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.

Computational and Experimental Investigations of Osmium-Rich Borides Hf2MOs5B2 (M = Mn, Fe, Co): From Spin Glass to Room-Temperature Magnetic Behaviors.

The metal borides, Hf2MOs5B2 (M = Mn, Fe, Co), which are the first Os-rich quaternary variants of the prolific Ti3Co5B2 structure type, were investigated computationally and experimentally. In their crystal structures, osmium builds a network of prisms, in which the other elements are located. The magnetic M elements are found in face-connected Os8 square prisms leading to M-chains with intra- and interchain distances of about 3.0 and 6.5 Å, respectively. Density functional theory (DFT) showed that magnetic ordering is hugely favored for M = Mn and Fe but only slightly favored for M = Co. Experimental investigations then confirmed and extended the DFT predictions as a metamagnetic behavior was found for the M = Mn and Fe phases, whereby the antiferromagnetic interactions (TN = 19 and 90 K) found at low magnetic fields change to ferromagnetic at higher fields. A very broad transition (TN = 45 K) is found for M = Co, suggesting spin-glass behavior for this phase. For M = Fe, a hard-magnet hysteresis at 5 K is found with a 40 kA/m coercivity, and even at room temperature, a significant hysteresis is found. This study paves the way for the discovery of Os-based magnets in this structure type and other intermetallics.

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 ß-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 ß-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-Mo0.7W0.3B2 ,which indeed proved to be a better HER electrocatalyst than α-MoB2. Using the isoelectronic Cr instead of W led to the α-Cr1-xMoxB2 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-Cr0.4Mo0.6B2 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 VxBy 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.

Fe- and B-Chains in the Ti5-xFe1-yOs6+x+yB6 Structure Type Derived from Chemical Twinning of the Nb1-xOs1+xB Type: Experimental and Computational Investigations.

The complex metal-rich boride Ti5-xFe1-yOs6+x+yB6 (0 < x,y < 1), crystallizing in a new structure type (space group Cmcm, no. 63), was prepared by arc-melting. The new structure contains both isolated boron atoms and zigzag boron chains (B-B distance of 1.74 Å), a rare combination among metal-rich borides. In addition, the structure also contains Fe-chains running parallel to the B-chains. Unlike in previously reported structures, these Fe-chains are offset from each other and arranged in a triangular manner with intrachain and interchain distances of 2.98 and 6.69 Å, respectively. Density functional theory (DFT) calculations predict preferred ferromagnetic interactions within each chain but only small energy differences for different magnetic interactions between them, suggesting a potentially weak long-range order. This new structure offers the opportunity to study new configurations and interactions of magnetic elements for the design of magnetic materials.

Triangular Arrangement of Ferromagnetic Iron Chains in the High-TC Ferromagnet TiFe1-x Os2+x B2.

Transition-metal borides (TMBs) containing Bn -fragment (n>3) have recently gained interest for their ability to enable exciting magnetic materials. Herein, we show that the B4 -containing TiFe0.64(1) Os2.36(1) B2 is a new ferromagnetic TMB with a Curie temperature of 523(2) K and a Weiss constant of 554(3) K, originating from the chain of M3 -triangles (M=64 %Fe+36 %Os). The new phase was synthesized from the elements by arc-melting, and its structure was elucidated by single-crystal X-ray diffraction. It belongs to the Ti1+x Os2-x RuB2 -type structure (space group P 6 ‾ 2 m, no. 189) and contains trigonal-planar B4 boron fragments [B-B distance of 1.87(4) Å] interacting with M3 -triangles [M-M distances of 2.637(8) Šand 3.0199(2) Å]. The experimental results were supported by computational calculations based on the ideal TiFeOs2 B2 composition, which revealed strong ferromagnetic interactions within and between the Fe3 -triangles. This discovery represents the first magnetically ordered Os-rich TMB, thus it will help expand our knowledge of the role of Os in low-dimensional magnetism of intermetallics and enable the design of such materials in the future.

Metal-Mediated Directional Capping of Rod-Packing Metal-Organic Frameworks.

Two new rod-packing metal-organic frameworks (RPMOF) are constructed by regulating the in situ formation of the capping agent. In CPM-s7, carboxylate linkers extend 1D manganese-oxide chains in four additional directions, forming 3D RPMOF. The substitution of Mn2+ with a stronger Lewis acidic Co2+ , leads to an acceleration of the hydrolysis-prone sulfonate linker, resulting in presence of sulfate ions to reduce two out of the four carboxylate-extending directions, and thus forming a new 2D rod-packing CPM-s8. Density functional theory calculations and magnetization measurements reveal ferrimagnetic ordering of CPM-s8, signifying the potential of exploring 2D RPMOF for effective low-dimensional magnetic materials.

