Thermal Quenching Mechanism of Mn4+ in Na2SiF6, NaKSiF6, and K2SiF6 Phosphors: Insights from the First-Principles Analysis

This study aims to identify the key factors governing the thermal quenching of Mn4+ 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 Mn4+-doped Na2SiF6, NaKSiF6, and K2SiF6 hosts to explore how host properties and local coordination environments influence thermal quenching behavior. The Delta SCF method was used to model the geometric structures of the Mn(4+4)A(2) (ground) and E-2, T-4(2) (excited) states and the energies of the optical transitions between these states. Our results reveal that thermal quenching in Na2SiF6 and K2SiF6 phosphors occurs through thermally activated E-2 -> T-4(2) -> (4)A(2) crossover. In contrast, thermal quenching in NaKSiF6 is due to other nonradiative decay pathways. Investigations of the mechanical stability of these fluorides show that NaKSiF6 is mechanically unstable. We suggest that this property of the host limits the luminescence efficiency of the embedded Mn4+ ions. We also determined the reason for the difference in the intensity of the E-2 -> (4)A(2) emission transition (ZPL) in the systems. These findings advance our fundamental understanding of the thermal quenching mechanism of Mn4+ ion luminescence in fluorides, and the results can aid future discoveries of technologically useful phosphors through high-throughput design methodologies.