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Effect of polyhedral co-substitution on the crystal structure and luminescence color of Sr2(Al1–xMgx)(Al1–xSi1+x)O7: Eu2+

Wang Qing-Ling Dilare·Halimulati Shen Yu-Ling

Effect of polyhedral co-substitution on the crystal structure and luminescence color of Sr2(Al1–xMgx)(Al1–xSi1+x)O7: Eu2+

Wang Qing-Ling, Dilare·Halimulati, Shen Yu-Ling,
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  • A series of Sr1.98(Al1–xMgx)(Al1–xSi1+x)O7: 2%Eu2+ phosphors is prepared by the high-temperature solid-state reaction method, the crystal structures and luminescent properties of the prepared phosphors are investigated by measuring the X-ray diffraction, luminescent spectra and optical microscope. The isomorphic compounds of Sr2Al2SiO7 and Sr2MgSi2O7 contain tetrahedra including [MgO4], [SiO4] and [AlO4]. Although the valences of the [MgO4]6–, [SiO4]4– and [AlO4]5– groups are different, the charge imbalance occurs when the [MgO4]6– and [SiO4]4– substitutes of [AlO4]5– and [AlO4]5–, respectively. While the groups are co-substituted, the charge imbalance disappears. And the larger volume of [MgO4] and the smaller volume of [SiO4] together replaces the similar volume of [AlO4], resulting in the decrease of [(Si/Al)O4] and increase of [(Mg/Al)O4]. Moreover, the decrease of unit cell parameters c and the increase of a and V due to the increased replacement of Mg2+ (0.57 Å for CN = 4) by Al3+(0.39 Å for CN = 4) and Si4+ (0.26 Å for CN = 4) by Al3+ (0.39 Å for CN = 4) cause the ambient temperature to change, the crystal field splitting of the Eu2+ cation to be weakened, and the emission spectra to be blue-shifted from 503 nm to 467 nm, which are closely related to the local coordination environment of the Eu2+, in addition, this reveals that the emission color of this series of phosphors can be tuned from green with color coordinate (0.2384, 0.3919) to blue (0.1342, 0.1673) by adjusting the chemical compositions via the [MgO4]6– and [SiO4]4– groups’ co-substitution for [AlO4]5–. The full width at half maximumof emission band is 120 nm when x = 0, the photoluminescence emission width decreases monotonically from 89 to 50 nm as x is increased from 0.25 to 1. In other words, the full width at half maximum of emission band exhibits a decreasing trend. The internal quantum efficiency is enhanced with increasing x in Sr1.98(Al1–xMgx)(Al1–xSi1+x)O7: 2%Eu2+ phosphors. These results verify that the groups’ substitutions are enhanced with polyhedron changing in the solid solutions and contribute largely to the luminescence properties of the phosphor.
      Corresponding author: , aierkenjiang@sina.com
    [1]

    Chen M Y, Xia Z G, Molokeev M S, Wang T, Liu Q L 2017 Chem. Mater. 29 1430

    [2]

    Li S X, Wang L, Zhu Q Q, Tang D M, Liu X J, Cheng G F, Lu L, Taked T, Hirosaki N, Huang Z R, Xie R J 2016 J. Mater. Chem. C 4 11219

    [3]

    Li S X, Wang L, Tang M M, Cho Y J, Liu X J, Zhou X T, Lu L, Zhang L, Taked T, Hirosaki N, Xie R J 2018 Chem. Mater. 30 494

    [4]

    Xia Z G, Liu G K, Wen J G, Mei Z G, Balasubramanian M, Molokeev M S, Peng L C, Gu L, Miller D J, Liu Q L, Poeppelmeier K R 2016 J. Am. Chem. Soc. 138 1158

    [5]

    Xia Z G, Poeppelmeier K R 2017 Accounts Chem. Res. 50 1222

    [6]

    Ji H P, Huang Z H, Xia Z G, Molokeev M S, Atuchin V V, Fang M H, Liu Y G 2015 J. Phys. Chem. 119 2038

    [7]

    Xia Z G, Liu Q 2016 Prog. Mater. Sci. 84 59

    [8]

    Ye S, Xiao F, Pan Y X, Ma Y Y, Zhang Q Y 2011 Mat. Sci. Eng. R. 71 1

    [9]