Rare-Earth-Free Magnets: Enhancing Magnetic Anisotropy and Spin Exchange Toward High-TC Hf2MIr5B2 (M = Mn, Fe).

Designing new rare-earth-free (REF) permanent magnetic materials (PMM) to replace the high performing but critically restrained rare-earth-based PMM remains a great challenge to the scientific community. Here, we report on the rational design of new REF PMM, Hf2MIr5B2 (M = Fe, Mn) via a theory-experiment combined approach. Density functional theory (DFT) predicted strong interchain M-M spin-exchange coupling and large magnetocrystalline anisotropy energies (EMAE) for the new compounds, suggesting potential intrinsic PMM properties. Subsequent experimental bulk syntheses and magnetic characterizations established the highest ordering temperature (TC ∼ 900 K) for Hf2FeIr5B2 and the highest intrinsic coercivity (HC) value for Hf2MnIr5B2 (HC = 62.1 kA/m) reported to date for Ti3Co5B2-type compounds. Importantly, at room temperature both phases show significant coercivities due to intrinsic factors only, hinting at their huge potential to create REF PMM by improving extrinsic factors such as controlling the microstructure and the domain orientation.

A Delicate Balance between Antiferromagnetism and Ferromagnetism: Theoretical and Experimental Studies of A2 MRu5 B2 (A=Zr, Hf; M=Fe, Mn) Metal Borides.

Metal-rich borides with the Ti3 Co5 B2 -type structure represent an ideal playground for tuning magnetic interactions through chemical substitutions. In this work, density functional theory (DFT) and experimental studies of Ru-rich quaternary borides with the general composition A2 MRu5 B2 (A=Zr, Hf, M=Fe, Mn) are presented. Total energy calculations show that the phases Zr2 FeRu5 B2 and Hf2 FeRu5 B2 prefer ground states with strong antiferromagnetic (AFM) interactions between ferromagnetic (FM) M-chains. Manganese substitution for iron lowers these antiferromagnetic interchain interactions dramatically and creates a strong competition between FM and AFM states with a slight preference for AFM in Zr2 MnRu5 B2 and for FM in Hf2 MnRu5 B2 . Magnetic property measurements show a field dependence of the AFM transition (TN ): TN is found at 0.1 T for all phases with predicted AFM states whereas for the predicted FM phase it is found at a much lower magnetic field (0.005 T). Furthermore, TN is lowest for a Hf-based phase (20 K) and highest for a Zr-based one (28 K), in accordance with DFT predictions of weaker AFM interactions in the Hf-based phases. Interestingly, the AFM transitions vanish in all compounds at higher fields (>1 T) in favor of FM transitions, indicating metamagnetic behaviors for these Ru-rich phases.

Highly Efficient Spin-Orbit Torque and Switching of Layered Ferromagnet Fe3GeTe2.

Among van der Waals (vdW) layered ferromagnets, Fe3GeTe2 (FGT) is an excellent candidate material to form FGT/heavy metal heterostructures for studying the effect of spin-orbit torques (SOT). Its metallicity, strong perpendicular magnetic anisotropy built in the single atomic layers, relatively high Curie temperature (Tc ∼ 225 K), and electrostatic gate tunability offer a tantalizing possibility of achieving the ultimate high SOT limit in monolayer all-vdW nanodevices. In this study, we fabricate heterostructures of FGT/Pt with 5 nm of Pt sputtered onto the atomically flat surface of ∼15-23 nm exfoliated FGT flakes. The spin current generated in Pt exerts a damping-like SOT on FGT magnetization. At ∼2.5 × 1011 A/m2 current density, SOT causes the FGT magnetization to switch, which is detected by the anomalous Hall effect of FGT. To quantify the SOT effect, we measure the second harmonic Hall responses as the applied magnetic field rotates the FGT magnetization in the plane. Our analysis shows that the SOT efficiency is comparable with that of the best heterostructures containing three-dimensional (3D) ferromagnetic metals and much larger than that of heterostructures containing 3D ferrimagnetic insulators. Such large efficiency is attributed to the atomically flat FGT/Pt interface, which demonstrates the great potential of exploiting vdW heterostructures for highly efficient spintronic nanodevices.

Unexpected Correlation Between Boron Chain Condensation and Hydrogen Evolution Reaction (HER) Activity in Highly Active Vanadium Borides: Enabling Predictions.

