Rare-earth Hexaborides Research Articles

Low-temperature fabrication of high-entropy rare earth hexaboride powders

Herein, we report a low-temperature method for synthesizing high-entropy rare earth hexaboride powders by reaction among rare earth oxides, B4C, and Al powders. Al was used as a reducing and decarburization agent in this process, which further reduced the Gibbs free energy change for the reaction of RexOy with B4C to form REB6. By investigating the influence of reacting temperature and amount of Al added on preparation of LaB6, the optimum experimental conditions of this method were determined to be 1300 °C and 2 times the stoichiometric ratio of Al. Theoretical supports were proposed to verify the feasibility for formation of high-entropy borides. Then, multicomponent hexaborides La0.5Ce0.5B6, La0.33Ce0.33Nd0.33B6, and La0.25Ce0.25Nd0.25Sm0.25B6 as well as high-entropy hexaboride La0.2Ce0.2Nd0.2Sm0.2Eu0.2B6 powders were successfully prepared by doping with Ce, Nd, Sm, and Eu on LaB6. RE elements with different radius were uniformly distributed in the multicomponent boride powders without obvious segregation. More importantly, the lattice distortion and stress induced by the element substitution contribute to the reduction of the average powder particle size. Finally, the DTA-TG curves show that rare earth hexaborides have good oxidation resistances when the temperature is below about 1000 °C.

Valence instability and collapse of ferromagnetism in EuB[formula omitted] at high pressures

Despite the simplicity of their cubic crystal lattice, rare-earth hexaborides display complex physical properties including a (long debated) onset of metallization via magnetic polaron formation at Tc1≈ 15 K preceding ferromagnetic ordering at Tc2≈ 12 K. In this work, we used applied pressure to tune the interplay between electronic structure and magnetism in EuB6. We probed the magnetism, valence, and structure of EuB6 under quasi-hydrostatic pressures up to 30 GPa using X-ray techniques. Our findings show evidence for collapse of ferromagnetism above 20 GPa following a monotonic increase of mean Eu valence. While X-ray diffraction measurements in the paramagnetic state at room temperature show that the lattice retains cubic symmetry, a measurable quadrupole interaction seen by time-domain synchrotron Mössbauer spectroscopy suggests a lowering of symmetry associated with magnetic ordering, becoming more prominent across the magnetic transition. The interplay between conduction band electron count and magnetism observed under applied pressure in EuB6 opens possibilities for fine-tuning metallization and magnetic properties of similar Eu-based semi-metal systems.

Screw dislocation growth defects enhancing electromagnetic wave absorption performance of GdB6

In this study, a purity of up to 98.36 wt. % GdB6 with screw dislocation growth morphology were successfully fabricated using an in-situ reaction approach. The terraced-like structure of the GdB6 enhanced the interfacial polarization and multiple interface scattering of the electromagnetic waves. As a result, the as-obtained high purity GdB6 powder presented superior electromagnetic wave absorption performance. The minimum reflection loss value can be −49.24 dB at the frequency of 13.12 GHz with a thin matching thickness of 1.44 mm. In addition, the widest effective absorption bandwidth can be up to ∼4 GHz (14–18 GHz) with a thin matching thickness of 1.25 mm. The remarkable electromagnetic wave absorption performance of GdB6 ceramics could be attributed to conductance loss and polarization loss enhanced by the screw dislocation growth defects, as well as synergistic effects with magnetic loss. The as-obtained GdB6 with synergistic dielectric loss and magnetic loss is to be a novel single-phase with excellent electromagnetic wave absorbing material, which will pave the way on the investigation of rare earth hexaborides for the ultra-high temperature environment applications.

