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Tuning of the magnetic properties of intermetallic materials is increasingly gaining interest in the chemistry community. We report on the successful synthesis and magnetic characterizations of the two new quaternary boride series MRh6- nRu nB3 (M = Co, Ni; 1 ≤ n ≤ 5). Powder samples have been synthesized by arc-melting the elements under purified argon in a water-cooled copper crucible. The products were characterized by powder X-ray diffraction through Rietveld refinements as well as EDX measurements. All phases crystallize in the hexagonal Th7Fe3 structure type (space group P63 mc, no. 186, Z = 2). In the Co-based series, the lattice parameters decrease linearly with increasing ruthenium content, whereas in the Ni-based series they remain nearly constant for the Rh-rich side then decrease with increasing Ru content. Significant ferromagnetic interactions are observed for the Co-based series, and they decrease linearly with increasing Ru content with magnetic moments decreasing from 0.74 µB (CoRh5RuB3) to 0.13 µB (CoRhRu5B3) at 5 K and 5 T. Pauli paramagnetism dominates the Ni-based series. [ABSTRACT FROM AUTHOR]

Transition-metal borides (TMBs) have recently attracted attention as excellent hydrogen evolution (HER) electrocatalysts in bulk crystalline materials. Herein, we show for the first time that VB and V 3 B 4 have high electrocatalytic HER activity. Furthermore, we show that the HER activity (in 0.5 m H 2 SO 4 ) increases with increasing boron chain condensation in vanadium borides: Using a -23 mV overpotential decrement derived from -0.296 mV (for VB at -10 mA cm -2 current density) and -0.273 mV (for V 3 B 4 ) we accurately predict the overpotential of VB 2 (-0.204 mV) as well as that of unstudied V 2 B 3 (-0.250 mV) and hypothetical "V 5 B 8 " (-0.227 mV). We then derived an exponential equation that predicts the overpotentials of known and hypothetical V x B y phases containing at least a boron chain. These results provide a direct correlation between crystal structure and HER activity, thus paving the way for the design of even better electrocatalytic materials through structure-activity relationships., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Abundant transition metal borides are emerging as substitute electrochemical hydrogen evolution reaction (HER) catalysts for noble metals. Herein, an unusual canonic-like behavior of the c lattice parameter in the AlB 2 -type solid solution Cr 1- x Mo x B 2 (x = 0, 0.25, 0.4, 0.5, 0.6, 0.75, 1) and its direct correlation to the HER activity in 0.5 M H 2 SO 4 solution are reported. The activity increases with increasing x, reaching its maximum at x = 0.6 before decreasing again. At high current densities, Cr 0.4 Mo 0.6 B 2 outperforms Pt/C, as it needs 180 mV less overpotential to drive an 800 mA cm -2 current density. Cr 0.4 Mo 0.6 B 2 has excellent long-term stability and durability showing no significant activity loss after 5000 cycles and 25 h of operation in acid. First-principles calculations have correctly reproduced the nonlinear dependence of the c lattice parameter and have shown that the mixed metal/B layers, such as (110), promote hydrogen evolution more efficiently for x = 0.6, supporting the experimental results., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Two different boron layers, flat (graphene-like) and puckered (phosphorene-like), found in the crystal structure of Mo 2 B 4 show drastically different activities for hydrogen evolution, according to Gibbs free energy calculations of H-adsorption on Mo 2 B 4 . The graphene-like B layer is highly active, whereas the phosphorene-like B layer performs very poorly for hydrogen evolution. A new Sn-flux synthesis permits the rapid single-phase synthesis of Mo 2 B 4 , and electrochemical analyses show that it is one of the best hydrogen evolution reaction active bulk materials with good long-term cycle stability under acidic conditions. Mo 2 B 4 compensates its smaller density of active sites if compared with highly active bulk MoB 2 (which contains only the more active graphene-like boron layers) by a 5-times increase of its surface area.

The new quaternary boride series FeRh 6-n Ru n B 3 (n = 1-5) was synthesized by arc melting and characterized by powder and single-crystal X-ray diffraction (XRD), energy-dispersive X-ray analysis, and superconducting quantum interference device magnetometry. Single-crystal structure refinement showed the distribution of the iron atoms in two of three possible crystallographic 4d metal sites in the structure (Th 7 Fe 3 -type, space group P6 3 mc). Rietveld refinements of the powder XRD data indicated single-phase synthesis of all the members. A linear decrease of the lattice parameters and the unit cell volume with increasing Ru content was found, indicating Vegard's behavior. Susceptibility measurements show decreasing Curie temperature and magnetic moment (μ a 5T ) recorded at 5 T with increasing Ru content from T C = 295 K and μ a 5T = 3.35 μ B (FeRh 5 RuB 3 ) to T C = 205 K and μ a 5T = 0.70 μ B (FeRhRu 5 B 3 ). The measured coercivities lie between 1.0 and 2.2 kA/m indicating soft to semihard magnetic materials.