The Synthesis, Morphology And Luminescence Properties Of A New Mn⁴⁺ Doped Rare Earth Fluoride Red Phosphor

White light emitting diodes?LEDs? is deemed to be the next-generation green lighting source owing to their admirable properties,such as energy saving,high efficiency,environmental friendliness,long service life,and so forth.Currently,the most popular white LED in the market is manufactured using an InGaN-based blue LED chip with the well-known commercial Y₃Al₅O₁₂:Ce³⁺(YAG:Ce³⁺) yellow phosphor.However,this type of white LED suffers from a high correlated color temperature?CCT>4500 K? and a low color-rendering index?CRI,Ra<80? which seriously limit its application in indoor lighting and wide color gamut backlight display.To improve the optical performance,the red phosphors are suitable for compensating the red component of the white LEDs.The transition metal Mn⁴⁺ doped fluoride red phosphors have rapidly developed into a new generation red-light candidate material for white LEDs,for their dominant blue absorption band?⁴60 nm? matched with the blue LED chip,red emission spectrum?⁶30 nm? adapted to the human eyes' sensitivity,high luminous efficiency and color purity,as well as simple synthesis.Nevertheless,for all Mn⁴⁺ activated fluorides reported to date,there are still low synthetic yields and poor morphologies,which seriously limits its further development.For this purpose,on the basis of summarizing the existing Mn⁴⁺-activated fluorides,this paper innovatively introduces rare earth element Sc from the perspective of matrix structure,and selects the new system of K₂AScF₆?A=K,Na? rare earth fluoride as the research object with the aim of designing and synthesizing Mn⁴⁺-activated fluoride red-light materials with high efficiency,high yield and controllable morphology to meet the application of high-quality white LED devices.The main works and results are as follows:?1?A novel K₃ScF₆:Mn⁴⁺ red phosphor with controllable uniform morphology was synthesized by a facile two-step co-precipitation method.The prototype crystallizes in space group Fm3?m with a cubic structure and the lattice parameters are fitted well to be a=b=c=8.4859?8??and V=611.074?2??³.The Mn⁴⁺ ions occupy Sc³⁺ sites and locate at the centers of the distorted[ScF₆]octahedrons.The DFT calculation shows that K₃ScF₆ possesses a band gap of approximately 6.15 eV,which can provide sufficient space to accommodate impurity(Mn⁴⁺)energy levels.By controlling the doping amount of Mn⁴⁺,the K₃ScF₆:Mn⁴⁺ phosphors show a morphological evolution from cube to tapered octahedron.Under blue light excitation,the K₃ScF₆:Mn⁴⁺phosphors exhibit intense sharp line red fluorescence?the strongest peak located at 631 nm? with a high color purity,and its quantum efficiency is 67.18%.The optimal conditions for the synthesis of K₃ScF₆:Mn⁴⁺ were as follows:the reaction temperature is 60?,the reaction time is 2 h,and the Mn⁴⁺ doping amount is 0.97 mol%.Besides,the K₃ScF₆:Mn⁴⁺phosphors have an excellent thermal cycling performance.Furthermore,a warm white LED fabricated with a blue chip merged with the mixtures of K₃ScF₆:Mn⁴⁺ and the commercial YAG:Ce³⁺yellow phosphors exhibits wonderful color quality with lower CCT?3250 K? and higher CRI?Ra=86.4?.?2?By changing the alkali metal cation at the A-site,a novel Mn⁴⁺-doped narrow-band red-emitting hexafluoscandate phosphor with ultra-high synthetic yield?¹00%? and uniform spherical morphology,e.i.,K₂NaScF₆:Mn⁴⁺,was synthesized through a simple two-step co-precipitation route.Its crystal structure is consistent with K₃ScF₆,and the octahedral Sc³⁺ sites were occupied by Mn⁴⁺ ions,resulting in a very small amount of alkali metal cation vacancies and consequent slight shrinkage of the lattice structure.Compared to the K₃ScF₆,the K₂NaScF₆ matrix possesses a wider electronic band gap of approximately 6.46 eV,which ensures the location of the Mn⁴⁺ energy levels in the host band gap.Besides,the Mn⁴⁺ ions in K₂NaScF₆ host lattice suffer from a strong crystal field strength and a weak nephelauxetic effect.K₂NaScF₆:Mn⁴⁺ phosphor has an ultra-high synthetic yield?¹00%? and a uniform spherical morphology with extremely narrow size distribution?¹.17?m?.The excess of KHF₂ and the low solubility of K₂NaScF₆ matrix are the key to an ultra-high synthetic yield of K₂NaScF₆:Mn⁴⁺ phosphor.Under blue light illumination,the K₂NaScF₆:Mn⁴⁺ phosphor exhibits intense sharp-line red fluorescence peaked at 630 nm with a strong ZPL,and its quantum efficiency is 70.3%.The optimal Mn⁴⁺-doping amount in K₂NaScF₆ matrix is 1.94mol% and the intensity of ZPL could be tailored through control of the Mn⁴⁺ amount in K₂NaScF₆ matrix.Meanwhile,the K₂NaScF₆:Mn⁴⁺ phosphor shows a small thermal quenching behavior,which maintains 82%of the initial integrated PL intensity at 425 K.By employing the K₂NaScF₆:Mn⁴⁺ as an efficient red emitter,two pc-WLEDs with ultra-wide color gamut?NTSC value of 126.3% and Rec.2020 value of 94.3%for LED-1,NTSC value of 130.9% and Rec.2020 value of 97.8% for LED-2?were obtained successfully.