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

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 ΔSCF method was used to model the geometric structures of the Mn⁴⁺⁴A₂ (ground) and ²E, ⁴T₂ (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 ²E → ⁴T₂ → ⁴A₂ 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 ²E → ⁴A₂ 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.