Structure And Luminescence Properties Of AM₄（PO₄）₆:Eu²⁺（A=Ca,Sr,Ba,Mg,Zn,M=Zr,Hf）Blue-green Phosphors For White LED Applications

Recently,a new type of phosphate matrix material,Na superionic conductor（NASICON）type compound,the general formula is Na₃Zr₂Si₂PO₁₂,due to its flexible frame structure and adjustable chemical composition has become a research hotspot in the field of w-LED.In this paper,the classical Eu²⁺ion is used as the luminescence center,new NASICON compound AM₄（PO₄）₆（A=Ca,Sr,Ba,M=Zr,Hf）and AHf₄（PO₄）₆（A=Mg,Zn）with the same chemical formula but withβ-Fe₂（SO₄）₃ structure are used as the matrix.The crystal structure and luminescent properties of AM₄（PO₄）₆（A=Ca,Sr,Ba,Mg,Zn,M=Zr,Hf）blue-green phosphors were studied.The main results are as follows:（1）Ca_1-xSr_xZr₄（PO₄）₆:0.03Eu²⁺（0≤x≤1）blue-green phosphor samples with different Ca/Sr ratios were prepared by high-temperature solid-state method.XRD diffraction and Rietveld refinement results show that samples are continuous solid solution in the entire range of x.The lattice constants（a,b,c,V）and Ca/Sr-O bond length change monotonically with the Sr²⁺content x.With the increase of x value,under the combined effect of crystal field effect and occupancy of Eu²⁺,the emission spectrum is blue-shifted（492 nm→463nm）,the bandwidth is narrowed（87 nm→68 nm）,and under the excitation of ultraviolet light（365 nm）,the luminescence gradually changed from blue-green to blue.Due to the influence of Eu²⁺occupancy,the fluorescence lifetime and temperature characteristics of the sample show a nonlinear relationship with the x value,and reach the maximum when x=0.2.In addition,the thermal quenching of this series of phosphor samples is severe and requires further optimization before they can be used.（2）A high-temperature solid-state method was used to prepare AHf₄（PO₄）₆:Eu²⁺(A=Ca,Ca_0.5Sr_0.5,Sr,Sr_0.5Ba_0.5,Ba)blue-violet phosphor samples.When cation at the A site changes from Ca→Sr→Ba,XRD diffraction and Rietveld refinement results indicate that the sample maintained a single-phase structure,the lattice constants（a,b,c,V）and A-O bond lengths change monotonously,proving the formation of a continuous solid solution.Under excitation at 330 nm,samples show strong luminescence,when the cation at the A site changes from Ca→Sr→Ba,the emission spectrum gradually shifts blue（A=Ca,452nm;A=Sr,436 nm;A=Ba,421 nm）,the bandwidth narrows linearly,and the luminescence gradually changes from blue to blue-violet.Besides,the temperature characteristics of this series of samples are good,especially when A=Ba,the luminous intensity at 150℃can maintain 88%of the normal temperature.The above results indicate that this series of phosphors have good application prospects.（3）A new type of AHf₄（PO₄）₆:x Eu²⁺（A=Mg,Zn）blue phosphor sample was prepared.XRD diffraction and Rietveld refinement results indicate that the synthesized sample hasβ-Fe₂（SO₄）₃ structure,the space group is P2₁/n,and the optimal synthesis temperature of phosphor is 1200℃.The forbidden band widths of the matrix material AHf₄（PO₄）₆（A=Mg,Zn）are 4.57 and 4.40 e V,respectively.The quenching of Eu²⁺in AHf₄（PO₄）₆:x Eu²⁺（A=Mg,Zn）comes from the dipole-dipole interaction,and the quenching concentration is higher(C⁺_Eu2=10 mol%).Both phosphor materials have good thermal stability and can maintain69%of the luminous intensity at room temperature at 150℃.Therefore,this series of phosphors are expected to provide blue light sources for ultraviolet-based w-LEDs.