    Shang M M, Liang S S, Qu N R, Lian H Z, Lin J 2017 Chem. Mater. 29 1813

    [10]

    Dubey S, Deshmukh P, Satapathy S, Singh M K, Gupta P K 2016 Luminesence 32 839

    [11]

    赵永旺, 苏全帅, 张超, 安胜利 2016 稀土 37 85

    Zhao Y W, Su Q S, Zhang C, An S L 2016 Chinese Rare Earths 37 85

    [12]

    赵永旺, 张超, 赵文广, 安胜利 2017 稀土 38 87

    Zhao Y W, Zhang C, Zhao G W, An S L 2017 Chinese Rare Earths 38 87

    [13]

    Shuang Y M, Zhu F L, Wang J D 2008 J. Func. Mater. 39 1078

    [14]

    Xia Z G, Ma C G, Molokeev M S, Liu Q L, Rickert K, Poeppelmeier K R 2015 J. Am. Chem. Soc. 137 12494

    [15]

    Lu F C, Bai L J, Dang W, Yang Z P, Lin P 2015 ECS J. Solid State Sc. 4 27

    [16]

    Tam T T H, Hung N Y, Lien N D K, Kien N D T, Huy P T 2016 Sci. Adv. Mater. 1 204

    [17]

    梁敬魁 2011 粉末衍射法测定晶体结构(上册)(北京:科学出版社) 第78页

    Liang J K 2003 Determination of Crystal Structure by Powder Diffraction (Vol. 1) (Beijing: Science Press) p78 (in Chinese)

    [18]

    Denault K A, George N C, Paden S R, Brinkley S, Mikhailovsky A A, Neuefeind J, DenBaars S P, Seshadri R J 2012 Mater. Chem. 22 18204

    [19]

    Denault K A, Brgoch J, Gaultois M W, Mikhailovsky A, Petry R, Winkler H, DenBaars S P, Seshadri R 2014 Chem. Mater. 26 2275

    [20]

    Guo Y, Park S H, Choi B C, Jeong J H, Kim J H 2018 J. Alloy. Compd. 742 159

  • 图 1  (a) 样品C的XRD精修图谱;(b) Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1)荧光粉的XRD图谱和2θ范围为15°—23°的峰位移动放大图;(c)晶胞参数随浓度x变化曲线图

    Figure 1.  (a) Rietveld refinement of C; (b) XRD patterns of the Sr1.98(Al1–xMgx)(Al1–xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1), the right inset is the magnified XRD patterns for 2θ region form 15° to 23°; (c) give the cell parameters (a/b, c) and volume (V), respectively, as a function of x concentration

    图 2  Sr1.98(Al1-xMgx)(Al1-xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1)荧光粉的归一化激发光谱

    Figure 2.  Normalized excitation spectra of Sr1.98(Al1xMgx) (Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1) phosphors

    图 3  (a), (b)在波长365 nm激发下样品A, C, E的光学显微镜图像和Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1)荧光粉的归一化发射光谱

    Figure 3.  (a) Optical microscope image of A, C, E excited at a wavelength of 365 nm and (b) normalized emission spectra of Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1) phosphors under 365 nm UV light excitation

    图 4  Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1)在365 nm激发波长下的色坐标(插图为实物照片)

    Figure 4.  Color coordinates of Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1) under λex = 365 nm in the CIE chromaticity diagram (the insets show the corresponding digital photos)

    图 5  在荧光粉Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1)中, 随着x的增加, 多面体[(Si/Al)O4]的收缩和多面体[(Mg/Al)O4]的膨胀对发光中心多面体的扭曲

    Figure 5.  Increasing of x leads to polyhedral [(Si/Al)O4] shrinkage and polyhedral [(Mg/Al)O4] expansion distortion the luminescent center polyhedron of Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1) phosphors

    图 6  Eu2+的5d能级变化示意图, VB, CB和CFS分别代表价带, 导带和晶体场劈裂

    Figure 6.  Schematics of the changes in 5d energy levels of the activator. CFS, CB, and VB are crystal field splitting, the conduction band, and the valence band, respectively