Evolution of surface conductivity in SmB6 under nonmagnetic (Yb2+) and magnetic (Eu2+) doping

Among of rare-earth (RE) hexaborides only two compounds SmB6 and YbB6 are discussed in literature to be members of a new class of 3D topological insulators. However, their ground states originate due to different physical mechanisms, including Kondo 4f-5d hybridization and 5d-2p band inversion, respectively. Here we report a comparative study of magnetotransport (resistivity and transverse magnetoresistance) measured on high quality single crystals of YbxSm1-xB6 and EuxSm1-xB6 solid solutions (x ≤ 0.05) at temperatures 1.7 − 300 K in magnetic fields up to 82 kOe. The choice of dopant was determined by the fact that the presence of magnetic/nonmagnetic (Eu2+/Yb2+) impurity in parent SmB6 matrix should lift/not lift the topological protection of surface states. Based on the two-gap paradigm the x-evolution of electron spectra in YbxSm1-xB6 and EuxSm1-xB6 was studied. Our data show that both the 4f lattice coherence (Eg) and the intrinsic gap (Ea) related to many-body states survive under RE doping at least for x ≈ 0.02 − 0.024. We also suggest that the point x(Eu) = 0.05 can be treated as an upper limit of the small gap closing in EuxSm1-xB6 materials. In YbxSm1-xB6 family a negative linear transverse magnetoresistance (TMR) was detected for the first time in the regime of surface conductivity (T < T∗ ≈ 5 K). The TMR anomaly at T∗ caused possibly by the topological protection of surface states in SmB6 is found to survive in Eu-doped compounds but disappears almost completely for Yb-doped compositions in the same fixed magnetic fields. This paradoxical observation is not consistent with general predictions of the topological Kondo insulator (TKI) model.

Medium- and High-Entropy Rare Earth Hexaborides with Enhanced Solar Energy Absorption and Infrared Emissivity.

The development of a new generation of solid particle solar receivers (SPSRs) with high solar absorptivity (0.28-2.5 μm) and high infrared emissivity (1-22 μm) is crucial and has attracted much attention for the attainment of the goals of "peak carbon" and "carbon neutrality". To achieve the modulation of infrared emission and solar absorptivity, two types of medium- and high-entropy rare-earth hexaboride (ME/HEREB6) ceramics, (La0.25Sm0.25Ce0.25Eu0.25)B6 (MEREB6) and (La0.2Sm0.2Ce0.2Eu0.2Ba0.2)B6 (HEREB6), with severe lattice distortions were synthesized using a high-temperature solid-phase method. Compared to single-phase lanthanum hexaboride (LaB6), HEREB6 ceramics show an increase in solar absorptivity from 54.06% to 87.75% in the range of 0.28-2.5 μm and an increase in infrared emissivity from 76.19% to 89.96% in the 1-22 μm wavelength range. On the one hand, decreasing the free electron concentration and the plasma frequency reduces the reflection and ultimately increases the solar absorptivity. On the other hand, the lattice distortion induces changes in the B-B bond length, leading to significant changes in the Raman scattering spectrum, which affects the damping constant and ultimately increases the infrared emissivity. In conclusion, the multicomponent design can effectively improve the solar energy absorption and heat transfer capacity of ME/HEREB6, thus providing a new avenue for the development of solid particles.

Thermal plasma processing of technologically important materials

Thermal plasma is one of the upcoming powerful tools used for materials processing. It covers a wide range of technological applications such as synthesis of various refractory ceramic materials, metals and alloys, deposition of coatings, high temperature processing of materials as well as disintegration of waste materials. Representative technologically important material systems viz rare earth hexaboride (e.g. GdB6) and carbonaceous materials are focus of the present manuscript. Both the material systems have been processed using DC thermal plasma route and characterized thoroughly for structural, morphological, surface properties using XRD, TEM, XPS respectively. Morphology of GdB6 has been tailored by varying plasma parameters during synthesis. Further, these GdB6 powder were investigated for electron emission performance using Field Electron Emission and maximum current density of 0.5 mA/cm2 was noted for the nanocrystalline GdB6 sample. Feasibility of thermal plasmas for production of nanocrystalline GdB6 and processing of a bio-waste to obtain technologically important carbonaceous materials has also been explored.

Fabrication of polycrystalline LaB6 tips array by femtosecond laser and its field emission performance

Femtosecond laser processing technology has been used to achieve controlled processing of microtips array on the surface of polycrystalline LaB6 blocks. Under the condition of ensuring the integrity of the geometric morphology of the tips array, the tips with a height of 10 μm and a radius of curvature of 1 μm can be processed. The distribution of tips in the array is uniform, the morphology is complete, the crystal structure and physical phase of LaB6 have not changed after processing, which breaks through the technical problems of difficult processing and poor geometry consistency about polycrystalline rare-earth hexaboride tips emitter, and the processing area is no longer limited by the small size of single crystals. The field emission performance test indicated that the turn-on electric field is 2.8 V/μm, and the current density is 467.9 mA/cm2 when the applied electric field reached 9.4 V/μm.

