Thermal Quenching\nMechanism of Mn<sup>4+</sup> in Na<sub>2</sub>SiF<sub>6</sub>, NaKSiF<sub>6</sub>, and K<sub>2</sub>SiF<sub>6</sub> Phosphors: Insights from\nthe First-Princip

This study aims to identify the key factors governing\nthe thermal quenching of Mn<sup>4+</sup> ion luminescence in fluoride-based\nphosphor materials used as red emitters in modern-day phosphor-converted\nLED devices. Here, we employ first-principles calculations for Mn<sup>4+</sup>-doped Na<sub>2</sub>SiF<sub>6</sub>, NaKSiF<sub>6</sub>,\nand K<sub>2</sub>SiF<sub>6</sub> hosts to explore how host properties\nand local coordination environments influence thermal quenching behavior.\nThe ΔSCF method was used to model the geometric structures of\nthe Mn<sup>4+4</sup>A<sub>2</sub> (ground) and <sup>2</sup>E, <sup>4</sup>T<sub>2</sub> (excited) states and the energies of the optical\ntransitions between these states. Our results reveal that thermal\nquenching in Na<sub>2</sub>SiF<sub>6</sub> and K<sub>2</sub>SiF<sub>6</sub> phosphors occurs through thermally activated <sup>2</sup>E → <sup>4</sup>T<sub>2</sub> → <sup>4</sup>A<sub>2</sub> crossover. In contrast, thermal quenching in NaKSiF<sub>6</sub> is\ndue to other nonradiative decay pathways. Investigations of the mechanical\nstability of these fluorides show that NaKSiF<sub>6</sub> is mechanically\nunstable. We suggest that this property of the host limits the luminescence\nefficiency of the embedded Mn<sup>4+</sup> ions. We also determined\nthe reason for the difference in the intensity of the <sup>2</sup>E → <sup>4</sup>A<sub>2</sub> emission transition (ZPL) in\nthe systems. These findings advance our fundamental understanding\nof the thermal quenching mechanism of Mn<sup>4+</sup> ion luminescence\nin fluorides, and the results can aid future discoveries of technologically\nuseful phosphors through high-throughput design methodologies.