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 V3 B4 have high electrocatalytic HER activity. Furthermore, we show that the HER activity (in 0.5 m H2 SO4 ) 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 V3 B4 ) we accurately predict the overpotential of VB2 (-0.204 mV) as well as that of unstudied V2 B3 (-0.250 mV) and hypothetical "V5 B8 " (-0.227 mV). We then derived an exponential equation that predicts the overpotentials of known and hypothetical Vx By 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.

Effect of Substitution on the Hysteretic Phase Transition in a Bistable Phenalenyl-Based Neutral Radical Molecular Conductor.

The ability to tune the physical properties of bistable organic functional materials by means of chemistry can facilitate their development for molecular electronic switching components. The butylamine-containing biphenalenyl boron neutral radical, [Bu]2 B, crystalline compound has recently attracted significant attention by displaying a hysteretic phase transition accompanied by simultaneous bistability in magnetic, electrical, and optical properties close to room temperature. In this report, substitutional doping was applied to [Bu]2 B by crystallizing solid solutions of bistable [Bu]2 B and its non-radical-containing counterpart [Bu]2 Be. With increasing doping degree, the hysteretic phase transition is gradually suppressed in terms of reducing the height, but conserves the width of the hysteresis loop as observed through magnetic susceptibility and electrical conductivity measurements. At the critical doping level of about 6 %, the abrupt transformation of the crystal structure to that of the pure [Bu]2 Be crystal packing was observed, accompanied by a complete collapse of the hysteresis loop. Further study of the structure-properties relationships of bistable neutral radical conductors based on the [Bu]2 B host can be conducted utilizing a variety of biphenalenyl-based molecular conductors.

From 3D to 2D: Structural, Spectroscopic and Theoretical Investigations of the Dimensionality Reduction in the [PtAl2 ]δ- Polyanions of the Isotypic MPtAl2 Series (M=Ca-Ba, Eu).

Four new MPtAl2 (M=Ca, Sr, Ba, Eu) compounds, adopting the orthorhombic MgCuAl2 -type structure, have been synthesized from the elements using tantalum ampoules. All compounds are obtained as platelet-shaped crystallites and exhibit an increasing moisture sensitivity with increasing size of the formal M cation. Structural investigations indicate a pronounced elongation of the crystallographic b-axis, which results in a significant distortion of the [PtAl2 ]δ- polyanion. Within the polyanion, layer-like arrangements can be found with bonding Pt-Al interactions within the slab; the increase of the b-axis can be attributed to increasing Al-Al distances and therefore decreasing interactions between the slabs, caused by the differently-sized formal M cations. While the alkaline earth (M=Ca, Sr) representatives exhibit Pauli paramagnetism, BaPtAl2 shows diamagnetic behavior, finally EuPtAl2 is ferromagnetic with TC =54.0(5) K. The effective magnetic moment indicates that the Eu atoms are in a divalent oxidation state, which is confirmed by 151 Eu Mössbauer spectroscopic investigations. Measurements below the Curie-temperature show a full magnetic hyperfine field splitting with Bhf =21.7(1) T. 27 Al and 195 Pt magic-angle spinning NMR spectroscopy corroborates the presence of single crystallographic sites for the Pt and Al atoms. The large 27 Al nuclear electric quadrupolar coupling constants confirm unusually strong electric field gradients, in agreement with the structural distortions and the respective theoretical calculations. X-ray photoelectron spectroscopy has been utilized to investigate the charge transfer within the polyanion. The Pt 4f binding energy decreases with decreasing electronegativity / ionization energy of the alkaline earth elements, suggesting an increasing electron density at the Pt atoms. Theoretical investigations underline the platinide character of the investigated compounds by Bader charge calculations. The analysis of the integrated crystal orbital Hamilton population (ICOHP) values, electron localization function (ELF) and isosurface analyses lead to a consistent structural picture, indicating stable layer-like arrangements of the [PtAl2 ]δ- polyanion.

An Old Dog with New Tricks - Additions to the Cesium Lithium Chloride System: Cs3Li2Cl5 and the Hydrated Cs3LiCl4 · 4H2O.

