The Synthesis,Optical Properties And LED Device Performance Of K₂NaAlF₆:Mn⁴⁺ Red Phosphor

The Mn4+-doped fluoride red phosphors have been regarded as the fourth generation of warm white LED red phosphors due to their unique optical properties,i.e.y strong absorption at blue light spectral region and intense sharp lines emission at?630 nm,which are promsing for applications in high-efficiency warm white LEDs and next-generation wide gamut displays.In this thesis,we developed a modified room-temperature coprecipitation method using methanol as precipitating agent for synthesizing K2NaAlF6:Mn4+ red phosphor,which has a high synthetic yield of-77%and is convinient for mass-scale production.The synthesized K2NaAlF6:Mn4+(2.31 at.%)red phosphor exhibited a broad absorption band peaked at 466 nm and sharp emission lines centered at 631 nm,which are ideal for WLEDs application.To analyze the electronic/vibronic structures of Mn4+ in the K2NaAlF6 host lattice,we measured the high-resolution emission spectra of the K2NaAlF6:Mn4+(2.31 at.%)sample under the excitation at 460 nm at extremely low temperatures(e.g.,3,10,20 and 30 K).We observed three zero-phonon lines(ZPLs)in the 3 K emission spectra,which are originated from three Mn4+ ions with different local site symmetry.The occurrence of multi-sites occupation of Mn4+ ions are probably due to the charge imbalance between Mn4+ and Al3+ ions in K2NaAlF6 crystal.When a Mn4+ ion replaced an Al3+ ion,a Na+ or K+ hole was correspondingly generated to achieve the charge balance.Ideally,the Mn4+ions occupy the crystallographic sites of Al3+ ions in K2NaAlF6 crystals have a local site symmetry of Oh.Nevertheless,the local site symmetry of Mn4+ ions were probably reduced from Oh to lower one when Na+ or K+ holes were neighbouring to them.The K2NaAlF6:Mn4+(2.31 at.%)red phosphor has good luminescent thermal stability.At 150?,the photoluminescence intensity remains 80.3%of that at 25?.Finally,the 461 nm blue chip,the commercial green powder GAL545,and the synthesized K2NaAlF6:Mn4+(2.31 at.%)red phosphor were used to fabricated a warm white LED with a low CCT(3911K),a high CRI(Ra=93),and a luminous efficacy of 89 lm/W.These results indicate that K2NaAlF6:Mn4+(2.31 at.%)red phosphor is promising for high-efficiency warm white LEDs and wide-gamut displays.