The luminescence of Mn4+ in K2GeF6

In this work, we report on the spectroscopic properties of Mn4+ in the fluoride host, K2GeF6. The Mn4+ luminescence intensity and lifetime are measured as a function of temperature. From the data, we have determined the onset temperature and activation energy for thermal quenching of luminescence. The optical properties and thermal quenching behavior of Mn4+ in K2GeF6 are compared with those reported for commercial Na2SiF6:Mn4+ and K2SiF6:Mn4+ (TriGain®) phosphors. This comparative study sheds light on the mechanism of thermal quenching of the Mn4+ ion luminescence in the A2XF6 (A = K, Na; X = Si, Ge) family of materials. We provide evidence that the activation energy and the onset temperature of thermal quenching for the 2E→ 4T2→ 4A2 crossover process is correlated with the energy of the 4T2 level and argue that this is a robust electronic structure-property relationship in host fluorides with low defect density. The K2GeF6 is reported to undergo structural phase transitions in the temperature ranges of 220–240 K and 150–170 K. We have probed the phase transitions by temperature dependent Raman spectroscopy. The evidence for structural phase transition in our Raman experiments is lacking. © 2025 Elsevier B.V.