Phase-transition-induced giant enhancement of red emission in Mn⁴⁺-doped fluoride elpasolite phosphors

Mn⁴⁺-Doped red fluoride phosphors have attracted significant interest of researchers because of their excellent luminescence properties that can address the issue of the lack of red light components in commercial white light-emitting-diodes (WLEDs). Herein, we synthesized two novel Mn⁴⁺-based phosphors based on two different K₂LiAlF₆ phases (cubic and trigonal). The quantum efficiency of cubic K₂LiAlF₆:Mn⁴⁺ was only 3.2%, but it underwent a dramatic increase to 87.5% when the trigonal phase changed to cubic K₂LiAlF₆. The trigonal phase was gradually transformed to the cubic phase by controlling the HF volume concentration; this led to an increase in the Mn content and enhancement of the emission intensity. The analysis of the experimental spectra of the synthesized phosphors was facilitated by the crystal field calculations of the K₂LiAlF₆:Mn⁴⁺ energy levels. A good agreement between the theoretical and experimental data was achieved. Special attention was paid to the structure of these two phases and the phase transition mechanism. The morphologies, compositions, and temperature-dependent photoluminescence properties were investigated in detail. The as-prepared phosphors showed excellent thermal and chromatic stabilities. Finally, the optimized K₂LiAlF₆:Mn⁴⁺ phosphor was blended with the YAG:Ce³⁺ yellow phosphor and a blue LED to fabricate WLEDs. The color rendering index (Ra) of this new WLED is 86, and the correlated color temperature (CCT) is 3498 K. These findings indicate that the K₂LiAlF₆:Mn⁴⁺ phosphor is a promising red phosphor for applications in WLEDs.