Electrochemical Synthesis of Rare-Earth Hexaborides in Chloride–Oxide Melts

Electrochemical Synthesis of Rare-Earth Hexaborides in Chloride–Oxide Melts

Metastable phase formation in europium hexaboride on compression to 187 GPa

Transition-metal and rare-earth borides are of considerable interest due to their electronic, mechanical, and magnetic properties as well as their structural stability under extreme conditions. Here, we report on a series of high-pressure Raman and x-ray diffraction experiments on the cubic rare-earth hexaboride EuB6 to an ultrahigh pressure of 187 GPa in a diamond anvil cell. In EuB6, divalent europium ions occupy the corners of the cubic structure, which encloses a rigid boron-bonded cage. So far, no structural phase transitions have been reported, while the nanoindentation studies indicate amorphization in nanoscale shear bands during plastic deformation. Our x-ray diffraction studies have revealed that the ambient cubic phase of EuB6 shows broadening and splitting of diffraction peaks starting at 72 GPa and the broadening continuing to 187 GPa. The high-pressure phase is recovered on decompression, and the Raman spectroscopy of the recovered sample from 187 GPa shows a downward frequency shift and broadening of T2g, Eg, and A1g modes of boron octahedron. The density functional theory simulations of EuB6 at 100 GPa have identified five possible lowest energy crystal structures. The experimental x-ray diffraction data at high pressures is compared with the theoretical predictions and the role of structural distortions induced by shear stresses is also discussed.

Synthesis of substitutional hexaboride of lanthanum and cerium La1–xCexB6 via aluminothermic reduction

Rare-earth hexaborides (REB6) are vital raw materials for cathode materials and high temperature structural ceramics that are widely applied as high-frequency electron tubes and ceramics adaptive for extreme environment, respectively. In this work, single phase substitutional solid solution REB6 (LaB6, La0.75Ce0.25B6, La0.5Ce0.5B6, La0.25Ce0.75B6 and CeB6) powders were prepared with the raw materials of La2O3, CeO2, B4C and Al powders, after calcining at 1773 K for 4 h and the following alkaline leaching. All substitutional solid solution products have homogeneous distributions of La and Ce in particles. Through microscopic morphology analysis, it is discovered that the formation of solid solution is beneficial for reducing the particle size of product, relative to LaB6 and CeB6. Moreover, Al flux plays an important role in decarbonizing reaction, and C contents of all products are below 0.4 wt%.

Magnetization tunable Weyl states in EuB6

The interplay between magnetism and topological band structure offers extraordinary opportunities to realize rich exotic magnetic topological phases such as axion insulators, Weyl semimetals, and quantum anomalous Hall insulators, and therefore has attracted fast growing research interest. The rare-earth hexaboride $\mathrm{Eu}{\mathrm{B}}_{6}$ represents an interesting magnetic topological phase with tunable magnetizations along different crystallographic directions, while the correlation with the topological properties remains scarcely explored. In this work, combining magnetotransport measurements and first principles calculations, we demonstrate that $\mathrm{Eu}{\mathrm{B}}_{6}$ exhibits versatile magnetic topological phases along different crystallographic directions, which tightly correlate with the varied magnetizations. Moreover, by virtue of the weak magnetocrystalline anisotropy and the relatively strong coupling between the local magnetization and the conduction electrons, we show that the magnetic ground state of the system can be directly probed by the anisotropy in the magnetotransport properties. Our work thus introduces an excellent platform to study the rich topological phases that are tunable by magnetic orders.

