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

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