Rediscovering the Crystal Chemistry of Borides

Evaluation of analytical uncertainty in quantitative determination of elements - The case of boron

BACKGROUND

The uncertainty including accuracy and precision is the most vital factor that determines the overall quality of quantitative analysis. The uncertainty has been, however, evaluated relatively within the same analytical technique. Given this background, the present study evaluates the uncertainty on quantitative elemental analysis with a quasi-absolute approach. The objectives of this study are (1) to investigate the analytical uncertainty of prompt gamma-ray analysis (PGA), a chemical interference-free method in principle, on the quantitative analysis of boron and (2) to evaluate the applicability of inductively coupled plasma optical emission spectrometry (ICP-OES), a common technique for quantitative elemental analysis including boron.

RESULTS

PGA provided analytical quantity that is equivalent to the true quantity. The quantity values determined for a series of boron-containing samples are all well above the detection limit of the PGA system, the quantity resolution of which is also much smaller than the minimum difference in quantity among the samples. These facts confirm that the evaluation of analytical uncertainty with the present PGA system is statistically meaningful. The analytical uncertainty in both methods was adequately evaluated by comparing the results from PGA and ICP-OES for a series of boron-containing materials with different physical/chemical properties (i.e. CrB2, B4C, and solidified products of stainless steel-B4C melt) and the major sources of uncertainty in both methods are specified. The conditions for sample preparation/pretreatment were optimized to lower the uncertainty.

SIGNIFICANCE AND NOVELTY

This study proposes a new concept to perform the quasi-absolute evaluation of analytical uncertainty by employing a chemical interference-free technique. The proposed concept is not limited to the combination of PGA and ICP-OES as demonstrated in this study, but it is applicable to any combination of any analytical methods for any element. Hence, the concept demonstrated in this study could be beneficial to a wide range of analytical chemistry.

Crystal structures, bonding and electronic structures of α- and β-Ir2B3-x compounds

The binary boron-rich compounds α-Ir2B3-x and β-Ir2B3-x, formerly denoted as α- and β-Ir4B5, were synthesized via both arc melting followed by annealing at 800 °C (900 °C) and high-temperature thermal treatment of mixtures of the elements. X-ray structure analysis of α-Ir2B3-x was performed on a single crystal (space group C2/m, a = 10.5515(11) Å, b = 2.8842(3) Å, c = 6.0965(7) Å, β = 91.121(9)°). The orthorhombic structure of β-Ir2B3-x was confirmed by X-ray powder diffraction (space group Pnma; a = 10.7519(3) Å, b = 2.83193(7) Å, c = 6.0293(1) Å). The α-Ir2B3-x structure exhibits ordered arrangements of iridium atoms. The structure is composed of corrugated layers of boron hexagons (interlinked via external B-B bonds) alternating with two corrugated layers of iridium along the c-direction; an additional boron atom (Oc. 0.46(7)) is located between iridium layers in Ir6 trigonal prisms. The boron partial structure in β-Ir2B3-x is composed of ribbons made up of slightly corrugated quadrilateral units running along the b-direction in the channels formed by 8 iridium atoms each. DFT calculations revealed a number of bands crossing the Fermi level, predicting metallic behaviors of the two compounds. β-Ir2B3-x is characterized by a pseudogap around the Fermi level and a smaller eDOS of 0.6405 states per eV per f.u. at the Fermi level, as compared to the α-Ir2B3-x value of 1.405 states per eV per f.u. The calculated electron localization functions revealed strong covalent bonds between boron atoms in the core part of the B6 hexagons, metallic B-B bonds within the quadrilateral boron partial structure and mixed covalent and metallic interactions between iridium and boron atoms. Structural relationships of α-Ir2B3-x and β-Ir2B3-x with ReB2-type structures as well as the common structural features with layered binary borides with CrB-type related structures have been discussed.

Heterogeneous Ni-Boride/Phosphide Anchored Amorphous B-C Layer for Overall Water Electrocatalysis

The rational design of efficient and economical bifunctional electrocatalysts remained a challenge for overall water electrolysis. In this work, the Ni-boride/ phosphide particles anchored amorphous B-doped carbon layer with hierarchical porous characteristics in Ni foam (Ni3P/Ni3B/B-C/NF) was fabricated for overall water splitting. The Boroncarbide (B4C) power was filled and fixed in the NF interspace through the electroplating and electroless plating, and then annealed in vacuum high temperature. The amorphous B-C layer derived from the B4 C not only speeded up the electron transport, but also cooperate with Ni-boride/phosphide to enhance the electrocatalytic activity for HER and OER synergistically. Furthermore, the hierarchical porous architecture of Ni3P/Ni3B/B-C/NF increased space utilization to load more active materials. The self-supported Ni3P/Ni3B/B-C/NF electrode possessed a low overpotential of 212 and 280 mV to deliver 100 mA cm-2 for HER and OER, respectively, and high stability for 48 h. In particular, the electrolyzer constituted with the Ni3P/Ni3B/B-C/NF bifunctional electrocatalyst only required a voltage of 1.59 V at 50 mA cm-2 for water electrocatalysis under alkaline medium, and demonstrated long-term stability for 48 h. This study provides a new technical path for the development of bifunctional of transition metal borides to promote the application of hydrogen production from water splitting.

Heterogeneous-Structured Molybdenum Diboride as a Novel and Promising Anode for Lithium-Ion Batteries

With the development of electric vehicles, exploiting anode materials with high capacity and fast charging capability is an urgent requirement for lithium-ion batteries (LIBs). Borophene, with the merits of high capacity, high electronic conductivity and fast diffusion kinetics, holds great potential as anode for LIBs. However, it is difficult to fabricate for the intrinsic electron-deficiency of boron atom. Herein, heterogeneous-structured MoB2 (h-MoB2) with amorphous shell and crystalline core, is prepared by solid phase molten salt method. As demonstrated, crystalline core can encapsulate the honeycomb borophene within two adjacent Mo atoms, and amorphous shell can accommodate more lithium ions to strengthen the lithium storage capacity and diffusion kinetics. According to theoretical calculations, the lithium adsorption energy in MoB2 is about -2.7 eV, and the lithium diffusion energy barrier in MoB2 is calculated to be 0.199 eV, guaranteeing the enhanced adsorption capability and fast diffusion kinetic behavior of Li+ ions. As a result, h-MoB2 anode presents high capacity of 798 mAh g-1 at 0.1 A g-1, excellent rate performance of 183 mAh g-1 at 5 A g-1 and long-term cyclic stability for 1200 cycles. This work may inspire ideas for the fabrication of borophene analogs and two-dimensional metal borides.

