results can aid future discoveries of technologically useful phosphors through high-throughput design methodologies.