    表 1  Sr1.98(Al1–xMgx)(Al1xSi1+x)O7: 2%Eu2+(0≤x ≤ 1)荧光粉发射波长的半高宽, 色坐标值和内量子效率

    Table 1.  The PL bands, color coordinate value, and internal quantum efficiency of Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1) phosphors

    样品 发射光谱 色坐标 IQY
    λem/nm FWHM/nm x y
    A 503 120 0.2384 0.3919 4.3%
    B 472 89 0.1972 0.2951 6.1%
    C 470 61 0.1675 0.2312 7.3%
    D 468 54 0.1463 0.1873 9.0%
    E 468 50 0.1342 0.1673 23%
    DownLoad: CSV
  • [1]

    Chen M Y, Xia Z G, Molokeev M S, Wang T, Liu Q L 2017 Chem. Mater. 29 1430

    [2]

    Li S X, Wang L, Zhu Q Q, Tang D M, Liu X J, Cheng G F, Lu L, Taked T, Hirosaki N, Huang Z R, Xie R J 2016 J. Mater. Chem. C 4 11219

    [3]

    Li S X, Wang L, Tang M M, Cho Y J, Liu X J, Zhou X T, Lu L, Zhang L, Taked T, Hirosaki N, Xie R J 2018 Chem. Mater. 30 494

    [4]

    Xia Z G, Liu G K, Wen J G, Mei Z G, Balasubramanian M, Molokeev M S, Peng L C, Gu L, Miller D J, Liu Q L, Poeppelmeier K R 2016 J. Am. Chem. Soc. 138 1158

    [5]

    Xia Z G, Poeppelmeier K R 2017 Accounts Chem. Res. 50 1222

    [6]

    Ji H P, Huang Z H, Xia Z G, Molokeev M S, Atuchin V V, Fang M H, Liu Y G 2015 J. Phys. Chem. 119 2038

    [7]

    Xia Z G, Liu Q 2016 Prog. Mater. Sci. 84 59

    [8]

    Ye S, Xiao F, Pan Y X, Ma Y Y, Zhang Q Y 2011 Mat. Sci. Eng. R. 71 1

    [9]

    Shang M M, Liang S S, Qu N R, Lian H Z, Lin J 2017 Chem. Mater. 29 1813

    [10]

    Dubey S, Deshmukh P, Satapathy S, Singh M K, Gupta P K 2016 Luminesence 32 839

    [11]

    赵永旺, 苏全帅, 张超, 安胜利 2016 稀土 37 85

    Zhao Y W, Su Q S, Zhang C, An S L 2016 Chinese Rare Earths 37 85

    [12]

    赵永旺, 张超, 赵文广, 安胜利 2017 稀土 38 87

    Zhao Y W, Zhang C, Zhao G W, An S L 2017 Chinese Rare Earths 38 87

    [13]

    Shuang Y M, Zhu F L, Wang J D 2008 J. Func. Mater. 39 1078

    [14]

    Xia Z G, Ma C G, Molokeev M S, Liu Q L, Rickert K, Poeppelmeier K R 2015 J. Am. Chem. Soc. 137 12494

    [15]

    Lu F C, Bai L J, Dang W, Yang Z P, Lin P 2015 ECS J. Solid State Sc. 4 27

    [16]

    Tam T T H, Hung N Y, Lien N D K, Kien N D T, Huy P T 2016 Sci. Adv. Mater. 1 204

    [17]

    梁敬魁 2011 粉末衍射法测定晶体结构(上册)(北京:科学出版社) 第78页

    Liang J K 2003 Determination of Crystal Structure by Powder Diffraction (Vol. 1) (Beijing: Science Press) p78 (in Chinese)

    [18]

    Denault K A, George N C, Paden S R, Brinkley S, Mikhailovsky A A, Neuefeind J, DenBaars S P, Seshadri R J 2012 Mater. Chem. 22 18204

    [19]

    Denault K A, Brgoch J, Gaultois M W, Mikhailovsky A, Petry R, Winkler H, DenBaars S P, Seshadri R 2014 Chem. Mater. 26 2275

    [20]

    Guo Y, Park S H, Choi B C, Jeong J H, Kim J H 2018 J. Alloy. Compd. 742 159

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  • Received Date:  26 December 2018
  • Accepted Date:  06 March 2019
  • Available Online:  01 May 2019
  • Published Online:  20 May 2019

