Synthesis, characterization, and fluorescence properties of SrAl2B2O7:Eu2+ and SrAl2B2O7:Eu2+, Li+

The blue-emitting phosphor SrAl2B2O7:Eu2+ has the potential to be used in full-spectrum LED devices. However, the reducing atmosphere is extremely demanding in the complex preparation process of SrAl2B2O7:Eu2+. Thereby, the synthesis of SrAl2B2O7:Eu2+ with an economic approach is important. In this work, a new synthesis route was realized; SrAl2O4:Eu2+ and H3BO3 were used as solid precursors to synthesize SrAl2B2O7:Eu2+ by solid-state reaction method, and only carbon powder is needed as a source of reducing atmosphere. The effects of different synthesis routes on the crystal structure, morphology, and luminescence properties of SrAl2O4:Eu2+ were studied, as well as the effects of Li+ doping on luminescence properties were investigated. The results show that under the new synthesis route, SrAl2B2O7:Eu2+ has good luminescence properties, and doping Li+ will improve the luminescence intensity.


Introduction
As there is an increasing requirement for health lighting and high-quality white LED, full-spectrum pc-LED white lighting has been proposed, full-spectrum white light with high color rendering index and appropriate color temperature.Plant growth lights, operating room library lighting, camera flash, film and television lights, and eye protection lights all have extremely high requirements for full-spectrum white LED lighting.Ultraviolet LED chips excited red green blue (RGB) phosphors, which is one way to obtain high-quality full-spectrum white LED lighting [1,2,3] .
Blue-emitting phosphor BaMgAl10O17:Eu 2+ (BAM) [4] is currently the most mature and widely used for white LED with high luminous efficiency and pure chrominance.BaMgAl10O17:Eu 2+ excitation range is 235-425nm.Its peak value is 355 nm, and its emission peak is 450nm.But the half peak width is narrow, and the thermal stability is poor.
SrAl2B2O7 was first reported by Keszler [5] , SrAl2B2O7 with a non-centrosymmetric structure.Then Lucas [6] studied the structure of SrAl2B2O7 and the low-temperature luminescence characteristics of SrAl2B2O7:Eu 2+ phosphor and showed the largest luminescence band at 415nm excited at 325nm.SrAl2B2O7:Eu 2+ has the potential to be used as full-spectrum white-blue phosphors.However, the preparation of SrAl2B2O7:Eu 2+ is very complicated and requires multiple reactions.Due to the low reaction temperature, H2 is also needed as the reducing atmosphere.Camardello [7] synthesized Sr1-xEuxAl2B2O7 by the solid state reaction technique, raw materials fired at 700℃/800℃/900℃/ for 5h under a reducing atmosphere of 2% H2-98% N2 gas mixture, the optimal doping concentration of Eu 2+ is 0.08.Zhou [6] synthesized SrAl2B2O7:Eu 2+ , Mn 2+ through the solid-state reaction method, materials calcined respectively at 800℃ and 900℃ for 4h under the reducing atmosphere of /N2 reducing atmosphere has high requirements for the sealing safety of experimental equipment in the laboratory.It is even more difficult if large quantities of raw materials are needed.The use of carbon powder in high-temperature solid reactions to form CO atmosphere is a simple way with low cost and high safety.It is used in many Eu 2+ -doped phosphors [8,9,10] .A new efficient synthesis route of SrAl2B2O7:Eu 2+ without 5%H2/95%N2 reduction atmosphere was explored, and the effect of Li + doped on the luminescence properties of SrAl2B2O7:Eu 2+ was also studied.

Preparation of Sr0.92Al2B2O7:xEu 2+ phosphors
Sr1-xAl2O4:xEu 2+ phosphors as precursors were synthesized through the solid-state reaction method, the starting materials for sample preparations included SrCO3(A.R. grade), Al2O3(A.R. grade), H3BO3(A.R. grade), Eu2O3(99.99%),carbon powder is placed around the raw material as a source of reducing atmosphere.The raw material in the molar ratio was mixed homogeneously in an agate mortar, heated to 1300℃ in 3h, and held for 3h under the reducing CO atmosphere produced by the combustion of carbon powder.Then prepared Sr1-xAl2O4:xEu 2+ was weighed, and the molar value was calculated, Sr1-xAl2O4: xEu 2+ and H3BO3 has a molar ratio of Sr:B=1:2.The well-grinded sample was slowly heated to 700-950℃ at a rate of 20.0℃/h and kept at 700-950℃ for 2.5 hours respectively to find the optimal reaction temperature, finally, Sr0.92Al2B2O7:0.08Eu2+ samples were prepared.

