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.