Optimal Doping Content of Red Emitting Y₄GeO₈∶Bi³⁺,Eu³⁺ Phosphor Designed by Response Surface Methodology Enhanced Publication

Optimal Doping Content of Red Emitting Y 4 GeO 8 ∶Bi 3+ ,Eu 3+ Phosphor Designed by Response Surface Methodology 效应面优化模型获取Y 4 GeO 8 ∶Bi 3+ ,Eu 3+ 红色荧光粉掺杂浓度 1 first-author 徐淑君(1997-)，女，安徽池州人，硕士研究生，2020年于安庆师范大学获得学士学位，主要从事稀土离子掺杂荧光粉的制备与发光性能的研究。E-mail: 2838110096@qq.com http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019425&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019427&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019443&type= 徐淑君 (1997-)，女，安徽池州人，硕士研究生，2020年于安庆师范大学获得学士学位，主要从事稀土离子掺杂荧光粉的制备与发光性能的研究。E-mail: 2838110096@qq.com 2838110096@qq.com 1 1 1 1 1 corresp *E-mail: ghh@zjnu.cn *E-mail: ghh@zjnu.cn 郭海(1980-)，男，江西吉水人，博士，教授，博士生导师，2005年于中国科学技术大学获得博士学位，主要从事新型稀土光学功能材料的研究。E-mail: ghh@zjnu.cn http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019430&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019432&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019452&type= 郭海 (1980-)，男，江西吉水人，博士，教授，博士生导师，2005年于中国科学技术大学获得博士学位，主要从事新型稀土光学功能材料的研究。E-mail: ghh@zjnu.cn ghh@zjnu.cn Department of Physics, Zhejiang Normal University, Jinhua 321004, China Department of Physics, Zhejiang Normal University, Jinhua 321004, China 浙江师范大学　物理系,　浙江　金华　321004 浙江师范大学　物理系,　浙江　金华　321004 Herein, a more direct method, response surface methodology, is used to predict the strongest red emission of Y 4 GeO 8 ∶Bi 3+ ,Eu 3+ samples. The concentrations of Bi 3+ and Eu 3+ of the optimal sample were predicted to be 31.03% and 67.36%(in mole ratio), respectively. The optimal sample was prepared and photoluminescent properties were measured and characterized. Y 4 GeO 8 ∶31.03%Bi 3+ ,67.36%Eu 3+ has strongest red emission. The difference between the intensity of experimental value and theoretical value is very small. The color coordinate of the as-prepared sample is (0.645 7, 0.349 0), which is very close to the color coordinate of standard red light (0.670 0, 0.330 0). The luminous color of the sample is pure, and the calculated color purity reaches 98%. What’s more, the internal quantum efficiency of the sample is as high as 72.5%. In a word, this paper provides an approach for searching the optimal doping concentration of phosphors with the strongest luminescence directly, which can be used in exploring all types of co-doped phosphors. 使用更直接的方法（效应面优化模型）预测了Y 4 GeO 8 ∶Bi 3+ ，Eu 3+ 样品的最强红光发射。预测最佳样品掺杂的Bi 3+ 离子和Eu 3+ 离子浓度分别为31.03%和67.36%（摩尔分数）。制备最佳样品后对其光致发光性能进行了测试和表征。荧光粉Y 4 GeO 8 ∶31.03%Bi 3+ ，67.36%Eu 3+ 具有最强的红光发射，并且强度的实验值和理论值之间的差值很小。优化样品的色坐标为（0.645 7，0.349 0），计算出的色纯度为98%，内量子效率高达72.5%。本文提供了一种直接寻找发光最强的荧光粉最佳掺杂浓度的方法，可用于探索各种类型的共掺杂荧光粉。 Y 4 GeO 8 ∶Bi 3+ ,Eu 3+ phosphors response surface methodology luminescent intensity Y 4 GeO 8 ∶Bi 3+ ,Eu 3+ 荧光粉 效应面优化模型 发光强度 Introduction 1 In the field of solid-state lighting, white light emitting diode(W-LED) has attracted extensive attention because of its high efficiency,no pollution and long service lifetime [ 1 1 - 4 4 1-4 ］ . Generally, W-LED can be made of blue light chip and yellow phosphor, YAG∶Ce 3+ . However, the lack of red light leads to low color rendering index and high color temperature. In this regard, some researchers have proposed a feasible method for fabricating W-LED using ultraviolet(UV) chip and red, green and blue phosphors [ 5 5 - 6 6 5-6 ］ . No matter which method is used to prepare W-LED, the red-light component is very important. At present, some researchers are committed to exploring red phosphors [ 7 7 - 8 8 7-8 ］ . For example, Yue et al . reported red-emitting Sr 3 La(AlO) 3 (BO 3 ) 4 ∶Eu 3+ phosphors with high color purity of 92.4% [ 9 9 ] . Guo et al . synthesized red-emitting Na 3 Sc 2 (PO 4 )∶Eu 3+ phosphors with quantum efficiency of 49% [ 10 10 ] . Wang et al . prepared red-emitting Gd 2 InSbO 7 ∶Eu 3+ phosphors with Commission Internationale de L'Eclairage(CIE) of (0.629 0, 0.371 0) [ 11 11 ] . All the red phosphors studied above have good luminescent properties. Usually, researchers change the concentration of doped ions by the control variable method to seek the strongest luminescence. But this traditional method cannot find the strongest