In Situ Construction of Biphasic Boride Electrocatalysts on Dealloyed Bulk Ni-Mo Alloy as Self-Supporting Electrode for Water Splitting at High Current Density

Nickel-molybdenum-boron (Ni-Mo-B)-based catalysts with biphasic interfaces are highly advantageous in bifunctional electrocatalytic activity in alkaline water-splitting. However, it remains an ongoing challenge to obtain porous Ni-Mo alloy substrates that provide stable adhesion to catalysts, ensuring the long-term performance of bifunctional self-supporting electrodes at a high current density. Herein, a porous Ni-Mo alloy substrate was effectively obtained by a cost-effective dealloying process on a commercial Ni-Mo alloy with high-energy crystal planes. Subsequently, the Mo2NiB2/Ni3B bifunctional catalyst was in situ synthesized on this substrate via boriding heat treatment, resulting in outstanding catalytic activity and stability. Density functional theory (DFT) calculations reveal that the abundant biphasic interfaces and surface-reconstructed sites of the Mo2NiB2/Ni3B catalyst can decrease the energy barriers for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Thus, the designed self-supporting electrodes show bifunctional catalytic activity with overpotentials of 151 mV for HER and 260 mV for OER at a current density of 10 mA cm-2. Markedly, the assembled water electrolyzer can be driven up to 10 mA cm-2 at 1.64 V and maintain catalytic activity at a high current density of 1000 mA cm-2 for 100 h. The new strategy is expected to provide a low-cost scheme for designing self-supporting bifunctional electrodes with high activity and excellent stability and contribute to the development of hydrogen energy technology.

Unveiling a Family of Dimerized Quantum Magnets, Conventional Antiferromagnets, and Nonmagnets in Ternary Metal Borides

Dimerized quantum magnets are exotic crystalline materials where Bose-Einstein condensation of magnetic excitations can happen. However, known dimerized quantum magnets are limited to only a few oxides and halides. Here, we unveil 9 dimerized quantum magnets and 11 conventional antiferromagnets in ternary metal borides MTB4 (M = Sc, Y, La, Ce, Lu, Mg, Ca, and Al; T = V, Cr, Mn, Fe, Co, and Ni), where T atoms are arranged in structural dimers. Quantum magnetism in these compounds is dominated by strong antiferromagnetic (AFM) interactions between Cr (Cr and Mn for M = Mg and Ca) atoms within the dimers, with much weaker interactions between the dimers. These systems are proposed to be close to a quantum critical point between a disordered singlet spin-dimer phase, with a spin gap, and the ordered conventional Néel AFM phase. They greatly enrich the materials inventory that allows investigations of the spin-gap phase. Conventional antiferromagnetism in these compounds is dominated by ferromagnetic Mn (Fe for M = Mg and Ca) interactions within the dimers. The predicted stable and nonmagnetic (NM) YFeB4 phase is synthesized and characterized, providing a scarce candidate to study Fe dimers and Fe ladders in borides. The identified quantum, conventional, and NM systems provide a platform with abundant possibilities to tune the magnetic exchange coupling by doping and study the unconventional quantum phase transition and conventional magnetic transitions. This work opens new avenues for studying novel magnetism in borides arising from spin dimers and establishes a theoretical workflow for future searches for dimerized quantum magnets in other families of materials.

Recent Developments and Future Perspectives of Molybdenum Borides and MBenes

Metal borides have received a lot of attention recently as a potentially useful material for a wide range of applications. In particular, molybdenum-based borides and MBenes are of great significance, due to their remarkable properties like good electronic conductivity, considerable stability, high surface area, and environmental harmlessness. Therefore, in this article, the progress made in molybdenum-based borides and MBenes in recent years is reviewed. The first step in understanding these materials is to begin with an overview of their structural and electronic properties. Then synthetic technologies for the production of molybdenum borides, such as high-temperature/pressure methods, physical vapor deposition (PVD), chemical vapor deposition (CVD), element reaction route, molten salt-assisted, and selective etching methods are surveyed. Then, the critical performance of these materials in numerous applications like energy storage, catalysis, biosensors, biomedical devices, surface-enhanced Raman spectroscopy (SERS), and tribology and lubrication are summarized. The review concludes with an analysis of the current progress of these materials and provides perspectives for future research. Overall, this review will offer an insightful reference for the understanding molybdenum-based borides and their development in the future.

Two-dimensional Mo1-xB2 with ordered metal vacancies obtained for advanced thermoelectric applications based on first-principles calculations

The study and development of high thermoelectric properties is crucial for the next generation of microelectronic and wearable electronics. Derived from the recent experimental realization of layers of transition metal molybdenum and boride, we report the theoretical realization of advanced thermoelectric properties in two-dimensional (2D) transition metal boride Mo1-xB2 (x = 0, 0.05, 0.10, 0.125, 0.15)-based defect sheets. The introduction of metal vacancies results in stronger d-p exchange interactions and hybridization between the Mo-d and B-p atoms. Meanwhile, the ordered metal vacancies enabled transition metal borides (n-type Mo0.9B2) to widen the d-bandwidth and raise the d-band center, leading to a relatively high carrier mobility of 3262 cm2 V-1 s-1 and conductivity twice that of a bug-free n-type MoB2 layer, which indicates that it presents good electronic and thermal transport properties. Furthermore, investigations of the thermoelectric performance exhibit a maximum ZT of up to 3.29, which is superior to those of currently reported 2D materials. Modulation by defect engineering suggests that 2D transition metal boride sheets with ordered metal vacancies have promising applications in microelectronics, wearable electronics and thermoelectric devices.

Dendritic boron and nitrogen doped high-entropy alloy porous carbon fibers for high-efficiency hydrogen evolution reaction

Among various electrocatalysts, high-entropy alloys (HEAs) have gained significant attention for their unique properties and excellent catalytic activity in the hydrogen evolution reaction (HER). However, the precise synthesis of HEA catalysts in small sizes remains challenging, which limits further improvement in their catalytic performance. In this study, boron- and nitrogen-doped HEA porous carbon nanofibers (HE-BN/PCNF) with an in situ-grown dendritic structure were successfully prepared, inspired by the germination and growth of tree branches. Furthermore, the dendritic fibers constrained the growth of HEA particles, leading to the synthesis of quantum dot-sized (1.67 nm) HEA particles, which also provide a pathway for designing HEA quantum dots in the future. This work provides design ideas and guiding suggestions for the preparation of borated HEA fibers with different elemental combinations and for the application of dendritic nanofibers in various fields.

