Preparation And Luminescent Properties Of Mn⁴⁺ Doped K₂Ta_1-xIn_xF₇ Red Phosphors Based On Core-shell Structure

White light emitting diode（LED）has become one of the most promising strategic emerging industries due to its low energy consumption,long life,short response time,energy saving and environment-friendly.The most common white light synthesis method is realized by blue light excitation of yellow phosphor Y₃Al₅O₁₂:Ce³⁺(YAG:Ce³⁺).However,the lack of red light components in the spectrum shows low color rendering index（CRI）and high color temperature（CCT）.Mn⁴⁺doped fluoride phosphors usually exhibit a series of narrow-band sharp red emission at about 630 nm,which can effectively improve the color rendering index of white LED.Compared with rare earth doped red phosphors,the cost is low,but there is still a problem of easy hydrolysis.The development of Mn⁴⁺doped fluoride red phosphors with high water resistance and excellent luminescent properties has become one of the research hotspots in recent years.In this study,a series of shell-free K₂Ta_1-xIn_xF₇:Mn⁴⁺(KTIF:Mn⁴⁺)red phosphors were synthesized by using K₂Ta F₇ with asymmetric octahedral structure as the matrix and Mn⁴⁺as the luminescence center.The partial Ta⁵⁺in the matrix was replaced by In³⁺by saturated solution crystallization method.The substitution of heterovalent ions further distorted the polyhedron structure,thus effectively improving the luminescence intensity of the phosphor.Furthermore,K₂Ta_1-xIn_xF₇:Mn⁴⁺@K₂Ta_1-xIn_xF₇ red phosphors with homogeneous core-shell structure were prepared by surface passivation method using weak reducing agent citric acid.The results are as follows:1.A series of shell-free K₂Ta_0.7In_0.3F₇:Mn⁴⁺red phosphors were synthesized by introducing In³⁺to replace part of Ta⁵⁺in the asymmetric octahedral K₂Ta F₇ matrix,while and Mn⁴⁺was selected as the luminescent center.Through composition and process optimization,the optimal ratio of In³⁺to Ta⁵⁺is 3:7,the optimal precipitation time is 4 hours,and the optimal doping concentration of Mn⁴⁺is 7 mol%.The isovalent substitution of In³⁺further distorts the[Ta F₇]^2-octahedron structure and effectively enhances the emission intensity of Mn⁴⁺zero phonon line（ZPL）.EDS test shows that KTIF:7mol%Mn⁴⁺is composed of K,Ta,In,F and Mn elements.XRD and refinement show that the structure match with the PDF#19-0997 standard card.And In³⁺and Mn⁴⁺occupy the position of Ta⁵⁺.XPS test results show that the surface of KTIF:7mol%Mn⁴⁺contains the characteristic peak of Mn,indicating that the surface of KTIF:7mol%Mn⁴⁺contains Mn element.The KTIF:7mol%Mn⁴⁺phosphor has two obvious broadband excitation peaks between the 300～400 nm ultraviolet region and the 400～550 nm blue region,corresponding to the ⁴A_2g→⁴T_1g and ⁴A_2g→⁴T_2g spin-allowed transitions of Mn⁴⁺.Under the excitation of 468 nm,the strongest emission peak of the phosphor is located at 628 nm,which is attributed to the spin-forbidden ²E_g→⁴A_2g transition of Mn⁴⁺.The color coordinates,color purity and quantum efficiency of KTIF:7mol%Mn⁴⁺are（x=0.6888,y=0.3110）,99.94%and 81.90%respectively.And the LED packaged with commercial blue chip can emit bright red light.2.K₂Ta_0.7In_0.3F₇:7mol%Mn⁴⁺phosphor with the best luminescent properties was used as the core,and K₂Ta_0.7In_0.3F₇:7mol%Mn⁴⁺@K₂Ta_0.7In_0.3F₇(abbreviated as KTIF:7mol%Mn⁴⁺@CA,CA represents citric acid passivation to construct KTIF shell)red phosphor with homogeneous core-shell structure was prepared by surface passivation method using weak reducing agent citric acid.The optimum synthesis conditions were citric acid concentration of 1.44 mol/L,time of reaction were 0.5 hours.The results of the structure of KTIF:7mol%Mn⁴⁺@CA showed that the citric acid passivation treatment had no significant effect on the crystal structure of KTIF:Mn⁴⁺.XPS results showed that the signal of Mn was not detected on the surface of KTIF:7mol%Mn⁴⁺@CA,indicating that the shell with no Mn or less Mn was successfully constructed.Compared with KTIF:7mol%Mn⁴⁺phosphor,the position of excitation and emission peaks of KTIF:7mol%Mn⁴⁺@CA did not change,but the fluorescence intensity increased to 150%,which was due to the core-shell structure reducing the surface defects of the phosphor,thereby reducing the non-radiative transition between Mn⁴⁺ions.The color coordinates,color purity and quantum efficiency of KTIF:7mol%Mn⁴⁺@CA are（x=0.6930,y=0.3069）,99.97%and 78.74%respectively.And the LED packaged with commercial blue chip can emit bright red light.3.After the shell-free KTIF:7mol%Mn⁴⁺and shell-containing KTIF:7mol%Mn⁴⁺@CA phosphors were immersed in water for 10 days,the color of KTIF:7mol%Mn⁴⁺became brown due to the hydrolysis of Mn⁴⁺ions,while the color of KTIF:7mol%Mn⁴⁺@CA did not change significantly.And the fluorescence intensity remained 90%of KTIF:7mol%Mn⁴⁺@CA of the initial value,showing good water resistance.The KTIF:7mol%Mn⁴⁺@CA red phosphor and commercial YAG:Ce³⁺yellow phosphor were mixed and encapsulated into a white LED with a commercial blue chip.The Ra,CRI and CCT were 84.0,77.7 and 3821 K,respectively.Compared with the white LED without red phosphor（Ra,CRI and CCT were 69.5,57.9 and 5970K,respectively）,the color rendering was effectively improved.