Controlled morphology and improved photoluminescence of red emitting K₂LiAlF₆:Mn⁴⁺ nano-phosphor by co-doping with alkali metal ions

Trace alkali metal ions doped elpasolite K₂LiAlF₆:X⁺, Mn⁴⁺ (X = Li, Na, K) was studied as a red phosphor for white light emitting diodes (WLEDs). It exhibited red emission consisted of a series of narrow bands in the spectral range from 610 to 650 nm. The excitation bands were mainly centered at 360 nm and 465 nm, which matches well with emission of the UV and blue LED chips. The energy levels of Mn⁴⁺ in K₂LiAlF₆ were calculated using the exchange charge model of crystal field. The effects of trace alkali metal ions (Li⁺, Na⁺, K⁺) doping on the morphology and photoluminescence properties were investigated in detail. The emission intensity of K₂LiAlF₆:0.5 %K⁺, 3 %Mn⁴⁺ was enhanced obviously, which is 25 times higher than that one of the samples without alkali metal ions.