Preparation of Sr1-yAl2B2O7:Eu2+, yLi+ phosphors
Such as the previous preparation method: H3BO3(A.R.grade), Sr0.92Al2O4:0.08Eu2+ ,yLi + (y=0.04,0.06, 0.08, 0.1) precursors and carbon powder were prepared as raw materials, Sr0.92Al2B2O7:0.08Eu2+ ,yLi + was prepared according to the optimum preparation conditions.The results indicate that SrAl2B2O7 can be synthesized from SrAl2O4 a n d H 3 BO3 through high-temperature solid phase reaction, and the best reaction condition is to calcination at about 950℃ for 2.5 h.This indicates that the reaction of excess H3BO3 with SrAl2O4 will not result in the formation of other heterophase, and SrAl2B2O7 can be directly produced.Figure 1.(c) shows the XRD patterns of Sr0.92B2O7:0.08Eu2+ and yLi + phosphors doped with Li + .When Li + doping concentration is lower than 0.08, the crystal lattice of Sr0.92Al2B2O7:0.08Eu2+ and yLi + will not change significantly.However, as the doping concentration of Li + increases to more than 0.08 and a weak impurity peak appears at 25.02°, the radius of Li + is 0.076nm, the radius of Sr 2+ is 0.118nm, and the radius of Al 3+ is 0.0535 nm.The difference between the radius of Li + and Sr 2+, and Al 3+ is similar, but Al 3+ is trivalent.

Results and discussion
In the lattice of SrAl2B2O7, AlO4 tetrahedrons exist.The atomic spacing between Al 3+ and the coordination O2-is 1.7661 A and 1.666A, and the spacing between Sr 2+ and the coordination O2atom is 2.547 Å and 3.292 Å. Li + is more inclined to have an unequal substitution with Sr 2+ , replacing Sr 2+ in the lattice.Figure 2 shows through the traditional preparation method of SrAl2B2O7 and microscopic morphology under the new synthesis route, SrAl2B2O7 grains prepared by the traditional one-step method show many sharp edges and corners, but the SrAl2B2O7 grains prepared by the new method are smooth, it has better crystallinity.

Fluorescence property of Sr1-xAl2B2O7:xEu 2+ ,Li +
Figure 3 shows the emission spectra and fluorescence attenuation maps of Sr1-x-yAl2B2O7:xEu 2+ , yLi + excited photoexcitation at 325 nm.Li + enhances the absorption of SrAl2B2O7:Eu 2+ at 300-400 nm and the emission intensity of 417 nm, corresponding to the 4f7→4f65d1( 8 S7/2) transition of Eu 2+ .There are no studies on the effect of alkali metal ions on the luminescence properties of Eu 2+ fluorescent materials in the existing reports.Li + doping may play a role: Li + replaces the Sr 2+ position, enhances the energy absorption of the matrix, and transfers energy to Eu 2+ .Due to charge imbalance, excessive Li + produces defects in the matrix, resulting in excessive lattice shrinkage, resulting in defects, destroying the original crystal structure, and reducing the luminescence intensity.

Conclusion
The low-cost and high-efficiency preparation of SrAl2B2O7:Eu 2+ phosphor was realized by a new synthesis route.This method reduces the equipment requirements, increases the production safety, reduces the reaction times and reaction time, improves the efficiency of SrAl2B2O7:Eu 2+ phosphor preparation, and can prepare pure SrAl2B2O7:Eu 2+ phosphor.The results show that the new synthesis path does not oxidize Eu 2+ due to the secondary reaction.Li + doping can improve the luminescence properties of SrAl2B2O7:Eu 2+ , and appropriate Li + doping can enhance the excitation and emission intensity of SrAl2B2O7:Eu 2+ .The optimal doping concentration is 0.06, and excessive Li + will destroy the lattice structure and generate an impurity phase.The new synthesis path of SrAl2B2O7:Eu 2+ is valuable for further research and mass production.

Figure 1 (
Figure 1(a) and (b)show the XRD patterns of Sr0.92Al2O4:0.08Eu2+ and H3BO3 after solid phase reaction at different reaction temperatures for 2.5h.By comparing with the standard XRD card (JCPDS No.47-0812), it can be obviously found that with the increase of calcination temperature, the diffraction peak corresponding to SrAl2O4 gradually disappears, while the SrAl2B2O7 diffraction peak gradually appears and increases with the increase of reaction temperature.All the diffraction peaks were well matched to standard SrAl2B2O7(JCPDS No.47-0812), and the SrAl2B2O7 diffraction peaks decreased in width and increased in intensity due to the increased crystallinity of SrAl2B2O7 phase.The results indicate that SrAl2B2O7 can be synthesized from SrAl2O4 a n d H 3 BO3 through high-temperature solid phase reaction, and the best reaction condition is to calcination at about 950℃ for 2.5 h.This indicates that the reaction of excess H3BO3 with SrAl2O4 will not result in the formation of other heterophase, and SrAl2B2O7 can be directly produced.Figure1.(c) shows the XRD patterns of Sr0.92B2O7:0.08Eu2+ and yLi + phosphors doped with Li + .When Li + doping concentration is lower than 0.08, the crystal lattice of Sr0.92Al2B2O7:0.08Eu2+ and yLi + will not change significantly.However, as the doping concentration of Li + increases to more than 0.08 and a weak impurity peak appears at 25.02°, the radius of Li + is 0.076nm, the radius of Sr 2+ is 0.118nm, and the radius of Al 3+ is 0.0535 nm.The difference between the radius of Li + and Sr 2+, and Al 3+ is similar, but Al 3+ is trivalent.In the lattice of SrAl2B2O7, AlO4 tetrahedrons exist.The atomic spacing between Al 3+ and the

Figure 2 .
Figure 2. Morphologies of SrAl2B2O7.(a) Prepared by traditional methods.(b)Prepared by a new synthetic route method.