Crystal Structure Refinement and Bonding Patterns of CrB₄: A Boron-Rich Boride with a Framework of Tetrahedrally Coordinated B Atoms

Crystals of chromium tetraboride, a recently proposed candidate superhard material, have been grown for the first time to allow for a first structure refinement of the compound [orthorhombic, space group Immm (No. 71), a = 474.82(8) pm, b = 548.56(8) pm, and c = 287.17(4) pm, R value (all data) = 0.018]. The previously proposed structure model is confirmed, and accurate interatomic distances are presented for the first time. First-principles electronic structure calculations emphasize the unique framework of three-dimensionally linked B atoms that are tetrahedrally coordinated and carry a slightly negative charge. All B–B bonding is of the 2-center 2-electron type. CrB4 is metallic with a pseudogap at the Fermi level

Crystal Structure Refinement and Bonding Patterns of CrB₄: A Boron-Rich Boride with a Framework of Tetrahedrally Coordinated B Atoms

Crystals of chromium tetraboride, a recently proposed candidate superhard material, have been grown for the first time to allow for a first structure refinement of the compound [orthorhombic, space group Immm (No. 71), a = 474.82(8) pm, b = 548.56(8) pm, and c = 287.17(4) pm, R value (all data) = 0.018]. The previously proposed structure model is confirmed, and accurate interatomic distances are presented for the first time. First-principles electronic structure calculations emphasize the unique framework of three-dimensionally linked B atoms that are tetrahedrally coordinated and carry a slightly negative charge. All B–B bonding is of the 2-center 2-electron type. CrB4 is metallic with a pseudogap at the Fermi level

Metallic Iron Nanocatalysts for the Selective Acetylene Hydrogenation under Industrial Front-End Conditions

The need for nontoxic, cheap, earth-abundant catalysts, which can be sustainably produced and implemented, is essential to many processes. In this work we present unsupported iron nanoparticles as an efficient catalyst for selective acetylene hydrogenation under industrially relevant front-end conditions. Additionally, the selectivity and the activity of this catalyst can be easily moderated by the addition of carbon monoxide. The iron nanoparticles were prepared in an environment completely free of water or air using condensed ammonia at −78 °C. State of the art X-ray diffraction and scanning electron microscopy were used to determine the crystal structure, morphology, and purity. The catalyst showed stable performance over several experiments and, other than an agglomeration of the unsupported and unstabilized particles, no changes to the catalyst were detected before and after the reactions

Low-Temperature Synthesis and Magnetostructural Transition in Antiferromagnetic, Refractory Nanoparticles: Chromium Nitride, CrN

Nanostructured chromium nitride (CrN), both a hard material and a high-melting compound that is used in the medical industry and for new energy-harvesting applications, was synthesized phase-pure for the first time via low-temperature solution synthesis in liquid ammonia. TEM analysis confirms the nanoscale character of CrN. The antiferromagnetic properties of the agglomerates of nanoparticles are discussed in comparison to literature data on the bulk materials. SQUID and DSC measurements show the transition from paramagnetic to antiferromagnetic at 258.5 K. In situ low-temperature X-ray diffraction patterns confirm the magnetostructural phase transition at this temperature, not seen before for nanoscale CrN. This structural distortion was calculated earlier to be driven by magnetic stress. The bottom-up synthesis of CrN allows for the production of nearly oxygen- and carbon-free and highly dispersed fine particles

Nanoscale Iron Nitride, ε‑Fe₃N: Preparation from Liquid Ammonia and Magnetic Properties

ε-Fe3N shows interesting magnetism but is difficult to obtain as a pure and single-phase sample. We report a new preparation method using the reduction of iron­(II) bromide with elemental sodium in liquid ammonia at −78 °C, followed by annealing at 573 K. Nanostructured and monophasic oxygen-free iron nitride, ε-Fe3N, is produced according to X-ray diffraction and transmission electron microscopy experiments. The magnetic properties between 2 and 625 K were characterized using a vibrating sample magnetometer, revealing soft ferromagnetic behavior with a low-temperature average moment of 1.5 μB/Fe and a Curie temperature of 500 K. TC is lower than that of bulk ε-Fe3N (575 K) [Chem. Phys. Lett 2010, 493, 299], which corresponds well with the small particle size within the agglomerates (15.4 (±4.1) nm according to TEM, 15.8(1) according to XRD). Samples were analyzed before and after partial oxidation (Fe3N–FexOy core–shell nanoparticles with a 2–3 nm thick shell) by X-ray diffraction, transmission electron microscopy, electron energy-loss spectroscopy, and magnetic measurements. Both the pristine Fe3N nanoparticles and the oxidized core–shell particles showed shifting and broadening of the magnetic hysteresis loops upon cooling in a magnetic field

