Facile synthesis, morphology and photoluminescence of a novel red fluoride nanophosphor K₂NaAlF₆:Mn⁴⁺

A novel red-emitting fluoride phosphor, K₂NaAlF₆:Mn⁴⁺, with nanoscale particle size was synthesized via a cation exchange route. This phosphor absorbs UV and blue light and emits red light at about 630 nm; thus, it has been regarded as an ideal red phosphor for WLEDs. The reaction parameters were systematically investigated and an optimized sample was obtained. K₂NaAlF₆:Mn⁴⁺ showed better photoluminescence properties and its emission wavelength was blue shifted with respect to that of K₂LiAlF₆:Mn⁴⁺ due to differences in the neighbouring alkali metal ions and different distances between the cation and the ligands. These results are in agreement with the trend determined by the recently introduced parameter β₁, which describes the energy of the ²E_g → ⁴A_2g transition as a function of the nephelauxetic effect. Finally, these results could further contribute to the optimization of red-emitting phosphors based on Mn⁴⁺ ions.