Molten-Salt Electrochemical Preparation of Co2B/MoB2 Heterostructured Nanoclusters for Boosted pH-Universal Hydrogen Evolution

Boosting the hydrogen evolution reaction (HER) activity of α-MoB2 at large current densities and in pH-universal medium is significant for efficient hydrogen production. In this work, Co2B/MoB2 heterostructured nanoclusters are prepared by molten-salt electrolysis (MSE) and then used as a HER catalyst. The composition, structure, and morphology of Co2B/MoB2 can be modulated by altering the stoichiometries of raw materials and synthesis temperatures. Impressively, the obtained Co2B/MoB2 at optimized conditions exhibits a low overpotential of 297 and 304 mV at 500 mA cm-2 in 0.5 m H2SO4 and 1 m KOH, respectively. Moreover, the Co2B/MoB2 catalyst possesses a long-term catalytic stability of over 190 h in both acidic and alkaline medium. The excellent HER performance is due to the modified electronic structure at the Co2B/MoB2 heterointerface where electrons are accumulated at the Mo sites to strengthen the H adsorption. Density functional theory (DFT) calculations reveal that the formation of the Co2B/MoB2 heterointerface decreases the H adsorption and H2O dissociation free energies, contributing to the boosted HER intrinsic catalytic activity of Co2B/MoB2. Overall, this work provides an experimental and theoretical paradigm for the design of efficient pH-universal boride heterostructure electrocatalysts.

Dehydrogenation of diborane on small Nbn+ clusters

The reactivity of Nbn+ (1 ≤ n ≤ 21) clusters with B2H6 is studied by using a self-developed multiple-ion laminar flow tube reactor combined with a triple quadrupole mass spectrometer (MIFT-TQMS). The Nbn+ clusters were generated by a magnetron sputtering source and reacted with the B2H6 gas under fully thermalized conditions in the downstream flow tube where the reaction time was accurately controlled and adjustable. The complete and partial dehydrogenation products NbnB1-4+ and NbnB1-4H1,2,4+ were detected, indicative of the removal of H2 and likely BHx moieties. Interestingly, these NbnB1-4+ and NbnB1-4H1,2,4+ products are limited to 3 ≤ n ≤ 6, suggesting that the small Nbn+ clusters are relatively more reactive than the larger Nbn>6+ clusters under the same conditions. By varying the B2H6 gas concentrations and the reactant doses introduced into the flow tube, and by changing the reaction time, we performed a detailed analysis of the reaction dynamics in combination with the DFT-calculated thermodynamics. It is demonstrated that the lack of cooperative active sites on the Nb1+ cations accounts for the weakened dehydrogenation efficiency. Nb2+ forms partial dehydrogenation products at a faster rate. In contrast, the Nbn>6+ clusters are subject to more flexible vibrational relaxation which disperse the energy gain of B2H6-adsorption and thus are unable to overcome the energy barriers for subsequent hydrogen atom transfer and H2 release.

Quasi-Periodic Growth of One-Dimensional Copper Boride on Cu(110)

An unexplored material of copper boride has been realized recently in two-dimensional form at a (111) surface of the fcc copper crystal. Here, one-dimensional (1-D) boron growth was observed on the Cu(110) surface, as probed by atomically resolved scanning probe microscopy. The 1-D copper boride was composed of quasi-periodic atomic chains periodically aligned parallel to each other, as confirmed by Fourier transform analysis. The 1-D growth unexpectedly proceeded across surface steps in a self-assembled manner and extended over several 100 nm. The long-range formation of a 1-D quasi-periodic structure on a surface has been theoretically modeled as a 1-D quasi-crystal and the predicted conditions matched the structural parameters obtained by the experimental work here. The quasi-periodic 1-D copper boride system enabled a way to examine 1-D quasi-crystallinity on an actual material.

Insights into the Unanticipated Chemical Reactivity of Functionalized Nanosheets Derived from TiB2

Titanium diboride (TiB2) is a member of the AlB2-type layered metal boride family; the materials of this family are receiving renewed research interest owing to their amenability to nanoscaling. Earlier, we showed that TiB2 can be nanoscaled to yield quasi 2D nanostructures following a dissolution-recrystallization approach. This approach yielded nanosheets that were chemically functionalized with oxy-functional groups. Also, these nanosheets could inherently form a gel-like substance. In this work, we show that these functionalized nanosheets can interact with ascorbic acid in a way that first imparts a characteristic orange hue to the original yellowish nanosheet dispersion. Second, this interaction results in the loss of gel-like behavior of the nanosheet dispersion. We utilize several spectroscopic techniques such as UV-visible, FT-IR, NMR, EPR, XPS, and XANES to unravel this unexplored chemical interaction. The findings show that both titania as well as oxy-boron species react with ascorbic acid, leading to a profound modification of the nanosheets. This modification results in an augmented electrochemical response, implying that the modified nanosheets can be used in novel applications. This study is therefore a step toward gaining an even deeper understanding of the chemical opportunities that these nanoscaled borides can provide.

Hardness and superconductivity in tetragonal LiB4 and NaB4

Boron-based compounds have triggered substantial attention due to their multifunctional properties, incorporating excellent hardness and superconductivity. While tetragonal metal borides LiB4 and NaB4 with BaAl4-type structure and striking clathrate boron motif have been induced under compression, there is still a lack of deep understanding of their potential properties at ambient pressure. We herein conduct a comprehensive study on I4/mmm-structured LiB4 and NaB4 under ambient pressure via first-principles calculations. Remarkably, both LiB4 and NaB4 are found to possess high Vickers hardness of 39 GPa, which is ascribed to the robust boron framework with strong covalency. Furthermore, their high hardness values together with distinguished stability make them highly potential superhard materials. Meanwhile, electron-phonon coupling analysis reveals that both LiB4 and NaB4 are conventional phonon-mediated superconductors, with critical temperatures of 6 and 8 K at 1 atmosphere pressure (atm), respectively, mainly arising from the coupling of B 2p electronic states and the low-frequency phonon modes associated with Li-, Na-, and B-derived vibrations. This work provides valuable insights into the mechanical and superconducting behaviors of metal borides and will boost further studies of emergent borides with multiple functionalities.

Synthesis of Ni3 B/Ni via Vacuum-Induced for Ultrahigh Stable and Efficient Methanol Oxidation

Designing efficient catalysts to promote the electrochemical oxidation of anodes is the core of the development of electrochemical synthesis technologies, such as HER and CO2 RR. Here, a novel vacuum induction strategy is used to synthesize nickel boride/nickel (Ni3 B/Ni) heterostructure catalyst for electrochemical oxidation of methanol into formic acid. The catalyst has extremely high reactivity (only 146.9 mV overpotential at 10 mA cm-2 , the maximum current density reaches 555.70 mA mg-1 and 443.87 mA cm-2 ), ultra-high selectivity (Faraday efficiency of methanol conversion to formic acid is close to 100%), and ultra-long life (over 50 h at 100 mA cm-2 ). In-suit electrochemical impedance spectroscopy proved that MeOH is oxidized first and inhibits the phase transition of the electrocatalyst to the high-valent electrooxidation products, which not only enables the high selectivity of MeOH oxidation but also ensures high stability of the catalyst. The mechanism studies by density functional theory calculations show that the potential determining step, the formation of *CH2 O, occurs most favorably in the Ni3 B/Ni heterostructure. These results provide references for the development of MeOH oxidation catalysts with high activity, high stability, high selectivity, and low cost.