Effect of polyhedral co-substitution on the crystal structure and luminescence color of Sr2(Al1–xMgx)(Al1–xSi1+x)O7: Eu2+

    Corresponding author: aierkenjiang@sina.com
  • Key Laboratory of Mineral Luminescent Material and Microstructure of Xinjiang, Xinjiang Normal University, Urumqi 830054, China

Abstract: A series of Sr1.98(Al1–xMgx)(Al1–xSi1+x)O7: 2%Eu2+ phosphors is prepared by the high-temperature solid-state reaction method, the crystal structures and luminescent properties of the prepared phosphors are investigated by measuring the X-ray diffraction, luminescent spectra and optical microscope. The isomorphic compounds of Sr2Al2SiO7 and Sr2MgSi2O7 contain tetrahedra including [MgO4], [SiO4] and [AlO4]. Although the valences of the [MgO4]6–, [SiO4]4– and [AlO4]5– groups are different, the charge imbalance occurs when the [MgO4]6– and [SiO4]4– substitutes of [AlO4]5– and [AlO4]5–, respectively. While the groups are co-substituted, the charge imbalance disappears. And the larger volume of [MgO4] and the smaller volume of [SiO4] together replaces the similar volume of [AlO4], resulting in the decrease of [(Si/Al)O4] and increase of [(Mg/Al)O4]. Moreover, the decrease of unit cell parameters c and the increase of a and V due to the increased replacement of Mg2+ (0.57 Å for CN = 4) by Al3+(0.39 Å for CN = 4) and Si4+ (0.26 Å for CN = 4) by Al3+ (0.39 Å for CN = 4) cause the ambient temperature to change, the crystal field splitting of the Eu2+ cation to be weakened, and the emission spectra to be blue-shifted from 503 nm to 467 nm, which are closely related to the local coordination environment of the Eu2+, in addition, this reveals that the emission color of this series of phosphors can be tuned from green with color coordinate (0.2384, 0.3919) to blue (0.1342, 0.1673) by adjusting the chemical compositions via the [MgO4]6– and [SiO4]4– groups’ co-substitution for [AlO4]5–. The full width at half maximumof emission band is 120 nm when x = 0, the photoluminescence emission width decreases monotonically from 89 to 50 nm as x is increased from 0.25 to 1. In other words, the full width at half maximum of emission band exhibits a decreasing trend. The internal quantum efficiency is enhanced with increasing x in Sr1.98(Al1–xMgx)(Al1–xSi1+x)O7: 2%Eu2+ phosphors. These results verify that the groups’ substitutions are enhanced with polyhedron changing in the solid solutions and contribute largely to the luminescence properties of the phosphor.

    • 稀土掺杂无机材料形成的荧光粉具有良好的化学和光学性能, 在发光二极管(LEDs)、液晶显示器(LCDs)、激光器等方面有广泛应用[1-4]. 为了拥有高质量的荧光粉, 世界各地正在大力优化现有材料, 固溶是优化材料的方法之一, 包括单离子替代、阴/阳离子替代或者共替代. 共替代是同时替换两种或多种阳离子、阴离子、复杂阴离子、其他基本单元或空位[5]. 晶体结构的同构为人工制备固溶体材料提供了可行性, 激活剂周围局部晶体场强度的改变, 已被用于发射光谱的调节, 优化荧光粉的发光性能, 并改善宿主的刚性[6].

      在众多矿物中, 黄长石化合物的一般化学式为A2BC2X7. A(Na+, Sr2+, La3+等)可以是一价、二价或者三价, 可能会有1种或者2种不同的阳离子格位; B(Mg2+, Zn2+, Al3+等)和C(Al3+, Si4+, Ge4+等)为三价或者四价的阳离子; X(O2–, F, S2–等)为阴离子, 可供多种阴阳离子替换, 是当前一种有潜力的发光材料[7]. 离子或者多面体的替代, 需要选择一个对周围环境敏感的激活剂. Eu2+离子5d电子的外层呈裸露状态, 对晶体对称性、原子配位、共价、键长、晶体场强度改变非常敏感, 因此利用Eu2+电子结构特点, 可通过固溶体主体的化学成分的变化达到颜色可调[6,8]. 硅酸盐荧光粉的化学成分和晶体结构的修饰, 可以很容易得到颜色可调的硅酸盐荧光粉[9]. 例如Dubey等[10]研究了Sr2SiO4: Eu2+黄绿色荧光粉, 当Mg2+取代Sr2+离子, Sr2–xMgxSiO4: Eu2+荧光粉的发光颜色随着x的增加变为蓝色. 文献[11-13]研究了Sr2–xyCaxMgyAl2SiO7: Eu2+, Sr2–xCaxAl2SiO7: Eu2+和Sr2–xBaxAl2SiO7: Eu2+的发光性质. 这些都属于调节了等价阳离子, 使发光颜色改变. 然而, 第二配位球的阳离子不等价替换也开始被广泛关注.