Discovery of γ‑MnP₄ and the Polymorphism of Manganese Tetraphosphide

A new polymorph of MnP₄ was prepared by reaction of the elements via chemical vapor transport with iodine as transporting agent. The crystal structure was refined using single-crystal diffraction data (space group <i>Cc</i>, no. 9, <i>a</i> = 5.1049(8) Å, <i>b</i> = 10.540(2) Å, <i>c</i> = 10.875(2) Å, β = 93.80(2)°). The phase is called γ-MnP₄ as it is isostructural with γ-FeP₄. It is the fourth reported binary polymorph in the MnP₄ system, all of which are stacking variants of nets built with manganese and phosphorus atoms. In γ-MnP₄, there are two Mn–Mn distances (2.93 and 3.72 Å) arising from a Peierls-like distortion effectively forming Mn₂ dumbbells in the structure. Magnetic and electrical conductivity measurements show diamagnetism and a small anisotropic band gap (100–200 meV) with significantly enhanced conductivity along the crystallographic <i>a</i> axis. Calculations of the electronic and vibrational (phonon) structures show the P–P and Mn–P bonds within the nets are mainly responsible for the stability of the phase. The similar bonding motifs of the polymorphs give rise to the existence of numerous dynamically stable variants. The calculated Helmholtz energy shows the polymorph formation to be closely tied to temperature with the 6-MnP₄ structure favorable at low temperatures, the 2-MnP₄ favorable between approximately 800 and 2000 K, and 8-MnP₄ preferred at high temperatures

Metastable Ni₇B₃: A New Paramagnetic Boride from Solution Chemistry, Its Crystal Structure and Magnetic Properties

We trapped an unknown metastable boride by applying low-temperature solution synthesis. Single-phase nickel boride, Ni₇B₃, was obtained as bulk samples of microcrystalline powders when annealing the amorphous, nanoscale precipitate that is formed in aqueous solution of nickel chloride upon reaction with sodium tetrahydridoborate. Its crystal structure was solved based on a disordered Th₇Fe₃-type model (hexagonal crystal system, space group <i>P</i>6₃<i>mc</i>, no. 186, <i>a</i> = 696.836(4) pm, <i>c</i> = 439.402(4) pm), using synchrotron X-ray powder data. Magnetic measurements reveal paramagnetism, which is in accordance with quantum chemical calculations. According to high-temperature X-ray diffraction and differential scanning calorimetry this nickel boride phase has a narrow stability window between 300 and 424 °C. It crystallizes at ca. 350 °C, then starts decomposing to form Ni₃B and Ni₂B above 375 °C, and shows an exothermic effect at 424 °C

Possible Superhardness of CrB₄

Chromium tetraboride [orthorhombic, space group <i>Pnnm</i> (No. 58), <i>a</i> = 474.65(9) pm, <i>b</i> = 548.0(1) pm, <i>c</i> = 286.81(5) pm, and <i>R</i> value (all data) = 0.041], formerly described in space group <i>Immm</i>, was found not to be superhard, despite several theory-based prognoses. CrB₄ shows an almost temperature-independent paramagnetism, consistent with low-spin Cr^I in a metallic compound. Conductivity measurements confirm the metallic character

From MAX Phase Carbides to Nitrides: Synthesis of V₂GaC, V₂GaN, and the Carbonitride V₂GaC_1–<i>x</i>N<i>_x</i>

The research in MAX phases is mainly concentrated on the investigation of carbides rather than nitrides (currently >150 carbides and only <15 nitrides) that are predominantly synthesized by conventional solid-state techniques. This is not surprising since the preparation of nitrides and carbonitrides is more demanding due to the high stability and low diffusion rate of nitrogen-containing compounds. This leads to several drawbacks concerning potential variations in the chemical composition of the MAX phases as well as control of morphology, the two aspects that directly affect the resulting materials properties. Here, we report how alternative solid-state hybrid techniques solve these limitations by combining conventional techniques with nonconventional precursor synthesis methods, such as the “urea–glass” sol–gel or liquid ammonia method. We demonstrate the synthesis and morphology control within the V–Ga–C–N system by preparing the MAX phase carbide and nitridethe latter in the form of bulkier and more defined smaller particle structuresas well as a hitherto unknown carbonitride V2GaC1–xNx MAX phase. This shows the versatility of hybrid methods starting, for example, from wet chemically obtained precursors that already contain all of the ingredients needed for carbonitride formation. All products are characterized in detail by X-ray powder diffraction, electron microscopy, and electron and X-ray photoelectron spectroscopies to confirm their structure and morphology and to detect subtle differences between the different chemical compositions