Enhancing Intrinsic Magnetic Hardness by Modulating Antagonistic Interactions in the Rare-Earth-Free Magnetic Solid Solution Hf2 Fe1-δ Ru5-x Irx+δ B2

The quinary members in the solid solution Hf2 Fe1-δ Ru5-x Irx+δ B2 (x=1-4, VE=63-66) have been investigated experimentally and computationally. They were synthesized via arc-melting and analyzed by EDX and X-ray diffraction. Density functional theory (DFT) calculations predicted a preference for magnetic ordering in all members, but with a strong competition between ferro- and antiferromagnetism arising from interchain Fe-Fe interactions. The spin exchange and magnetic anisotropy energies predicted the lowest magnetic hardness for x=1 and 3 and the highest for x=2. Magnetization measurements confirm the DFT predictions and demonstrate that the antiferromagnetic ordering (TN =55-70 K) found at low magnetic fields changed to ferromagnetic (TC =150-750 K) at higher fields, suggesting metamagnetic behavior for all samples. As predicted, Hf2 FeRu3 Ir2 B2 has the highest intrinsic coercivity (Hc =74 kA/m) reported to date for Ti3 Co5 B2 -type phases. Furthermore, all coercivities outperform that of ferromagnetic Hf2 FeIr5 B2 , indicating the importance of AFM interactions in enhancing magnetic anisotropy in these materials. Importantly, two members (x=1 and 4) maintain intrinsic coercivities in the semi-hard range at room temperature. This study opens an avenue for controlling magnetic hardness by modulating antagonistic AFM and FM interactions in low-dimensional rare-earth-free magnetic materials.

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.

Electronic and Lattice Distortions Induce Elastic Softening in Refractory Multicomponent Borides

Borides are extensively employed in applications demanding exceptionally high hardness, which arises from the unique and strong crystallographic arrangement of boron atoms therein. Addition of multiprincipal elements in borides is expected to enhance their structural properties due to lattice distortion and high configurational entropy. In contrast, we unravel a phenomenon of elastic softening in refractory multicomponent borides from first-principle predictions, which concur with experimentally determined metrics in their single-phase multiprincipal element counterparts. The reductions in the bulk and Young's modulus of these compounds are attributed to the lengthening and distortion of the boron-boron bonds and angles, but more critically to the perturbation in the charge densities arising from the different cations and the consequential increase in statistical weights of the d5 configuration states of the transition metals present in the boride..

Nano-metal diborides-supported anode catalyst with strongly coupled TaOx/IrO2 catalytic layer for low-iridium-loading proton exchange membrane electrolyzer

The sluggish kinetics of oxygen evolution reaction (OER) and high iridium loading in catalyst coated membrane (CCM) are the key challenges for practical proton exchange membrane water electrolyzer (PEMWE). Herein, we demonstrate high-surface-area nano-metal diborides as promising supports of iridium-based OER nanocatalysts for realizing efficient, low-iridium-loading PEMWE. Nano-metal diborides are prepared by a novel disulphide-to-diboride transition route, in which the entropy contribution to the Gibbs free energy by generation of gaseous sulfur-containing products plays a crucial role. The nano-metal diborides, TaB2 in particular, are investigated as the support of IrO2 nanocatalysts, which finally forms a TaOx/IrO2 heterojunction catalytic layer on TaB2 surface. Multiple advantageous properties are achieved simultaneously by the resulting composite material (denoted as IrO2@TaB2), including high electrical conductivity, improved iridium mass activity and enhanced corrosion resistance. As a consequence, the IrO2@TaB2 can be used to fabricate the membrane electrode with a low iridium loading of 0.15 mg cm-2, and to give an excellent catalytic performance (3.06 A cm-2@2.0 V@80 oC) in PEMWE-the one that is usually inaccessible by unsupported Ir-based nanocatalysts and the vast majority of existing supported Ir-based catalysts at such a low iridium loading.

Exploring the Structural, Electronic, Magnetic, and Transport Properties of 2D Cr, Fe, and Zr Monoborides

Compared to other 2D materials, MBenes are at an early stage of investigation in terms of both experimental and theoretical approaches. However, their wide range of possible 2D structures leads to novel and challenging properties and consequent applications. From all the possible stoichiometries, we performed a theoretical study of orthorhombic and hexagonal M2B2 MBenes within the framework of density functional theory. We found that both symmetries of Cr2B2, Fe2B2, and Zr2B2 show metallic behavior and could be grown under certain conditions as they were demonstrated to be dynamically stable. Moreover, the values of the magnetic moment observed, in specific ferromagnetic cases exceeding 2.5μB/M2B2, make them suitable as robust 2D magnets. Our findings represent an important step in the understanding of MBenes and open several windows to future research in fields like energy conversion and storage, sensing, catalysis, biochemistry, and nanotechnology, among others.

Chemical Potential-Modulated Ultrahigh-Phase-Purity Growth of Ultrathin Transition-Metal Boride Single Crystals

Two-dimensional (2D) transition-metal borides (TMBs) are especially expected to exhibit excellent performance in various fields among electricity, superconductivity, magnetism, mechanics, biotechnology, battery, and catalysis. However, the synthesis of ultrathin TMB single crystals with ultrahigh phase purity was deemed extremely challenging and has not been realized till date. That is because TMBs have the most kinds of crystal structures among inorganic compounds, which possess generous phase structures with similar formation energies compared with other transition-metal compounds, attributing to the metalloid and electron-deficient characteristics of boron. Herein, for the first time, we demonstrate a chemical potential-modulated strategy to realize the precise synthesis of various ultrahigh-phase-purity (approximately 100%) ultrathin TMB single crystals, and the precision in the phase formation energy can reach as low as 0.01 eV per atom. The ultrathin MoB2 single crystals exhibit an ultrahigh Young's modulus of 517 GPa compared to other 2D materials. Our work establishes a chemical potential-modulated strategy to synthesize ultrathin single crystals with ultrahigh phase purity, especially those with similar formation energies, and undoubtedly provides excellent platforms for their extensive research and applications.