      基于结构单元的定义和替换, 结构单元替换前后的化合价之和相等, 但每个替代组分的化合价不一定相等[14]. Lu等[15]和Tam等[16]分别报道了Sr2Al2SiO7: Eu2+和Sr2MgSi2O7: Eu2+荧光粉的制备方法和发光性质, 发射峰分别位于500 nm和462 nm. 本文采用高温固相法, 以Sr2Al2SiO7为始端, 以Sr2MgSi2O7为终端, 得到Sr2(Al1xMgx)(Al1xSi1+x)O7(0 ≤ x ≤ 1)完全固溶体. 以Eu2+为激活剂形成的Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+固溶荧光粉, 固溶体中存在[MgO4]、[SiO4]和[AlO4]三种四面体, 虽然[MgO4]6–、[SiO4]4–和[AlO4]5–基团的化合价不同, 当[MgO4]6–替代[AlO4]5–,[SiO4]4–替代[AlO4]5–时会出现电荷失衡, 但基团同时替代会消除电荷失衡现象. 改变Mg2+与Si4+共替代Al3+的浓度, 实现一系列颜色从绿色到蓝色可调谐荧光粉.

    2.   实 验
    • 试剂: SrCO3(A.R.), SiO2(99.99%), Al2O3(A.R.), MgO(A.R.), Eu2O3(99.9%)药品均为上海阿拉丁生化科技股份有限公司生产.

      使用高温固相法, 两步合成Sr1.98(Al1xMgx)(Al1–xSi1+x)O7: 2%Eu2+系列荧光粉. 首先, 按照化学计量比称取药品, x = 0时SrCO3(0.8769 g), SiO2(0.1802 g), Al2O3(0.3058 g)和Eu2O3(0.0105 g), 其他样品按照相应比例进行计算, 并把x = 0, 0.25, 0.5, 0.75和1时的样分别品标记为A, B, C, D和E. 然后将称量好的药品放入玛瑙研钵中, 研磨30 min, 混合均匀后置于刚玉坩埚. 在箱式电阻炉空气氛围中进行第一次煅烧, 以5 ℃/min升温速率达到800 ℃保温6 h, 自然降至室温; 在玛瑙研钵中研磨10 min, 进行二次煅烧, 以3 ℃/min的升温速率在管式炉90%N2—10%H2气氛中1350 ℃保温3 h, 待冷却至室温后再次研磨得到待测样品, 装袋备用.

    • 采用日本岛津XRD-6100型粉末衍射仪进行物相鉴定. 测试条件如下: X光源为Cu-Kα射线(波长λ = 0.15406 nm), 工作电压为40 kV, 工作电流为30 mA, 扫描步速为5°/min, 采用连续扫描模式, 2θ范围为10°—70°, 其中粉晶结构精修的X射线衍射(XRD)数据采用步进扫描模式, 步速为0.02°/min, 每步停留5 s, 2θ范围10°—80°, 用GSAS软件对样品晶体结构进行精细修正, Diamond软件绘制晶体结构图. 用日本TS2-LS光学显微镜观察发光颜色, 用英国爱丁堡FLS920型稳态/瞬态荧光光谱仪测量样品的激发、发射光谱, 其测量范围为260—650 nm, 在测量过程中用450 W 氙灯(UshioUXL-500D)作为激发光源, 根据实验需求选取适当滤光片放置在光栅入口处以消除激发光源的杂散光. 色坐标用CIE1931软件进行计算, 内量子效率是使用日本滨松C11347量子效率仪测试.

