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Nanocrystalline rare earth hexaborides Nd1–xEuxB6 powders are successfully synthesized by the simple solid-state reaction in vacuum condition for the first time. The effect of Eu doping on the crystal structure, grain morphology, microstructure and optical absorption properties of nanocrystalline NdB6 are investigated by X-ray diffraction, scanning electron microscope (SEM), high resolution transmission electron microscopy (HRTEM) and optical absorption measurements. The results show that all the synthesized samples have a single-phase CsCl-type cubic structure with space group of Pm-3m. The SEM results show that the average grain size of the synthesized Nd1–xEuxB6 powders is 50 nm. The HRTEM results show that nanocrystalline Nd1–xEuxB6 has good crystallinity. The results of optical absorption show that the absorption valley of nanocrystalline Nd1–xEuxB6 is redshifted from 629 nm to higher than 1000 nm with the increase of Eu doping, indicating that the transparency of NdB6 is tunable. Additionally, the X-ray absorption near-edge structure spectra μ(E) around the Nd and Eu L3 edges for nanocrystalline NdB6 and EuB6 show that total valence of Nd ion is estimated at +3 in nanocrystalline NdB6 and total valence of Eu ion in nanocrystalline EuB6 is +2. Therefore, the Eu-doping into NdB6 effectively reduces the electron conduction number and it leads the plasma resonance frequency energy to decrease. In order to further qualitatively explain the influence of Eu doping on the optical absorption mechanism, the first principle calculations are used to calculate the band structure, density of states, dielectric function and plasma resonance frequency energy. The calculation results show that the electron band of NdB6 and EuB6 cross the Fermi energy, indicating that they are typical conductors. In addition, the plasmon resonance frequency can be described in the electron energy loss function. The plasmon resonance frequency energy of NdB6 and EuB6 are 1.98 and 1.04 eV, which are corresponding to the absorption valley of 626.26 and 1192.31 nm, respectively. This confirms that the first principle calculation results are in good consistence with the experimental optical absorption valley. Therefore, as an efficient optical absorption material, nanocrystalline Nd1–xEuxB6 powders can expand the optical application scope of rare earth hexaborides.
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Keywords:
- rare earth hexaboride /
- nanocrystalline material /
- optical absorption
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图 1 稀土六硼化物RB6 (R = Nd, Eu)的晶体结构, 其中左图为以硼(B)原子为中心的晶体结构, 右图为以稀土(R)原子为中心的晶体结构
Figure 1. Crystal structure of RB6 (R = Nd, Eu). Left panel shows the crystal structure centered on the boron atom. Right panel shows the crystal structure centered on rare earth atom.
图 2 反应温度为1150 ℃下制备的纳米Nd1–xEuxB6的XRD图谱
Figure 2. XRD patterns of the nanocrystalline Nd1–xEuxB6 prepared at 1150 ℃.
图 3 纳米晶Nd1–xEuxB6 (x = 0, 0.2, 0.4, 0.6, 0.8)的SEM照片
Figure 3. SEM images of nanocrystalline Nd1–xEuxB6 (x = 0, 0.2, 0.4, 0.6, 0.8).
图 4 (a) 纳米Nd0.4Eu0.6B6的TEM照片; (b) HRTEM照片和快速傅里叶变换照片; (c) 纳米Nd0.4Eu0.6B6的HAADF照片; (d)−(f) Nd0.4Eu0.6B6中的Nd, Eu和B元素分布
Figure 4. (a) TEM image of nanocrystalline Nd0.4Eu0.6B6; (b) HRTEM image and fast Fourier transform pattern; (c) HAADF image of Nd0.4Eu0.6B6; (d)−(f) elemental distribution of Nd, Eu and B for nanocrystalline Nd0.4Eu0.6B6.
图 5 纳米Nd1–xEuxB6粉末光吸收曲线
Figure 5. Optical absorption spectrum of nanocrystalline Nd1–xEuxB6 powder.
图 6 第一性原理计算的能带结构图 (a) NdB6; (b) EuB6
Figure 6. First-principle calculation results of band structure: (a) NdB6; (b) EuB6.
图 7 第一性原理计算的总态密度和部分态密度曲线 (a) NdB6; (b) EuB6
Figure 7. First-principle calculation results of total density of states (TDOS) and partial density of states (PDOS) curves: (a) NdB6; (b) EuB6.
图 8 介电函数的实部ε1和虚部ε2 (a) NdB6; (b) EuB6
Figure 8. Real part ε1 and imaginary part ε2 of the dielectric function: (a) NdB6; (b) EuB6.