Introducing Porosity into Refractory Molybdenum Boride through Controlled Decomposition of a Metastable Mo-Al-B Precursor

The high temperatures typically required to synthesize refractory compounds preclude the formation of high-energy morphological features, including nanoscopic pores that are beneficial for applications, such as catalysis, that require higher surface areas. Here, we demonstrate a low-temperature multistep pathway to engineer mesoporosity into a catalytic refractory material. Mesoporous molybdenum boride, α-MoB, forms through the controlled thermal decomposition of nanolaminate-containing sheets of the metastable MAB (metal-aluminum-boron) phase Mo₂AlB₂ and amorphous alumina. Upon heating, the Mo₂AlB₂ layers of the Mo₂AlB₂-AlO_x nanolaminate, which is derived from MoAlB, begin to bridge and decompose, forming inclusions of alumina in a framework of α-MoB. The alumina can be dissolved in aqueous sodium hydroxide in an autoclave, forming α-MoB with empty and accessible pores. Statistical analysis of the morphologies and dimensions of the pores reveals a correlation with grain size, which relates to the pathway by which the alumina inclusions form. The transformation of Mo₂AlB₂ to α-MoB is topotactic due to crystal structure relationships, resulting in a high density of stacking faults that can be modeled to account for the observed experimental diffraction data. Porosity was validated by comparing surface areas and demonstrating catalytic viability for the hydrogen evolution reaction.

A New Approach to the Synthesis of Nanocrystalline Cobalt Boride in the Course of the Thermal Decomposition of Cobalt Complexes [Co(DMF)6]2+ with Boron Cluster Anions

In the course of the study, nanocrystalline cobalt monoboride was prepared by thermal decomposition of precursors [Co(DMF)₆][An], where [An] = [B₁₂H₁₂]^2- (1), [trans-B₂₀H₁₈]^2- (2) or [B₁₀Cl₁₀]^2- (3) in an argon atmosphere. Three new salt-like compounds 1-3 were prepared when Co(NO₃)₂ was allowed to react with (Et₃NH)₂[An]. Compound 1 is new; the structures of compounds 2 and 3 have been previously reported. Samples 1-3 were annealed at 900 °C in argon to form samples 1a-3a, which were characterized by single crystal XRD for 1 and powder XRD for 1-3. Powder XRD on the products showed the formation of BN and CoB for 1a in a 1:1 ratio; 2a gave a higher CoB:BN ratio but an overall decreased crystallinity. For 3a, only CoB was found. IR spectra of samples 1a-3a as well as X-ray spectral fluorescence analysis for 3a confirmed these results. The nanoparticular character of the decomposition products 1a-3a was shown using TEM; quite small particle sizes of about 10-15 nm and a quite normal size distribution were found for 1a and 2a, while the decomposition of 3 gave large particles with 200-350 nm and a broad distribution.

Planar Elongated B12 Structure in M3B12 Clusters (M = Cu-Au)

Here, it is shown that the M₃B₁₂ (M = Cu-Au) clusters' global minima consist of an elongated planar B₁₂ fragment connected by an in-plane linear M₃ fragment. This result is striking since this B₁₂ planar structure is not favored in the bare cluster, nor when one or two metals are added. The minimum energy structures were revealed by screening the potential energy surface using genetic algorithms and density functional theory calculations. Chemical bonding analysis shows that the strong electrostatic interactions with the metal compensate for the high energy spent in the M₃ and B₁₂ fragment distortion. Furthermore, metals participate in the delocalized π-bonds, which infers an aromatic character to these species.

The Synthesis and Characterisation of the High-Hardness Magnetic Material Mn2N0.86

High-quality P6₃22 Mn₂N_0.86 samples were synthesised using a high-pressure metathesis reaction, and the properties of the material were investigated. The measurements revealed that the Vickers hardness was 7.47 GPa, which is less than that predicted by commonly used theoretical models. At low air pressure, Mn₂N_0.86 and MnO coexist at 500 to 600 °C, and by excluding air, we succeeded in producing Mn₄N by heating Mn₂N_0.86 in nitrogen atmosphere; we carefully studied this process with thermogravimetry and differential scanning calorimetry (TG-DSC). This gives a hint that to control temperature, air pressure and gas concentration might be an effective way to prepare fine Mn-N-O catalysis. Magnetic measurements indicated that ferromagnetism and antiferromagnetism coexist within Mn₂N_0.86 at room temperature and that these magnetic properties are induced by nitrogen vacancies. Ab intio simulation was used to probe the nature of the magnetism in greater detail. The research contributes to the available data and the understanding of Mn₂N_0.86 and suggests ways to control the formation of materials based on Mn₂N_0.86.

Novel Class of Rhenium Borides Based on Hexagonal Boron Networks Interconnected by Short B2 Dumbbells

Transition metal borides are known due to their attractive mechanical, electronic, refractive, and other properties. A new class of rhenium borides was identified by synchrotron single-crystal X-ray diffraction experiments in laser-heated diamond anvil cells between 26 and 75 GPa. Recoverable to ambient conditions, compounds rhenium triboride (ReB₃) and rhenium tetraboride (ReB₄) consist of close-packed single layers of rhenium atoms alternating with boron networks built from puckered hexagonal layers, which link short bonded (∼1.7 Å) axially oriented B₂ dumbbells. The short and incompressible Re-B and B-B bonds oriented along the hexagonal c-axis contribute to low axial compressibility comparable with the linear compressibility of diamond. Sub-millimeter samples of ReB₃ and ReB₄ were synthesized in a large-volume press at pressures as low as 33 GPa and used for material characterization. Crystals of both compounds are metallic and hard (Vickers hardness, H _V = 34(3) GPa). Geometrical, crystal-chemical, and theoretical analysis considerations suggest that potential ReB _x compounds with x > 4 can be based on the same principle of structural organization as in ReB₃ and ReB₄ and possess similar mechanical and electronic properties.

Rapid and Energetic Solid-State Metathesis Reactions for Iron, Cobalt, and Nickel Boride Formation and Their Investigation as Bifunctional Water Splitting Electrocatalysts

Metal borides have long-standing uses due to their desirable chemical and physical properties such as high melting points, hardness, electrical conductivity, and chemical stability. Typical metal boride preparations utilize high-energy and/or slow thermal heating processes. This report details a facile, solvent-free single-step synthesis of several crystalline metal monoborides containing earth-abundant transition metals. Rapid and exothermic self-propagating solid-state metathesis (SSM) reactions between metal halides and MgB₂ form crystalline FeB, CoB, and NiB in seconds without sustained external heating and with high isolated product yields (∼80%). The metal borides are formed using a well-studied MgB₂ precursor and compared to reactions using separate Mg and B reactants, which also produce self-propagating reactions and form crystalline metal borides. These SSM reactions are sufficiently exothermic to theoretically raise reaction temperatures to the boiling point of the MgCl₂ byproduct (1412 °C). The chemically robust monoborides were examined for their ability to perform electrocatalytic water oxidation and reduction. Crystalline CoB and NiB embedded on carbon wax electrodes exhibit moderate and stable bifunctional electrocatalytic water splitting activity, while FeB only shows appreciable hydrogen evolution activity. Analysis of catalyst particles after extended electrocatalytic experiments shows that the bulk crystalline metal borides remain intact during electrochemical water-splitting reactions though surface oxygen species may impact electrocatalytic activity.