    3.   结 果
    • 图1(a)为Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(x = 0.5)的XRD精修图, 精修的标准数据卡片为Sr2Al2SiO7(PDF#38-1333). 图中“×”表示测量衍射数据, 红色线为计算出的衍射数据, 绿色的垂直线代表模拟衍射峰位置, 蓝色线表示测量值与计算值之间的偏差. 样品的剩余因子值为RP = 12%, RWP = 15%, V = 323.55(7) Å3, a = b = 7.8303(10) Å, c = 5.2769(7) Å. 图1(b)是Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1)荧光粉的XRD衍射图, 所有样品均维持四方晶系结构(空间群P-421m). 在放大局部2θ角度15°—23°, 可以看出随着x的增加, (110)和(111)晶面向低角度偏移, 而(001)晶面向高角度偏移.

      Figure 1.  (a) Rietveld refinement of C; (b) XRD patterns of the Sr1.98(Al1–xMgx)(Al1–xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1), the right inset is the magnified XRD patterns for 2θ region form 15° to 23°; (c) give the cell parameters (a/b, c) and volume (V), respectively, as a function of x concentration

      随着阳离子替换浓度的增加, XRD从Sr2Al2SiO7(PDF#38-1333)完全变为Sr2MgSi2O7(PDF#75-1736), 说明获得了一个完全固溶体. 这可以用取代离子与被取代离子的半径差来解释, 大半径的Mg2+(r = 0.57 Å, CN = 4)和小半径的Si4+(r=0.26 Å, CN = 4)共同替代Al3+(r = 0.39 Å, CN = 4)改变了晶胞参数, 用四方晶系面间距dhkl与晶面组(hkl)的关系公式[17]

      计算晶胞参数, 其变化曲线如图1(c), 随x的增加, aV在增大, c在减小, 表明晶胞在不同方向膨胀, 多面体[MgO4]和[SiO4]替代[AlO4]会引起晶格失真.

    • 图2为Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1)荧光粉的归一化激发光谱. 在503 nm波长监测下, 荧光粉的激发光谱在260—420 nm范围有两个宽带组成, 峰值分别位于287 nm和365 nm, 对应Eu2+的4f7—4f65d跃迁. [MgO4]和[SiO4]共同替代[AlO4]时没有影响激发光谱的形状.

      Figure 2.  Normalized excitation spectra of Sr1.98(Al1xMgx) (Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1) phosphors

      图3(a)是在365 nm波长激发下, 使用光学显微镜拍摄A, C, E样品的荧光图像, 荧光粉颗粒的发光颜色从绿色转变为蓝色. 图3(b)是Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1)归一化发射光谱图. 随着[SiO4]、[MgO4]替代[AlO4]多面体浓度的增大, 发射峰从503 nm蓝移至467 nm, 这主要归因于第二配位球的配位多面体替换导致发光中心晶体场发生了变化. x = 0时的发射光谱在400—650 nm范围内有一个很宽的发射带, 半高宽为120 nm, 多面体替换的进一步增加, 发射带的半高宽明显变小, x = 0.25, 0.5, 0.75和1时样品的发射峰在470 nm左右, 发射峰的位置几乎没有变化, 半高宽由89 nm逐渐减小至50 nm.

      Figure 3.  (a) Optical microscope image of A, C, E excited at a wavelength of 365 nm and (b) normalized emission spectra of Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1) phosphors under 365 nm UV light excitation

    • 图4是用CIE1931软件计算Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1)荧光粉在365 nm波长激发下发射光谱的色坐标. x = 0, 0.25, 0.5, 0.75, 1的色坐标分别为(0.24, 0.39), (0.20, 0.30), (0.17, 0.23), (0.14.0.19), (0.13, 0.16). 图4插图是在近紫外灯365 nm照射下的实物照片, 可以看出颜色从绿色转变为蓝色. Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1)荧光粉的发射波长、色坐标和365 nm波长下的内量子效率对比结果如表1所示.