The CsCl/LiCl system has been studied for over a century now. Numerous phases have been predicted - only three have hitherto been found. We present the synthesis and single-crystal structure of the cesium lithium pentachloride Cs3Li2Cl5, predicted earlier but with a different structure. The anhydrous new phase readily reacts to Cs3LiCl4 · 4H2O in air. The tetrahydrate can also be obtained through the simplest, most inexpensive and green synthesis possible: an immediate, room-temperature mechanosynthesis from only CsCl and LiCl (3 : 1) in air. Differential scanning calorimetry (DSC) and thermogravimetric analyses (TGA), as well as in situ temperature-dependent powder X-ray diffraction studies on this second ever reported ternary alkali chloride hydrate allowed for a revision of the CsCl/LiCl phase diagram. Density of states and total energy calculations further elucidate the new alkali chlorides and update the relative stability of previous structure predictions.

Boron: Enabling Exciting Metal-Rich Structures and Magnetic Properties.

Boron's unique chemical properties and its reactions with metals have yielded the large class of metal borides with compositions ranging from the most boron-rich YB66 (used as monochromator for synchrotron radiation) up to the most metal-rich Nd2Fe14B (the best permanent magnet to date). The excellent magnetic properties of the latter compound originate from its unique crystal structure to which the presence of boron is essential. In general, knowing the crystal structure of any given extended solid is the prerequisite to understanding its physical properties and eventually predicting new synthetic targets with desirable properties. The ability of boron to form strong chemical bonds with itself and with metallic elements has enabled us to construct new structures with exciting properties. In recent years, we have discovered new boride structures containing some unprecedented boron fragments (trigonal planar B4 units, planar B6 rings) and low-dimensional substructures of magnetically active elements (ladders, scaffolds, chains of triangles). The new boride structures have led to new superconducting materials (e.g., NbRuB) and to new itinerant magnetic materials (e.g., Nb6Fe1-xIr6+xB8). The study of boride compounds containing chains (Fe-chains in antiferromagnetic Sc2FeRu5B2), ladders (Fe-ladders in ferromagnetic Ti9Fe2Rh18B8), and chains of triangles (Cr3 chains in ferrimagnetic and frustrated TiCrIr2B2) of magnetically active elements allowed us to gain a deep understanding of the factors (using density functional theory calculations) that can affect magnetic ordering of such low-dimensional magnetic units. We discovered that the magnetic properties of phases containing these magnetic subunits can be drastically tuned by chemical substitution within the metallic nonmagnetic network. For example, the small hysteresis (measure of magnetic energy storage) of Ti2FeRh5B2 can be successively increased up to 24-times by gradually substituting Ru for Rh, a result that was even surpassed (up to 54-times the initial value) for Ru/Ir substitutions. Also, the type of long-range magnetic interactions could be drastically tuned by appropriate substitutions in the metallic nonmagnetic network as demonstrated using both experimental and theoretical methods. It turned out that Ru-rich and valence electron poor metal borides adopting the Ti3Co5B2 or the Th7Fe3 structure types have dominating antiferromagnetic interactions, while in Rh-rich (or Ir-rich) and valence electron rich phases ferromagnetic interactions prevail, as found, for example, in the Sc2FeRu5-xRhxB2 and FeRh6-xRuxB3 series. Fascinatingly, boron clusters (e.g., B6 rings) even directly interact in some cases with the magnetic subunits, an interaction which was found to favor the Fe-Fe magnetic exchange interactions in the ferromagnetic Nb6Fe1-xIr6+xB8. Using less expensive transition metals, we have recently predicted new itinerant magnets, the experimental proof of which is still pending. Furthermore, new structures have been discovered, all of which are being studied experimentally and computationally with the aim of finding new superconductors, magnets, and mechanically hard materials. A new direction is being pursued in our group, as binary and ternary transition metal borides show great promise as efficient water splitting electrocatalysts at the micro- and nanoscale.

Structural-Distortion-Driven Magnetic Transformation from Ferro- to Ferrimagnetic Iron Chains in B6 -based Nb6 FeIr6 B8.

We report on a structural distortion of kinetically stable B6 -based ferromagnetic Nb6 FeIr6 B8 that induces an unprecedented transformation of a ferromagnetic Fe chain into two ferrimagnetic Fe chains through superstructure formation. Density functional theory calculations showed that the ferromagnetic Fe-Fe intrachain interactions found in the undistorted structure become ferrimagnetic in the distorted superstructure, mainly because the two independent iron atoms building each chain interact antiferromagnetically and carry different magnetic moments. High-temperature SQUID magnetometry confirmed ferrimagnetic ordering at 525 K with a high and negative Weiss constant of -972 K indicating the presence of strong antiferromagnetic interactions, as predicted. This finding paves the way for the development of low-dimensional magnetic intermetallic systems based on Heisenberg ferrimagnetic chains, which have previously been studied only in molecular-based compounds.