图 9 能量损失函数曲线 (a) NdB6; (b) EuB6
Figure 9. Energy loss function curves: (a) NdB6; (b) EuB6.
图 10 纳米NdB6和EuB6同步辐射吸收图谱 (a) Nd-L3和Eu-L3边总X射线吸收光谱; (b) Nd-L3边局部放大; (c) Eu-L3边局部放大
Figure 10. Synchrotron radiation absorption spectrum of nanocrystalline NdB6 and EuB6: (a) total X-ray absorption spectra of Nd-L3 and Eu-L3; (b) partial enlargement of Nd-L3; (c) partial enlargement of Eu-L3.
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[1] Muz I, Kurban M 2020 J. Alloys Compd. 842 155983
Google Scholar
[2] Chen X B, Mao S S 2007 Chem. Rev. 107 2891
Google Scholar
[3] Wang X B, Ji F 2020 J. Nanosci. Nanotechnol. 20 7464
Google Scholar
[4] Xiao Q F, Zheng X P, Bu W B, Ge W Q, Zhang S J, Chen F, Xing H Y, Ren Q G, Fan W P, Zhao K L, Hua Y Q, Shi J L 2013 J. Am. Chem. Soc. 135 13041
Google Scholar
[5] Lv R C, Yang P P, He F, Gai S, Li C X, Dai Y L, Yang G X, Lin J 2015 ACS Nano 9 1630
[6] Yuan Y F, Zhang L, Hu L J, Wang W, Min G H 2011 J. Solid State Chem. 184 3364
Google Scholar
[7] Takeda H, Kuno H, Adachi K 2008 J. Am. Ceram. Soc. 91 2897
Google Scholar
[8] Schelm S, Smith G B 2003 Appl. Phys. Lett. 82 4346
Google Scholar
[9] Lai B H, Chen D H 2013 Acta Biomater. 9 7556
Google Scholar
[10] Chen M C, Lin Z W, Ling M H 2016 ACS Nano 10 93
Google Scholar
[11] Wang Y, Fang C, Li X, Li Z P, Liu B H 2019 J. Alloys Compd. 803 757
Google Scholar
[12] Xiao L H, Su Y C, Zhou X Z, Chen H Y, Tan J, Hu T, Yan J, Peng P 2012 Appl. Phys. Lett. 101 041913
Google Scholar
[13] 肖立华, 伏云昌, 苏玉长, 张鹏飞, 彭平 2011 原子与分子 28 0176
Xiao L H, Fu Y C, Su Y C, Zhang P F, Peng P 2011 J. At. Mol. Phys. 28 0176
[14] Xiao L H, Su Y C, Chen H Y, Jiang M, Liu S N, Hu Z X, Liu R F, Peng P, Mu Y L, Zhu D Y 2011 AIP Adv. 1 022140
Google Scholar
[15] Bao L H, Qi X P, Tana, Chao L M, Tegus O 2016 CrystEngComm 18 1223
Google Scholar
[16] Bao L H, Qi X P, Tana, Chao L M, Tegus O 2016 Phys. Chem. Chem. Phys. 18 19165
Google Scholar
[17] Bao L H, Chao L M, Li Y J, Ming M, Yibole B, Tegus O 2015 J. Alloys Compd. 651 19
Google Scholar
[18] Bao L H, Chao L M, Wei W, Tegus O 2015 Mater. Lett. 139 187
Google Scholar
[19] Bao L H, Wurentuya B, Wei W, Li Y J, Tegus O 2014 J. Alloys Compd. 617 235
Google Scholar
[20] Zhang X J, Tsai Y T, Wu S M, Lin Y C, Lee J F, Sheu H S, Cheng B M, Liu R S 2016 ACS Appl. Mater. Interfaces 8 19612
Google Scholar
[21] Kerisit S N, Prange M P 2020 Chem. Geol. 534 119460
Google Scholar
[22] Qi X P, Bao L H, Chao L M, Tegus O 2018 Physica B 530 312
Google Scholar
[23] Kimura S, Nanba T, Tomikawa M, Kunii S, Kasuya T 1992 Phys. Rev. B 46 12196
Google Scholar
[24] Choi Y G, Lee K A, Lee K S 2007 Met. Mater. Int. 13 269
Google Scholar
[25] Hernandez R E R, Marcos F R, Serrano A, Salas E, Hussainova I, Fernandez J F 2019 Nanomaterials 9 1473
Google Scholar
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