Bowl-shaped CuB12- Cluster. A viable Global Minimum with Twofold Aromaticity

A low-lying structure is revealed for the CuB 12 - cluster, which is bowl-shaped. It consists of a triangular CuB 2 base and a B 10 rim. Molecular dynamics simulations evidence its structural robustness; at an elevated temperature (600 K), the base rotates reversibly within the B 10 perimeter. Chemical bonding analysis detects 2σ- and 3π-delocalized bonds, suggesting double aromaticity, which is confirmed by two diatropic and concentric ring currents under an external magnetic field.

Transition Metal Non-Oxides as Electrocatalysts: Advantages and Challenges

The identification of hydrogen as green fuel in the near future has stirred global realization toward a sustainable outlook and thus boosted extensive research in the field of water electrolysis focusing on the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). A huge class of compounds consisting of transition metal-based nitrides, carbides, chalcogenides, phosphides, and borides, which can be collectively termed transition metal non-oxides (TMNOs), has emerged recently as an efficient class of electrocatalysts in terms of performance and longevity when compared to transition metal oxides (TMOs). Moreover, the superiority of TMNOs over TMOs to effectively catalyze not only OERs but also HERs and ORRs renders bifunctionality and even trifunctionality in some cases and therefore can replace conventional noble metal electrocatalysts. In this review, the crystal structure and phases of different classes of nanostructured TMNOs are extensively discussed, focusing on recent advances in design strategies by various regulatory synthetic routes, and hence diversified properties of TMNOs are identified to serve as next-generation bi/trifunctional electrocatalysts. The challenges and future perspectives of materials in the field of energy conversion and storage aiding toward a better hydrogen economy are also discussed in this review.

Structure Determination, Mechanical Properties, Thermal Stability of Co2MoB4 and Fe2MoB4

The precise determination of atomic position of materials is critical for understanding the relationship between structure and properties, especially for compounds with light elements of boron and single or multiple transition metals. In this work, the single crystal X-ray diffraction is employed to analyze the atomic positions of Co₂MoB₄ and Fe₂MoB₄ with a Ta₃B₄-type structure, and it is found that the lengths of B-B bonds connecting the two zig-zag boron chains are 1.86 Å and 1.87 Å, but previously unreported 1.4 Å. Co and Fe atoms occupy the same crystallographic position in lattice for the doped samples and the valence is close to the metal itself, and Co/Fe K-edge X-ray Absorption Fine Structure(XAFS) spectra of borides with different ratios of Co to Fe are collected to detect the local environment and chemical valence of Co and Fe. Vickers hardness and nano indentation measurements are performed, together with the Density Functional Theory (DFT) calculations. Finally, Co₂MoB₄ possess better thermal stability than Fe₂MoB₄ evaluated by Thermogravimetric Differential Thermal Analysis (TG-DTA) results.

Quasiparticle multiplets and 5felectronic correlation in prototypical plutonium borides

In this paper, we investigate the electronic structures of plutonium borides (PuB_x,x= 1, 2, 6, 12) to uncover the fascinating bonding behavior and orbital dependent correlations of 5fvalence electrons by using the density functional theory combined with single-site dynamical mean-field method. We not only reproduce the correlated topological insulator of PuB₆, but also predict the metallicity in PuB_x(x= 1, 2, 12). It is found that the band structure, density of states, hybridization functions all indicate partially itinerant 5fstates in PuB_x(x= 1, 2, 6, 12). Especially, quasiparticle multiplets induced noteworthy valence state fluctuations implying the mixed-valence behavior of plutonium borides. Moreover, the itinerant degree of freedom for 5felectrons in PuB_x(x= 1, 2, 12) is tuned by hybridization strength between 5fstates and conduction bands, which is affected by atomic distance in plutonium borides. Lastly, 5felectronic correlations encoded in the electron self-energy functions demonstrate moderate 5felectronic correlations in PuB₆and orbital selective 5felectronic correlations in PuB_x(x= 1, 2, 12). Consequently, the understanding of electronic structure and related crystal structure stability shall shed light on exploring novel 5felectrons states and ongoing experiment study.

Few-layer Mg-deficient borophene nanosheets: I2 oxidation and ultrasonic delamination from MgB2

By using I₂ as an oxidant and CH₃CN as a reaction medium, few-layer Mg-deficient borophene nanosheets (FBN) with a stoichiometric ratio of Mg_0.22B₂ are prepared by oxidizing MgB₂ in a mixture of CH₃CN and HCl for 14 days under nitrogen protection and followed by ultrasonic delaminating in CH₃CN for 2 h. The prepared FBN possess a two-dimensional flake morphology, and they show a clear interference fringe with a d-spacing of 0.251 nm corresponding to the (208) plane of rhombohedral boron. While maintaining the hexagonal boron networks of MgB₂, the FBN have an average thickness of about 4.14 nm (four monolayer borophene) and a lateral dimension of 500 nm, and the maximum Mg deintercalation rate can reach 78%. The acidity of the reaction system plays an important role; the HCl reaction system not only facilitates the oxidation of MgB₂ by I₂, but also increases the deintercalation ratio of Mg atoms. Etching of the Mg atom layer with HCl, the negative charge decrease of the boron layer by I₂ oxidation, and the Mg chelating effect from CH₃COOH due to the hydrolysis of CH₃CN in an HCl environment led to a high deintercalation rate of the Mg atom. Density functional theory (DFT) calculations further support the result that the maximum deintercalation rate of Mg atoms is about 78% while maintaining the hexagonal layer structure of boron. This research solves the problems of low Mg atom deintercalation rate and hexagonal boron structure destruction when using the precursor MgB₂ to produce borophene nanosheets, which is of great significance for large-scale novel preparation and application of borophene nanosheets.

Borophene as an emerging 2D flatland for biomedical applications: current challenges and future prospects

Recently, two-dimensional (2D)-borophene has emerged as a remarkable translational nanomaterial substituting its predecessors in the field of biomedical sensors, diagnostic tools, high-performance healthcare devices, super-capacitors, and energy storage devices. Borophene justifies its demand due to high-performance and controlled optical, electrical, mechanical, thermal, and magnetic properties as compared with other 2D-nanomaterials. However, continuous efforts are being made to translate theoretical and experimental knowledge into pragmatic platforms. To cover the associated knowledge gap, this review explores the computational and experimental chemistry needed to optimize borophene with desired properties. High electrical conductivity due to destabilization of the highest occupied molecular orbital (HOMO), nano-engineering at the monolayer level, chemistry-oriented biocompatibility, and photo-induced features project borophene for biosensing, bioimaging, cancer treatment, and theragnostic applications. Besides, the polymorphs of borophene have been useful to develop specific bonding for DNA sequencing and high-performance medical equipment. In this review, an overall critical and careful discussion of systematic advancements in borophene-based futuristic biomedical applications including artificial intelligence (AI), Internet-of-Things (IoT), and Internet-of-Medical Things (IoMT) assisted smart devices in healthcare to develop high-performance biomedical systems along with challenges and prospects is extensively addressed. Consequently, this review will serve as a key supportive platform as it explores borophene for next-generation biomedical applications. Finally, we have proposed the potential use of borophene in healthcare management strategies.