Nanocrystalline high-entropy hexaboride ceramics enable remarkable performance as thermionic emission cathodes

The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential, yet relevant studies have been rarely reported. In this work, we report nanocrystalline (La0.25Ce0.25Nd0.25Eu0.25)B6 high-entropy rare-earth hexaboride (HEReB6-1) ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time. The as-fabricated samples exhibited a highly dense (96.3%) nanocrystalline (94 nm) microstructure with major (001) fiber textures and good grain boundaries without any impurities, resulting in a remarkable mechanical, electrical, and thermionic emission performance. The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures; these were greater than the average values of corresponding binary rare-earth hexaborides, such as a Vickers hardness of 23.4 ± 0.6 GPa and a fracture toughness of 3.0 ± 0.4 MPa•m1/2 at room temperature. More importantly, they showed high emission current densities at elevated temperatures, which were higher than the average values of the corresponding binary rare-earth hexaborides. For instance, the maximum emission current density reached 48.3 A•cm−2 at 1873 K. Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes.

Shear band formation during nanoindentation of EuB6 rare-earth hexaboride

Research on rare-earth hexaborides mainly focuses on tuning their electronic structure from insulating-to-metallic states during high pressure experiments. However, the structural evolution that contributes to their mechanical failure is not well understood. Here, we examine the pressure-induced structural evolution of a model rare-earth hexaboride, EuB6, during nanoindentation. Transmission electron microscopy reveals that nanoscale amorphous shear bands, mediated by dislocations, play a decisive role in deformation failure. Density functional theory calculations confirm that amorphous bands evolve by breaking boron-boron bonds within B6 octahedra during shear deformation. Our results underscore an important damage mechanism in hard and fragile hexaborides at high shear pressures.

Weyl semimetal in the rare-earth hexaboride family supporting a pseudonodal surface and a giant anomalous Hall effect

Rare-earth hexaborides offer a rich tapestry for exploring the interplay of topological orders, magnetism, and electron-correlation effects associated with protected quantum phenomena. Here, using first-principles modeling, we identify in cerium hexaboride ($\mathrm{Ce}{\mathrm{B}}_{6}$) a topological state that harbors a pseudonodal surface (PNS) along with double-Weyl fermions. An analysis of the electronic states in the vicinity of the Fermi level reveals the presence of ${M}_{z}$-mirror-symmetry-protected band crossings that involve itinerant ($d$) and localized ($f$) states of Ce. These band crossings are found to remain nearly gapless as one goes away from the mirror plane, driving the formation of PNS states. We also demonstrate the presence of coexisting double-Weyl fermions with chiral charges of $\ifmmode\pm\else\textpm\fi{}2$ on the ${C}_{4z}$ rotational axis. By analyzing the Berry curvature field associated with the topological states, we predict that $\mathrm{Ce}{\mathrm{B}}_{6}$ supports a giant anomalous Hall conductivity of $\ensuremath{-}1840 {\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}{\text{cm}}^{\ensuremath{-}1}$ in the slightly electron-doped regime, which would be larger than the values reported to date in magnetic Weyl semimetals. Our study thus indicates that $\mathrm{Ce}{\mathrm{B}}_{6}$, which combines ferromagnetic order with topological physics in a heavy-fermion matrix, would provide a promising playground for investigating topological magnetic states with giant anomalous Hall responses.

Electronic structures and emission properties of typical binary single crystal REB&lt;sub&gt;6&lt;/sub&gt;