luminescence directly. Therefore, some researchers want to explore whether there is a direct way to find the strongest luminescence. Some researchers have found that the response surface methodology(RSM) can be used to determine an optimal value of some problems. And RSM has the advantages, such as displaying the relationships between independent and dependent variables, including error analysis, and saving time and low cost [ 12 12 - 14 14 12-14 ］ . For instance, Li et al . predicted the best conditions to reach the maximum relative density of calcia partially stabilized zirconia by RSM [ 15 15 ] . Luo et al . studied the effects of extraction temperature, extraction time, ratio of water and their interaction on the yield of urea dioxide total polysaccharide by RSM [ 16 16 ] . Theoretically, most of the methods are universal. In other words, RSM is also feasible in the field of luminescent materials. Due to their stable properties and high solubility of rare earth ions, germanate phosphors have stimulated the interest of researchers in recent years [ 17 17 - 19 19 17-19 ］ . For doped ions, Eu 3+ is a very excellent activator for red emission, but its intensity is very weak [ 20 20 - 23 23 20-23 ］ . It is necessary to introduce a sensitizer to enhance its luminous intensity [ 18 18 ] . Bi 3+ is an excellent sensitizer [ 24 24 - 25 25 24-25 ］ . The co-doping of Bi 3+ and Eu 3+ can improve the luminous intensity of phosphors [ 26 26 - 28 28 26-28 ］ . Therefore, the phosphor Y 4 GeO 8 ∶Bi 3+ ,Eu 3+ was selected as an example to optimize by RSM. The purpose of this study is to determine the optimal doping concentrations of Bi 3+ and Eu 3+ in Y 4 GeO 8 ∶Bi 3+ ,Eu 3+ phosphors through uniform design and RSM. Through simulation, the ions concentrations of Bi 3+ and Eu 3+ are 31.03% and 67.36%(in mole ratio), respectively. The CIE, color purity and quantum efficiency of Y 4 GeO 8 ∶31.03%Bi 3+ ,67.36%Eu 3+ phosphors were further studied. The color purity of the optimal sample is as high as 98%, and its internal quantum efficiency is 72.5%. Experiments 1 Sample Preparation 2 A series of Y 4 GeO 8 ∶Bi 3+ ,Eu 3+ phosphors were synthesized by solid-state reaction method. Firstly, the basic materials Y 2 O 3 (99.99%), GeO 2 (99.999%), Bi 2 O 3 (99.99%), Eu 2 O 3 (99.99%) and H 3 BO 3 (99.8%) were weighed conforming to the stoichiometric ratio, in which H 3 BO 3 was used as the cosolvent. Secondly, basic materials were ground in a mortar for 30 min. Alcohol was added during the grinding process to help the raw materials mix evenly. The uniformly blended material was then placed in an alumina crucible and sintered for 5 h at 1 400 ℃. Finally, after cooling, the sintered sample was ground into powder for further characterization. Characterizations 2 X-ray diffraction(XRD) patterns were characterized by Rigaku MiniFlex/600 using Cu Kα1 radiation. An Edinburgh FS5 spectrofluorometer with a 150 W Xe lamp was used to examine the samples' excitation and emission spectra. Samples with the same mass were added into the same sample holders. For spectra measurement, the position of the samples remains unchanged, and the spectrometer parameters remain unchanged for better comparison. Internal quantum efficiencies and temperature-dependent photoluminescence spectra were obtained using FS5 spectrofluorometer with a TCB1402C temperature controller(China) and a SC-30 integrating sphere, respectively. Uniform Design and RSM 2 Uniform design can narrow the experimental range of RSM according to the experimental results. So U 9 (9 2 ) test table was used to design the experiment. RSM was used to determine the best combination of experimental parameters(concentrations of Bi 3+ and Eu 3+ ) to obtain the strongest luminescence. RSM needs to be reflected in the coding space, so central composite design(CCD) was used to encode the experimental parameters. The data obtained by RSM were analyzed by variance, and the obtained data were judged by T-test and F-test. F-test was used to determine the credibility of the RSM's equation. T-test is used to determine whether the equation is loss of fitting [ 12 12 - 16 16 12-16 ］ . Results and Discussion 1 Results and Analysis of