Structural transformations in boron clusters induced by metal doping

In the last decades, experimental techniques in conjunction with theoretical analyses have revealed the surprising structural diversity of boron clusters. Although the 2D to 3D transition thresholds are well-established, there is no certainty about the factors that determine the geometry adopted by these systems. The structural transformation induced by doping usually yields a minimum energy structure with a boron skeleton entirely different from that of the bare cluster. This review summarizes those clusters no larger than 40 boron atoms where one or two dopants show a radical transformation of the structure. Although the structures of these systems are not easy to predict, they often adopt familiar shapes such as umbrella-like, wheel, tubular, and cages in various cases.

Nonprecious Metal Borides: Emerging Electrocatalysts for Hydrogen Production

ConspectusThe development of highly active noble-metal-free catalysts for the hydrogen evolution reaction (HER) is the focus of current fundamental research, aiming for a more efficient and economically affordable water-splitting process. While most HER catalysts are studied only at the nanoscale (small particle size and high surface area), metal borides (MBs) are mostly studied in bulk form. This offers a unique opportunity for designing highly efficient and nonprecious HER MBs electrocatalysts based on structure-activity relationships, especially because of their rich compositional and structural diversity.In this Account, we focus on the importance of boron and its substructures in achieving extraordinary HER performances and the importance of using structure-activity relationships to design next-generation MBs electrocatalysts. Studying the Mo-B system, we found that the HER activity of molybdenum borides increases with increasing boron content: from Mo₂B (no B-B bonds in the structure, least active) to α-MoB and β-MoB (zigzag boron chains, intermediate activity) and MoB₂ (planar graphene-like boron layer, most active). Density functional theory (DFT) calculations have shown that the (001) boron layer in hexagonal MoB₂ (α-MoB₂) is the most active surface and has similar HER activity behavior like the benchmark Pt(111) surface. However, puckering this flat boron layer to the chair-like configuration (phosphorene-like layer) drastically reduces its activity, thereby making the rhombohedral modification of MoB₂ (Mo₂B₄ or β-MoB₂) less active than α-MoB₂. This discovery was then further supported by studies of the Mo-W-B system. In fact, the binary WB₂, which also contains the puckered boron layer, is also less active than α-MoB₂, despite containing the more active transition metal W, which performs better in elemental form than Mo. To design a superior catalyst, the more active W was then stabilized in the hexagonal α-MoB₂ structure through the synthesis of α-Mo_0.7W_0.3B₂ ,which indeed proved to be a better HER electrocatalyst than α-MoB₂. Using the isoelectronic Cr instead of W led to the α-Cr_1-xMo_xB₂ solid solution, the HER activity of which followed unexpected canonic-like (or volcano-like) behavior that perfectly matched that of the c lattice parameter trend, thereby producing the best catalyst α-Cr_0.4Mo_0.6B₂ that outperformed Pt/C at high current density, thus underscoring the effectiveness of the structure-activity concept in designing highly active catalysts. This concept was further applied to the V-B system, leading to the discovery of an unexpected boron chain dependency of the HER activity that ultimately led to the prediction of new V_xB_y catalysts and their crystal structures and overpotentials. Finally, reducing the particle sizes of all of these bulk crystalline catalysts is also possible and offers an even greater potential as demonstrated for nanoscale a-MoB₂ and VB₂. Nevertheless, these crystalline nanomaterials are still highly agglomerated due to the high temperature required for their synthesis, thus the synthesis of highly dispersed MBs is an urgent goal that will enable the fulfillment of their extraordinary potential in the future.

The crystal structures, phase stabilities, electronic structures and bonding features of iridium borides from first-principles calculations

We present results of an unbiased structure search for the lowest energy crystalline structures of various stoichiometric iridium borides, using first-principles calculations combined with particle swarm optimization algorithms. As a result, besides three stable phases of C2/m-Ir₃B₂, Fmm2-Ir₄B₃, and Cm-Ir₄B₅, three promising metastable phases, namely, P2₁/m-Ir₂B, P2₁/m-IrB, and Pnma-Ir₃B₄, whose energies are within 20 meV per atom above the convex hull curve, are also identified at ambient pressure. The high bulk modulus of 301 GPa, highest shear modulus of 148 GPa, and smallest Poisson's ratio of 0.29 for C2/m-Ir₃B₂ make it a promising low compressible material. C2/m-Ir₃B₂ is predicted to possess the highest Vickers hardnesses, with a Vickers hardness of 13.1 GPa and 19.4 GPa based on Chen's model and Mazhnik-Oganov's model respectively, and a high fracture toughness of 5.17 MPa m^0.5. The anisotropic indexes and the three-dimensional surface constructions of Young's modulus indicate that Ir–B compounds are anisotropic with the sequence of the elastic anisotropy of Ir₂B > IrB > Ir₄B₅ > Ir₃B₄ > Ir₄B₃ > Ir₃B₂. Remarkably, these iridium borides are all ductile. We further find that the four Ir–B phases of P2₁/m-Ir₂B, C2/m-Ir₃B₂, P2₁/m-IrB, and Fmm2-Ir₄B₃ possess dominant Ir–B covalent bonding character, while strong B–B and Ir–B covalent bonds are present in Cm-Ir₄B₅ and Pnma-Ir₃B₄, which are responsible for their excellent mechanical properties.

We mainly probe into phase stabilities, structural characters, elastic anisotropy and bonding features of the iridium borides under ambient pressure.

Borophene Nanosheets as High-Efficiency Catalysts for the Hydrogen Evolution Reaction

Borophene has been predicted to have outstanding catalytic activity owing to its extreme electron deficiency and abundant active sites. However, no experimental results have been still reported for borophene application in high-efficiency catalysis. Here, a borophene nanosheet was prepared on a carbon cloth surface via chemical vapor deposition. The boron source is sodium borohydride and the carrier gas is hydrogen gas. The crystal structure of the borophene nanosheet highly matches that of a theoretical α'-borophene nanosheet. Borophene shows good electrocatalytic hydrogen evolution reaction (HER) ability with a 69 mV/dec Tafel slope and good cycling stability in a 0.5 M H₂SO₄ solution. The enhanced performance is ascribed to an abundant electrocatalytic active area and low resistance of charge transfer, which results from its rich surface active sites. The improvement has been revealed by first-principles calculations, which is originated from their inherent metallicity and abundant electrocatalytic active sites on the nanosheets' surface. Borophene's extraordinarily high activity and stability give rise to extensive investigation of the application of borophene in high-efficiency energy applications such as catalysts and batteries.