Graphene- and Phosphorene-like Boron Layers with Contrasting Activities in Highly Active Mo2B4 for Hydrogen Evolution.

Two different boron layers, flat (graphene-like) and puckered (phosphorene-like), found in the crystal structure of Mo2B4 show drastically different activities for hydrogen evolution, according to Gibbs free energy calculations of H-adsorption on Mo2B4. 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 Mo2B4, 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. Mo2B4 compensates its smaller density of active sites if compared with highly active bulk MoB2 (which contains only the more active graphene-like boron layers) by a 5-times increase of its surface area.

Chemical Tuning of Magnetic Properties through Ru/Rh Substitution in Th7Fe3-type FeRh6-nRunB3 (n = 1-5) Series.

The new quaternary boride series FeRh6-nRunB3 (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 (Th7Fe3-type, space group P63mc). 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 (µa5T) recorded at 5 T with increasing Ru content from TC = 295 K and µa5T = 3.35 µB (FeRh5RuB3) to TC = 205 K and µa5T = 0.70 µB (FeRhRu5B3). The measured coercivities lie between 1.0 and 2.2 kA/m indicating soft to semihard magnetic materials.

Unexpected Competition between Antiferromagnetic and Ferromagnetic States in Hf2MnRu5B2: Predicted and Realized.

Materials "design" is increasingly gaining importance in the solid-state materials community in general and in the field of magnetic materials in particular. Density functional theory (DFT) predicted the competition between ferromagnetic (FM) and antiferromagnetic (AFM) ground states in a ruthenium-rich Ti3Co5B2-type boride (Hf2MnRu5B2) for the first time. Vienna ab initio simulation package (VASP) total energy calculations indicated that the FM model was marginally more stable than one of the AFM models (AFM1), indicating very weak interactions between magnetic 1D Mn chains that can be easily perturbated by external means (magnetic field or composition). The predicted phase was then synthesized by arc-melting and characterized as Hf2Mn1-xRu5+xB2 (x = 0.27). Vibrating-scanning magnetometry shows an AFM ground state with TN ≈ 20 K under low magnetic field (0.005 T). At moderate-to-higher fields, AFM ordering vanishes while FM ordering emerges with a Curie temperature of 115 K. These experimental outcomes confirm the weak nature of the interchain interactions, as predicted by DFT calculations.

Network Formation by Condensed Tetrahedral [Au3Al] Units in Na2Au3Al: Crystal and Electronic Structure, Spectroscopic Investigations, and Physical Properties of an Ordered Ternary Auride.

Na2Au3Al, the first experimentally prepared compound in the ternary Na-Au-Al system, crystallizes in the cubic crystal system with space group P4132 (a = 771.42(2) pm). It can be described as a P-centered ternary ordered variant of the F-centered Laves phase MgCu2 and is isostructural to Mo3Al2C. A phase width was found for the series Na2Au4-xAlx allowing a successive substitution of Au by Al. The primitive structure forms for x ≥ 0.5. Na2Au3Al is diamagnetic at room temperature but metallic in nature, as seen from susceptibility and electrical resistivity measurements. Band structure calculations and X-ray photoelectron spectroscopy confirm the metallic nature of the title compound as states are found at the Fermi level of the DOS, along with its "auride" character. 23Na and 27Al solid-state-NMR investigations show the existence of both a disordered (x = 0.5 and 0.75) and a fully ordered (x = 1.0) representative within this series. Both COHP and Bader charge analyses suggest the presence of strong Au-Al interactions forming an anionic [Au3Al]δ- network, with the Na cations occupying the cavities.

Boron-Dependency of Molybdenum Boride Electrocatalysts for the Hydrogen Evolution Reaction.

Molybdenum-based materials have been considered as alternative catalysts to noble metals, such as platinum, for the hydrogen evolution reaction (HER). We have synthesized four binary bulk molybdenum borides Mo2 B, α-MoB, ß-MoB, and MoB2 by arc-melting. All four phases were tested for their electrocatalytic activity (linear sweep voltammetry) and stability (cyclic voltammetry) with respect to the HER in acidic conditions. Three of these phases were studied for their HER activity and by X-ray photoelectron spectroscopy (XPS) for the first time; MoB2 and ß-MoB show excellent activity in the same range as the recently reported α-MoB and ß-Mo2 C phases, while the molybdenum richest phase Mo2 B show significantly lower HER activity, indicating a strong boron-dependency of these borides for the HER. In addition, MoB2 and ß-MoB show long-term cycle stability in acidic solution.