Uniform Design 2 The U 9 (9 2 ) uniform design table is shown in Tab.1 Tab.1 . According to the results reported in the literature, the doping concentrations ranges of Bi 3+ and Eu 3+ ions are preliminarily selected as 10%-60% and 20%-120%, respectively [ 18 18 ] . Nine samples were uniformly designed according to U 9 (9 2 ), and the doping concentrations of Bi 3+ and Eu 3+ ions in each sample are located in column 2 and column 3 of Tab.1 Tab.1 , respectively. The integrated intensities of photoluminescence(PL) spectra in Fig.1 Fig.1 are calculated and analyzed. The integrated section is 560-800 nm. The results are presented in the fourth column( Y ) of Tab.1 Tab.1 . Comparing the results of the samples, it can be inferred that the optimal doping concentrations ranges of Bi 3+ and Eu 3+ are 10%-50% and 35%-95%, respectively. 1 1 Number 1 1 k 1 ( z 1 )/% 1 1 k 2 ( z 2 )/% 1 1 Y 1 1 1 1 1 1(0.10) 1 1 4(57.50) 1 1 871 775 1 1 2 1 1 2(7.59) 1 1 8(107.50) 1 1 1 485 240 1 1 3 1 1 3(15.08) 1 1 3(45.00) 1 1 3 854 920 1 1 4 1 1 4(22.56) 1 1 7(95.00) 1 1 6 032 930 1 1 5 1 1 5(30.05) 1 1 2(32.50) 1 1 5 243 530 1 1 6 1 1 6(37.53) 1 1 6(82.50) 1 1 6 264 890 1 1 7 1 1 7(45.02) 1 1 1(20.00) 1 1 2 786 500 1 1 8 1 1 8(52.51) 1 1 5(70.00) 1 1 2 191 900 1 1 9 1 1 9(60.00) 1 1 9(120.00) 1 1 1 331 170 Results and Test of RSM 2 Tab.2 Tab.2 illustrates the conversion of experimental parameters between coded space and real space, where z and k represent the concentrations of ions in real space and coding space, respectively [ 29 29 - 30 30 29-30 ］ . Thirteen samples were designed based on Tab.2 Tab.2 . The concentrations of Bi 3+ and Eu 3+ are listed in columns 3 and 4 of Tab.3 Tab.3 , in which the values in brackets and not in brackets represent the concentration in natural space and coding space, respectively. After thirteen samples were prepared, their PL spectra were measured. Fig.2 Fig.2 presents that the luminous intensity of each sample is different. The integrated luminous intensities are calculated according to spectra, and the results are shown in the last column of Tab.3 Tab.3 . 1 1 z j ( k j ) 1 1 z 1 (Bi 3+ )/% 1 1 z 2 (Eu 3+ )/% 1 1 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019488&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019494&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019491&type= ( r ) 1 1 50.00 1 1 95.00 1 1 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019509&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019500&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019512&type= + Δ j (1) 1 1 44.14 1 1 86.22 1 1 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019502&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019523&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019534&type= (0) 1 1 30.00 1 1 65.00 1 1 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019526&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019530&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019528&type= -Δ j (-1) 1 1 15.86 1 1 43.78 1 1 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019546&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019553&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019550&type= ( -r ) 1 1 10.00 1 1 35.00 1 1 Δ j =( http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019568&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019559&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019557&type= - http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019562&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019594&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019576&type= )/(2 r ) 1 1 14.14 1 1 21.22 1 1 k j =( z j - http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019596&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019604&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019601&type= /Δ j 1 1 k 1 =( z 1 -30.00)/14.14 1 1 k 2 =( z 2 -65.00)/21.22 1 1 Number 1 1 k 0 1 1 k 1 ( z 1 ) 1 1 k 2 ( z 2 ) 1 1 k 1 k 2 1 1 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019625&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019613&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019611&type= 1 1 