Direct MoB MBene domain formation in magnetron sputtered MoAlB thin films

Two-dimensional (2D) inorganic transition metal boride nanosheets are emerging as promising post-graphene materials in energy research due to their unique properties. State-of-the-art processing strategies are based on chemical etching of bulk material synthesized via solid-state reaction at temperatures above 1000 °C. Here, we report the direct formation of MoB MBene domains in a MoAlB thin film by Al deintercalation from MoAlB in the vicinity of AlO_x regions. Hence, based on these results a straightforward processing pathway for the direct formation of MoB MBene-AlO_x heterostructures without employing chemical etching is proposed here.

Unusual suppression of tungsten 5delectron depletion in superhard tungsten tetraboride solid solution with chromium under compression

The lattice compressibility and deformation in superhard tungsten tetraboride (WB₄) solid solution with chromium (Cr) are investigated by high-pressure x-ray diffraction and x-ray absorption fine structure (XAFS) spectroscopy up to 54 GPa. In contrast to pure WB₄, thec-axis softening is effectively suppressed in W_0.9Cr_0.1B₄, and less compressibility is shown for thea- andc-axes in the entire pressure range. Meanwhile, the white-line peak of W L₃-edge XAFS in W_0.9Cr_0.1B₄shows an absence of the sudden intensity drop as previously observed in WB₄at ∼21 GPa, suggesting a strong inhibition of W 5delectron depletion. This phenomenon is followed by an initial increase and then decrease for the W-B bond disorder, with the magnitude greatly lower than that of WB₄. Besides the apparent atomic size mismatch effect, these results imply that addition of Cr, which has the same number of valence electrons as W, can introduce an unexpected electronic structure change to strengthen the W-B bond via a modification of W vacancies and B trimers distribution in WB₄lattice. Our findings point out the great significance to precise manipulation of the intrinsic W vacancies and B trimers through different solute atoms to rational optimization of WB₄hardness.

Theoretical Design of Novel Boron-Based Nanowires via Inverse Sandwich Clusters

Borophene has important application value, boron nanomaterials doped with transition metal have wondrous structures and chemical bonding. However, little attention was paid to the boron nanowires (NWs). Inspired by the novel metal boron clusters Ln₂B_n⁻ (Ln = La, Pr, Tb, n = 7–9) adopting inverse sandwich configuration, we examined Sc₂B₈ and Y₂B₈ clusters in such novel structure and found that they are the global minima and show good stability. Thus, based on the novel structural moiety and first-principles calculations, we connected the inverse sandwich clusters into one-dimensional (1D) nanowires by sharing B−B bridges between adjacent clusters, and the 1D-Sc₄B₂₄ and 1D-Y₂B₁₂ were reached after structural relaxation. The two nanowires were identified to be stable in thermodynamical, dynamical and thermal aspects. Both nanowires are nonmagnetic, the 1D-Sc₄B₂₄ NW is a direct-bandgap semiconductor, while the 1D-Y₂B₁₂ NW shows metallic feature. Our theoretical results revealed that the inverse sandwich structure is the most energy-favored configuration for transition metal borides Sc₂B₈ and Y₂B₈, and the inverse sandwich motif can be extended to 1D nanowires, providing useful guidance for designing novel boron-based nanowires with diverse electronic properties.

Revealing the Unusual Boron-Pinned Layered Substructure in Superconducting Hard Molybdenum Semiboride

Improving the poor electrical conductivity of hard materials is important, as it will benefit their application. High-hardness metallic Mo₂B was synthesized by high-pressure and high-temperature methods. Temperature-dependent resistivity measurements suggested that Mo₂B has excellent metallic conductivity properties and is a weakly coupled superconductor with a T _c of 6.0 K. The Vickers hardness of the metal-rich molybdenum semiboride reaches 16.5 GPa, exceeding the hardness of MoB and MoB₂. The results showed that a proper boron concentration can improve the mechanical properties, not necessarily a high boron concentration. First-principles calculations revealed that the pinning effect of light elements is related to hardness. The high hardness of boron-pinned layered Mo₂B demonstrated that the design of high-hardness conductive materials should be based on the structure formed by light elements rather than high-concentration light elements.

Electron Precise Sodium Carbaboride Nanocrystals from Molten Salts: Single Sources to Boron Carbides

Boron-rich solids exhibit specific crystal structures and unique properties, which are only very scarcely addressed in nanoparticles. In this work, we address the original inorganic structural chemistry and reactivity of boron-rich nanoparticles, by reporting the first occurrence of sodium carbaboride nanocrystals based on the NaB₅C crystal structure. To design these sub-10 nm nano-objects, we use liquid-phase synthesis in molten salts at 900 °C. By combining a set of characterization tools including powder X-ray powder diffraction, transmission electron microscopy, solid-state nuclear magnetic resonance coupled to DFT modeling, and X-ray photoelectron spectroscopy, we demonstrate that these nanocrystals deviate from the ideal stoichiometry reported for the bulk compound. We suggest that the carbon and sodium contents compensate each other to ensure that the octahedral cluster-based framework is stabilized by fulfilling an electron counting rule. These nanocrystals encompass substituted octahedral covalent structural building units not reported in the related bulk compound. They then shed new light on the ability of nanoparticles to host wide solid solution ranges in covalent solids and then to yield new solids. We finally show that these nanocrystals are efficient single sources of boron and carbon to form a nanostructured boron carbide, thus paving the way to new nanostructured materials.

Ultra-incompressible High-Entropy Diborides

Transition metal borides are commonly hard and incompressible, offering great opportunities for advanced applications under extreme conditions. Recent studies show that the hardness of high-entropy borides may exceed that of their constituent simple borides due to the "cocktail effect". However, how high-entropy borides deform elastically remains largely unknown. Here, we show that two newly synthesized high-entropy diborides are ultra-incompressible, attaining ∼90% of the incompressibility of single-crystalline diamond and exhibiting a 50-60% enhancement over the density functional theory predictions. This unusual behavior is attributed to a Hall-Petch-like effect resulting from nanosizing under high pressure, which increases the bulk moduli through dynamic dislocation interactions and creation of stacking faults. The exceptionally low compressibility, together with their high phase stabilities, high hardness, and high electric conductance, renders them promising candidates for electromechanics and microelectronic devices that demand strong resistance to environmental impacts, in addition to traditional grinding and abrading.