Synthesis and optical absorption properties of nanocrystalline rare earth hexaborides Nd1–xEuxB6 powders

Abstract

Nanocrystalline rare earth hexaborides Nd_1–<i>_x</i>Eu<i>_x</i>B₆ 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 <i>Pm-</i>3<i>m</i>. The SEM results show that the average grain size of the synthesized Nd_1–<i>_x</i>Eu<i>_x</i>B₆ powders is 50 nm. The HRTEM results show that nanocrystalline Nd_1–<i>_x</i>Eu<i>_x</i>B₆ has good crystallinity. The results of optical absorption show that the absorption valley of nanocrystalline Nd_1–<i>_x</i>Eu<i>_x</i>B₆ 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 <i>μ</i>(<i>E</i>) around the Nd and Eu <i>L</i>₃ 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 Nd_1–<i>_x</i>Eu<i>_x</i>B₆ powders can expand the optical application scope of rare earth hexaborides.