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019630&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019619&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019617&type= 1 1 Y 1 1 1 1 1 1 1 1 1(44.14) 1 1 1(86.22) 1 1 1 1 1 1 1 1 1 1 1 4 292 510 1 1 2 1 1 1 1 1 1(44.14) 1 1 -1(43.78) 1 1 -1 1 1 1 1 1 1 1 1 4 130 000 1 1 3 1 1 1 1 1 -1(15.86) 1 1 1(86.22) 1 1 -1 1 1 1 1 1 1 1 1 4 022 890 1 1 4 1 1 1 1 1 -1(15.86) 1 1 -1(43.78) 1 1 1 1 1 1 1 1 1 1 1 3 112 560 1 1 5 1 1 1 1 1 r (50.00) 1 1 0(65.00) 1 1 0 1 1 r 2 1 1 0 1 1 3 695 560 1 1 6 1 1 1 1 1 - r (10.00) 1 1 0(65.00) 1 1 0 1 1 r 2 1 1 0 1 1 2 469 100 1 1 7 1 1 1 1 1 0(30.00) 1 1 r (95.00) 1 1 0 1 1 0 1 1 r 2 1 1 6 662 870 1 1 8 1 1 1 1 1 0(30.00) 1 1 - r (35.00) 1 1 0 1 1 0 1 1 r 2 1 1 6 459 910 1 1 9 1 1 1 1 1 0(30.00) 1 1 0(65.00) 1 1 0 1 1 0 1 1 0 1 1 6 867 920 1 1 10 1 1 1 1 1 0(30.00) 1 1 0(65.00) 1 1 0 1 1 0 1 1 0 1 1 7 766 760 1 1 11 1 1 1 1 1 0(30.00) 1 1 0(65.00) 1 1 0 1 1 0 1 1 0 1 1 7 818 470 1 1 12 1 1 1 1 1 0(30.00) 1 1 0(65.00) 1 1 0 1 1 0 1 1 0 1 1 6 987 520 1 1 13 1 1 1 1 1 0(30.00) 1 1 0(65.00) 1 1 0 1 1 0 1 1 0 1 1 8 082 376 1 1 D j =Σ http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019622&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019642&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019636&type= 1 1 13 1 1 8 1 1 8 1 1 4 1 1 12 1 1 12 1 1 — 1 1 B j =Σ x ij y ij 1 1 72 369 820 1 1 3 021 274.44 1 1 1 359 825.44 1 1 -747 820 1 1 27 887 280 1 1 41 803 520 1 1 — 1 1 b j 1 1 7 504 884 1 1 377 659.305 1 1 169 978.18 1 1 -186 955 1 1 -2 440 884.9 1 1 -701 355 1 1 — 1 1 S j 1 1 2.82×10 14 1 1 1.14×10 12 1 1 2.31×10 11 1 1 1.40×10 11 1 1 4.14×10 13 1 1 3.42×10 12 1 1 — 1 1 t j 1 1 30.96 1 1 1.97 1 1 0.89 1 1 0.69 1 1 11.87 1 1 3.41 1 1 — 1 1 α j 1 1 0.001 1 1 0.2 1 1 0.5 1 1 0.6 1 1 0.001 1 1 0.02 1 1 — A mathematical model, quadratic polynomial, is established based on the Tab.3 Tab.3 . The coefficients of each factor of the quadratic polynomial are given in the row of“ b j ”, and the quadratic regression equation of the coding space can be obtained as follow [ 30 30 ] : http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019648&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019651&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019668&type= Tab.4 Tab.4 shows the variance analysis of the F-test and T-test for the Eq.(1). The sum of squares and degrees of freedom can be obtained according to Tab.4 Tab.4 [ 29 29 ] . From the sum of squares and degrees of freedom, we can further judge whether the Eq.(1) is misfit and credible. The calculation is given by the following formulas: http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019662&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019664&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019666&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019684&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019685&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019676&type= where F lf , F r are the loss of fitting test and the degree of confidence test, respectively. It can be seen from the test results that the Eq.(1) does not loss fitting, and the degree of confidence reaches 99%, which proves that the regression model is consistent with the actual situation [ 12 12 , 14 14 , 29 29 - 30 30 29-30 ］ . The doping concentrations of Bi 3+ and Eu 3+ ions corresponding to Y 4 GeO 8 ∶Bi 3+ ,Eu 3+ phosphors with the best red luminous intensity can be obtained by converting Eq.(1) to real space. Notice:*** extremely significant( α ≤0.1), ** significant( α ≤0.3), * relatively significant( α > 0.3) (“Extremely significant” means that the existence of the term has a great impact on the Eq.(1). “Significant” means that the existence of the term has some impact on the Eq.(1). “Relatively significant” means that the term has little impact on the Eq.(1) and can be ignored). 