Binary rare earth hexaborides (REB&lt;sub&gt;6&lt;/sub&gt;) have different rare earth elements with different valence electron distributions, which lead to different strange physical properties and different emission properties. However, in the electron emission properties, whether PrB&lt;sub&gt;6&lt;/sub&gt;, NdB&lt;sub&gt;6&lt;/sub&gt;, SmB&lt;sub&gt;6&lt;/sub&gt; and GdB&lt;sub&gt;6&lt;/sub&gt; all have excellent emission properties remains to be further studied, and the physical mechanism affecting their emission properties needs investigating. In this paper, the electronic structures, work functions of typical binary single crystal REB&lt;sub&gt;6&lt;/sub&gt; (LaB&lt;sub&gt;6&lt;/sub&gt;, CeB&lt;sub&gt;6&lt;/sub&gt;, PrB&lt;sub&gt;6&lt;/sub&gt;, NdB&lt;sub&gt;6&lt;/sub&gt;, SmB&lt;sub&gt;6&lt;/sub&gt;, GdB&lt;sub&gt;6&lt;/sub&gt;) are studied by first principles calculations. The single crystal REB&lt;sub&gt;6&lt;/sub&gt; are prepared by optical zone melting method, and their thermionic electron emission properties are tested experimentally. The theoretical calculation results show that the typical binary REB&lt;sub&gt;6&lt;/sub&gt; have large densities of states near the Fermi level. The d-orbitals with broad distributions in conduction bands are beneficial to electron emission. The localized f-orbital electrons in valence bands are not conducive to their electron emission. The theoretical calculations of work functions of typical binary single crystal REB&lt;sub&gt;6&lt;/sub&gt; (100) surface are consistent with the analyses of their electronic structures. The theoretical calculation values of work functions are ordered as GdB&lt;sub&gt;6&lt;/sub&gt; (2.27 eV) &lt; CeB&lt;sub&gt;6&lt;/sub&gt; (2.36 eV) &lt; LaB&lt;sub&gt;6&lt;/sub&gt; (2.40 eV) &lt; PrB&lt;sub&gt;6&lt;/sub&gt; (2.58 eV) &lt; SmB&lt;sub&gt;6&lt;/sub&gt; (2.63 eV) &lt; NdB&lt;sub&gt;6&lt;/sub&gt; (2.91 eV). The experimental test results of thermionic electron emission of single crystal show that the experimental thermionic electron properties are consistent with the theoretical ones. The LaB&lt;sub&gt;6&lt;/sub&gt; and CeB&lt;sub&gt;6&lt;/sub&gt; both have good thermionic and field emission properties, and the GdB&lt;sub&gt;6&lt;/sub&gt; has excellent field emission properties.

Recent Developments on Rare-Earth Hexaboride Nanowires

With the rise of topological insulator samarium hexaboride (SmB6), rare-earth hexaboride (RB6) nanowires are the focus of the second wave of a research boom. Recent research has focused on new preparation methods, novel electronic properties, and extensive applications. Here, we review the recent developments in RB6 nanowires in the past five years. Two main synthesis methods (chemical vapor deposition and high-pressure solid-state) of RB6 nanowires are introduced and compared. Moreover, their electronic transport, magnetic properties, and superconducting properties are revealed. Furthermore, the applications of RB6 nanowires are presented, including as field emitters, photodetectors, and in energy storage. Finally, we detail further research directions for RB6 nanowires.

Work Function Trends and New Low-Work-Function Boride and Nitride Materials for Electron Emission Applications

LaB6 has been used as a commercial electron emitter for decades. Despite the large number of studies on the work function of LaB6, there is no comprehensive understanding of work function trends in the hexaboride materials family. In this study, we use Density Functional Theory (DFT) calculations to calculated trends of rare earth hexaboride work function and rationalize these trends based on the electronegativity of the metal element. We predict that alloying LaB6 with Ba can further lower the work function by ~0.2 eV. Interestingly, we find that alloyed (La, Ba)B6 can have lower work functions than either LaB6 or BaB6, benefitting from an enhanced surface dipole due to metal element size mismatch. In addition to hexaborides we also investigate work function trends of similar materials families, namely tetraborides and transition metal nitrides, which, like hexaborides, are electrically conductive and refractory and thus may also be promising materials for electron emission applications. We find that tetraborides consistently have higher work functions than their hexaboride analogues as the tetraborides having less ionic bonding and smaller positive surface dipoles. Finally, we find that HfN has a low work function of about 2.2 eV, making HfN a potentially promising new electron emitter material.