      Figure 4.  Color coordinates of Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1) under λex = 365 nm in the CIE chromaticity diagram (the insets show the corresponding digital photos)

      样品 发射光谱 色坐标 IQY
      λem/nm FWHM/nm x y
      A 503 120 0.2384 0.3919 4.3%
      B 472 89 0.1972 0.2951 6.1%
      C 470 61 0.1675 0.2312 7.3%
      D 468 54 0.1463 0.1873 9.0%
      E 468 50 0.1342 0.1673 23%

      Table 1.  The PL bands, color coordinate value, and internal quantum efficiency of Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1) phosphors

    4.   讨 论
    • 图5表示在Sr1.98(Al1-xMgx)(Al1-xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1)荧光体系中, Mg2+和Si4+取代Al3+时发光中心局部环境的变化. 以激活剂Eu2+为中心, 与其最邻近的原子称为第一配位球, 次近邻的原子为第二配位球. 第一配位球是8个O原子直接与Eu2+相连, 构成[EuO8]多面体. 第二配位球为Mg2+, Al3+和Si4+, 并与邻近的O原子形成配位多面体[MgO4]、[AlO4]、[SiO4], 多面体通过共用一个氧原子相互连接. 随着四面体替换浓度的增加, 大半径的Mg2+(r = 0.57 Å, CN = 4)取代小半径的Al3+(r = 0.39 Å, CN = 4)会使四面体膨胀, 小半径的Si4+(r = 0.26 Å, CN = 4)会取代大半径的Al3+(r = 0.39 Å, CN = 4)使四面体收缩. 第二配位球所形成的配位多面体的替代引起Eu周围环境的变化, 导致发射光谱出现蓝移现象.

      Figure 5.  Increasing of x leads to polyhedral [(Si/Al)O4] shrinkage and polyhedral [(Mg/Al)O4] expansion distortion the luminescent center polyhedron of Sr1.98(Al1xMgx)(Al1xSi1+x)O7: 2%Eu2+(0 ≤ x ≤ 1) phosphors

      以往的研究表明, 多面体畸变程度增加可能也会增加晶体场的劈裂[18]如Kristin等[19]报道SrxBa2−xSiO4: Eu2+随着Sr浓度的增加, 畸变增加导致晶体场劈裂产生红移. Xia等[14]研究Ca2(Al1–xMgx)(Al1–xSi1+x)O7Eu2+的光致发光, 随着x的增加, [(Ca/Eu)O8]扭曲度增大, 改变了5d能级劈裂程度, 使发射光谱红移. 而本文发现光致发光却向能量高的方向移动. 光致发光中的这种能量偏移可能归因于由晶格膨胀引起的5d轨道的晶体场分裂的减少[19]. Eu2+在Sr1.98(Al1xMgx)(Al1xSi1+x)O7中为8个氧原子的配位环境. 基于晶体场劈裂理论, 根据等(2)式[20]

      这里Dq代表5d能级劈裂程度, R是中心离子与配体之间键长, Z是阳离子的化合价, e代表电子电荷, r是d波函数的半径. [SiO4]、[MgO4]替代[AlO4]多面体浓度的增大, 计算[EuO8]多面体中Eu2+的劈裂程度, e, r, Z都是相等的, Dq正比于1/R5. 在[EuO8]多面体中, x = 0(R = 2.669 Å)时的Eu—O键长小于x = 1(R = 2.674 Å)时的键长, 5d能级劈裂减小, 光谱从503 nm蓝移至467 nm, 如图6.

      Figure 6.  Schematics of the changes in 5d energy levels of the activator. CFS, CB, and VB are crystal field splitting, the conduction band, and the valence band, respectively

    5.   结 论
    • 本文采用高温固相法, 以Sr2Al2SiO7为始端、Sr2MgSi2O7为终端、Eu2+为激活剂合成绿色到蓝色颜色可调的Sr2(Al1xMgx)(Al1xSi1+x)O7(0 ≤ x ≤ 1): 2%Eu2+荧光粉. Mg2+, Si4+, Al3+作为发光中心的第二配位球, 形成[MgO4]、[SiO4]和[AlO4]四面体, 当[MgO4]和[SiO4]共同替代[AlO4]时, 呈现晶胞参数aV增大, c减少的趋势, 使晶体结构发生了畸变, 导致Eu2+的周围晶体场环境改变, 从而光颜色从绿色到蓝色有一个可控的调节, 配位多面体的共替代对研究颜色可调的荧光粉有很大帮助.

Reference (20)

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