1 1 Source of variation 1 1 S j 1 1 f 1 1 t j 1 1 F -ratio 1 1 α j 1 1 Significance level 1 1 k 0 1 1 2.816×10 14 1 1 1 1 1 30.958 1 1 — 1 1 0.001 1 1 *** 1 1 k 1 1 1 1.141×10 12 1 1 1 1 1 1.971 1 1 — 1 1 0.2 1 1 ** 1 1 k 2 1 1 2.311×10 11 1 1 1 1 1 0.887 1 1 — 1 1 0.5 1 1 * 1 1 k 1 k 2 1 1 1.398×10 11 1 1 1 1 1 0.689 1 1 — 1 1 0.6 1 1 * 1 1 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019687&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019690&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019688&type= 1 1 4.143×10 13 1 1 1 1 1 11.874 1 1 — 1 1 0.001 1 1 *** 1 1 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019691&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019700&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019699&type= 1 1 3.421×10 12 1 1 1 1 1 3.412 1 1 — 1 1 0.02 1 1 *** 1 1 Residual( S R ) 1 1 2.774×10 12 1 1 7 1 1 — 1 1 — 1 1 — 1 1 — 1 1 Regression( S r ) 1 1 4.437×10 13 1 1 5 1 1 — 1 1 22.398 1 1 0.01 1 1 — 1 1 Loss of fitting( S lf ) 1 1 1.159×10 12 1 1 3 1 1 — 1 1 1.813 1 1 0.25 1 1 — 1 1 Error( S e ) 1 1 1.175×10 12 1 1 4 1 1 — 1 1 — 1 1 — 1 1 — 1 1 Sum( S ) 1 1 4.715×10 13 1 1 12 1 1 — 1 1 — 1 1 — 1 1 — By converting the coding space equation into natural space using the formula in Tab.2 Tab.2 , the binary quadratic regression equation between the doping concentration and luminescence intensity of Bi 3+ and Eu 3+ in natural space can be obtained. That is, replace the variable k in Eq.(1) with the variable z according to the formula in the last row of Tab.2 Tab.2 . The conversion result is as following: http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019716&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019706&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019720&type= According to the Eq.(4), the ions doping concentration corresponding to the sample with the strongest luminescence can be predicted by genetic algorithm.The concentrations of Bi 3+ and Eu 3+ of the optimal Y 4 GeO 8 ∶Bi 3+ ,Eu 3+ sample(labelled as Optimal-YGO) are 31.03% and 67.36%, respectively, and the predicted strongest luminescence intensity is 7 528 268. The 3D graph of the Eq.(4) shows the intensity of the samples more intuitively, as indicated in Fig.3 Fig.3 . We can infer the relationship between the luminous intensity of samples and the concentrations of Bi 3+ /Eu 3+ . Obviously, when the concentration of one ion is fixed, the luminous intensity will first increase and then decrease with the increase of the concentration of another ion. As shown in the figure, it reveals that the change of concentration of Bi 3+ has a greater impact on the luminous intensity than Eu 3+ . Characterization of Optimal-YGO Sample 2 The results of powder XRD patterns of the Optimal-YGO and the standard patterns of Y 4 GeO 8 (JCPDS No.21-1446) are presented in Fig.4 Fig.4 . The peaks of the sample are essentially consistent with those of the standard card, indicating that the incorporation of Bi 3+ and Eu 3+ has little affected the structure of the matrix, and red phosphor is successfully synthesized. The PL spectra of Optimal-YGO were measured in order to further investigate its properties. Fig.5(a) Fig.5(a) is the excitation spectra( λ em =627 nm) and the optimum excitation of the sample is 365 nm. Fig.5(b) Fig.5(b) is the corresponding emission spectra of the sample. It can be seen that the strongest emission of the sample under the excitation of 365 nm is 627 nm. A series of emission peaks of 565-750 nm come from 5 D 0 → 7 F J ( J =0-4) transitions of Eu 3+ , respectively [ 31 31 - 32 32 31-32 ］ . After calculation, the integral intensity of the emission spectrum is 7 520 120. Compared with the Eq.(4) simulation results, the error range is small. This proves that the RSM is highly reliable. After processing the measured emission spectra data, it is concluded that the CIE of the optimal sample is (0.645 7, 0.349 0). The internal quantum efficiency(IQE) of this sample is as high as 72.5%. Tab. 5 Tab. 5 lists some comparison results of as-prepared samples using the control variable method, and Fig.6 Fig.6 shows the corresponding data [ 18 18 ] . The IQE, external quantum efficiency(EQE) and absorption efficiency(Abs) can be given by the following equation [ 33 33 ] : http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019736&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019750&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019740&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019741&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019754&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019760&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019761&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019773&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019765&type= in which http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019767&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019779&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019769&type= is the integral intensity of emission light with the Optimal-YGO, http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019781&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019784&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019790&type= and http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019787&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019795&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019794&type= are the integral intensity of exciting light with and without Optimal-YGO, respectively. 1 1   1 1 CIE( x , y ) 1 1 IQE/% 1 1 EQE/% 1 1 Abs/% 1 1 Ref 1 1 Y 4 GeO 8 ∶25%Bi 3+ ,65%Eu 3+ 1 1 (0.643 7, 0.349 1) 1 1 61.3 1 1 37.4 1 1 61.0 1 1 [ 18 18 ] 1 1 Optimal-YGO 1 1 (0.645 7, 0.349 0) 1 1 72.5 1 1 40.3 1 1 55.6 1 1 This work 1 1 Standard red light 1 1 (0.670 0, 0.330 0) 1 1 — 1 1 — 1 1 — 1 1 [ 11 11 ] The color purity( P color ) of the sample can be studied by the following Eq.(8) to further investigate the quality of red emission of the sample [ 34 34 - 35 35 34-35 ］ : http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019833&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019835&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=28019836&type= where ( x , y ), ( x i , y i ) and ( x d , y d ) represent the chromaticity coordinates of the light source under test, equal-energy reference illuminant, and dominant-wavelength point, respectively. In this work, ( x , y )=(0.645 7, 0.349 0), ( x i , y i )=(0.333, 0.333) and ( x d , y d )=(0.650 7, 0.349 3). As a result, the color purity of YGO is about 98%, which is excellent compared with some reported phosphors, such as Sr 3 La(AlO) 3 (BO 3 ) 4 ∶Eu 3+ (92.4%) and Na 3 Sc 2 (PO 4 ) 3 ∶Eu 3+ (87%). The thermal stability of the optimal sample Optimal-YGO was characterized by temperature-dependent emission spectra. Fig.7(a) Fig.7(a) summarizes the change of the spectra of the sample with temperature. It can be seen that as the temperature rises, the sample's spectral intensity decreases. The normalization curve of the change of spectral intensity of the sample with the change of temperature is presented in Fig.7(b) Fig.7(b) . From the figure, the spectral intensity at 443 K is reduced to 50% of that at 293 K. Fig.8 Fig.8 displays the change of sample color coordinates with temperature. It can be seen that the color change of the sample is not obvious with the change of temperature. The insert shows the change of specific color coordinates of the sample. Conclusion 1 In this work, we successfully predicted the sample with the strongest luminescence through RSM more directly. The optimal concentrations of Bi 3+ and Eu 3+ were 31.03% and 67.36%, respectively. The optimal sample was successfully prepared. According to the measurement and calculation, the CIE of Optimal-YGO is (0.645 7, 0.349 0), the color purity is 98%, and the IQE is 72.5%. The above results not only show that Optimal-YGO has great application potential in the preparation of UV excited W-LED, but also show that RSM can predict the optimal doping contents of co-doped phosphors with the strongest luminescence. Response Letter is available for this paper at:http://cjl.lightpublishing.cn/thesisDetails#10.37188/CJL. 20220021. References WEI Y , XING G C , LIU K , et al . 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J. , 2021 , 412 : 128633-1-13 . Rare-earth-free Sr2YSb1-xO6∶xMn4+:synthesis, structure, luminescence behavior, thermal stability, and applications 10.37188/CJL.20220021 O482.31 A 1000-7032(2022)05-0633-09 Supported by National Natural Science Foundation of China (11974315) 国家自然科学基金（11974315）资助项目 2022-01-17 2022-02-07 2022-05-23 2022 发光学报 5 43