Synthesis of nanostructured lanthanum hexaboride via simple borothermal routes at low temperatures

Rare-earth hexaborides, especially lanthanum hexaboride (LaB6) are popular materials as thermionic cathodes. It is a great challenge to synthesise these hexaborides at a low temperature without any post-synthesis purification treatments. In this research work, we demonstrate a simple synthesis technique, namely borothermal method for preparing pure lanthanum hexaboride by reacting lanthanum oxide (La2O3) with boron (B). In this investigation, different pressed pellets were made using the starting powders mixed either by milling mildly for 30 min or high-energy milling (HEM) for 8/16 h. Later, the prepared pellets were put into a low-vacuum furnace at different temperatures. The synthesised products were characterised using X-ray diffraction (XRD), scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) and a Schottky device for their structural, chemical, morphological and thermionic properties. Both synthesis methods are found to produce LaB6 with a purity above 99% at a relatively low temperature of 1250 °C. High-energy ball milling does not result in reducing synthesis temperatures, rather introduces some iron impurities. From XRD and SEM investigations, the average crystallite size of pure LaB6 prepared by borothermal methods is observed to be in the range of 225–250 nm. From thermionic emission data, partially sintered LaB6 pellets prepared from powders via the simple borothermal and the borothermal method with HEM exhibit work functions of 2.86 ± 0.05 eV and 2.64 ± 0.08 eV, respectively. The results suggest that nanostructured LaB6 with a low work function can be easily prepared at lower temperatures for applications in thermionic energy converters, high-power electron devices and photonics.

Bulk electronic structure of lanthanum hexaboride ( LaB6 ) by hard x-ray angle-resolved photoelectron spectroscopy

In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter physics, and for their applications in advanced technological fields. Among these compounds, LaB$_6$ has a special place, being a traditional d-band metal without additional f- bands. In this paper we investigate the bulk electronic structure of LaB$_6$ using hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. By comparing La 3d core level spectra to cluster model calculations, we identify well-screened peak residing at a lower binding energy compared to the main poorly-screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulk-like band structure of LaB$_6$ determined by hard x-ray angle-resolved photoemission spectroscopy (HARPES). We interpret HARPES experimental results by the free-electron final-state calculations and by the more precise one-step photoemission theory including matrix element and phonon excitation effects. In addition, we consider the nature and the magnitude of phonon excitations in HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that one step theory of photoemission and HARPES experiments provide, at present, the only approach capable of probing true bulk-like electronic band structure of rare-earth hexaborides and strongly correlated materials.

Synthesis and optical absorption properties of nanocrystalline rare earth hexaborides Nd&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Eu&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;B&lt;sub&gt;6&lt;/sub&gt; powders

Nanocrystalline rare earth hexaborides Nd&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Eu&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;B&lt;sub&gt;6&lt;/sub&gt; 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 NdB&lt;sub&gt;6&lt;/sub&gt; 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 &lt;i&gt;Pm-&lt;/i&gt;3&lt;i&gt;m&lt;/i&gt;. The SEM results show that the average grain size of the synthesized Nd&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Eu&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;B&lt;sub&gt;6&lt;/sub&gt; powders is 50 nm. The HRTEM results show that nanocrystalline Nd&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Eu&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;B&lt;sub&gt;6&lt;/sub&gt; has good crystallinity. The results of optical absorption show that the absorption valley of nanocrystalline Nd&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Eu&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;B&lt;sub&gt;6&lt;/sub&gt; is redshifted from 629 nm to higher than 1000 nm with the increase of Eu doping, indicating that the transparency of NdB&lt;sub&gt;6&lt;/sub&gt; is tunable. Additionally, the X-ray absorption near-edge structure spectra &lt;i&gt;μ&lt;/i&gt;(&lt;i&gt;E&lt;/i&gt;) around the Nd and Eu &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;3&lt;/sub&gt; edges for nanocrystalline NdB&lt;sub&gt;6&lt;/sub&gt; and EuB&lt;sub&gt;6&lt;/sub&gt; show that total valence of Nd ion is estimated at +3 in nanocrystalline NdB&lt;sub&gt;6&lt;/sub&gt; and total valence of Eu ion in nanocrystalline EuB&lt;sub&gt;6&lt;/sub&gt; is +2. Therefore, the Eu-doping into NdB&lt;sub&gt;6&lt;/sub&gt; 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 NdB&lt;sub&gt;6&lt;/sub&gt; and EuB&lt;sub&gt;6&lt;/sub&gt; 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 NdB&lt;sub&gt;6&lt;/sub&gt; and EuB&lt;sub&gt;6&lt;/sub&gt; 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&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Eu&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;B&lt;sub&gt;6&lt;/sub&gt; powders can expand the optical application scope of rare earth hexaborides.