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

Nanocrystalline rare earth hexaborides Nd1-xEuxB6 powders are successfully synthesized by the simple solidstate 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 p(E) around the Nd and Eu L-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 Nd1-xEuxB6 powders can expand the optical application scope of rare earth hexaborides.