Synthesis, Hardness, and Electronic Properties of Stoichiometric VN and CrN

We report synthesis of single-crystal VN and CrN through high-pressure ion-exchange reaction routes. The final products are stoichiometric and have crystallite sizes in the range of 50–120 μm. We also prepared VN and TiN crystals using high-pressure sintering of nitride powders. On the basis of single-crystal indentation testing, the determined asymptotic Vickers hardness for TiN, VN, and CrN is 18 (1), 10 (1), and 16 (1) GPa, respectively. The relatively low hardness in VN indicates that the metallic bonding prevails due to the overfilled metallic σ bonds, although the cation−anion covalent hybridization in this compound is much stronger than that in TiN and CrN. All three nitrides are intrinsically excellent metals at ambient pressure. In particular, VN exhibits superconducting transition at <i>T</i>_c ≈ 7.8 K, which is slightly lower than the reported values for nitrogen-deficient or crystalline-disordered samples due to unsuppressed “spin fluctuation” in the well-crystallized stoichiometric VN. The magnetostructural transition in CrN correlates with a metal–metal transition at T_N = 240­(5) K and is accompanied by a ∼40% drop in electrical resistivity. In addition, more detailed electronic properties are presented with new insights into these nitrides

Comparative Study on Properties, Structural Changes, and Isomerization of Cis/Trans-Stilbene under High Pressure

The comparison of different stereoisomeric organic compounds under high pressure has been less investigated. Here, we chose different stereochemical configurations of cis/trans-stilbene to study the luminescence properties, polymerization reaction, and structural changes at 0–20 GPa by spectroscopy and XRD. No fluorescence enhancement occurred in cis-stilbene due to π–π stacking. At 16 GPa, the IR, UV–vis, and sample color changes show that it undergoes an irreversible polymerization, that C(sp2)–H changes to C(sp2 + sp3)–H. However, trans-stilbene undergoes fluorescence enhancement at 0–4 GPa due to the reduction of the torsion angle of the benzene ring and the CC bond leading to the formation of rigid planar molecules, which is further confirmed by the IR and XRD results. At 8 GPa, the new peaks in UV–vis and XRD results show the formation of new substances by structural change. However, the structure of trans-stilbene is more stable, which leads to the return to the raw state after releasing the pressure, and a reversible transformation occurs at high pressure. The cis-trans isomerization under high pressure was also briefly investigated by combining heating and laser irradiation. The cis → trans-stilbene transition can only happen under a fixed-range light irradiation, and the trans → cis-stilbene transition could not happen even under irradiation with a 360 nm laser, which may provide a new idea for synthesizing trans isomers with a higher purity

Phase-Transition Induced Elastic Softening and Band Gap Transition in Semiconducting PbS at High Pressure

We have investigated the crystal structure and phase stability, elastic incompressibility, and electronic properties of PbS based on high-pressure neutron diffraction, in-situ electrical resistance measurements, and first-principles calculations. The refinements show that the orthorhombic phase is structurally isotypic with indium iodide (InI) adopting a <i>Cmcm</i> structure (<i>B</i>33). The cubic-to-orthorhombic transition occurs at ∼2.1(1) GPa with a 3.8% volume collapse and a positive Clausius–Clapeyron slope. Phase-transition induced elastic softening is also observed, which is presumably attributed to the enhanced metallic bonding in the <i>B</i>33 phase. On the basis of band structure simulations, the cubic and orthorhombic phases are typical of direct and indirect semiconductors with band gaps of 0.47(1) and 1.04(1) eV, respectively, which supports electrical resistivity measurements of an abrupt jump at the structural transition. On the basis of the resolved structure for <i>B</i>33, the phase transition paths for <i>B</i>1→<i>B</i>33→<i>B</i>2 involve translation of a trigonal prism in <i>B</i>1 and motion of the next-nearest neighbor Pb atom into {SPb₇} coordination and subsequent lattice distortion in the <i>B</i>33 phase

Carbonization of Ethylenediamine Coimpregnated CoMo/Al₂O₃ Catalysts Sulfided by Organic Sulfiding Agent

Coimpregnating binary cobalt/molybdenum/alumina (CoMo/Al₂O₃) catalyst with adding ethylenediamine was studied for carbonization, sulfidation, and hydrodesulfurization using experimental methods. In order to understand the mechanism of carburization of active phases, theoretical CoMo/Carbon models were produced using density functional theory (DFT) method. The results from carbonization of the organic component indicate that the formation of support-like carbon species provokes dispersion of active particles and reduces interaction with support at the active sites (Co, Mo), thus enhancing the HDS activity. Theoretical results from DFT show that carbide-like Co–Mo–C structures are more stable, which can be formed by a simultaneous carburization and sulfidation at an unsaturated S or Mo edge of a (Co)­MoS_<i>x</i> slab with CH₃SSCH₃ as both carbon and sulfur source

Revisit of Pressure-Induced Phase Transition in PbSe: Crystal Structure, and Thermoelastic and Electrical Properties

Lead selenide, PbSe, an important lead chalcogenide semiconductor, has been investigated using <i>in-situ</i> high-pressure/high-temperature synchrotron X-ray diffraction and electrical resistivity measurements. For the first time, high-quality X-ray diffraction data were collected for the intermediate orthorhombic PbSe. Combined with <i>ab initio</i> calculations, we find a <i>Cmcm</i>, InI-type symmetry for the intermediate phase, which is structurally more favorable than the <i>anti</i>-GeS-type <i>Pnma</i>. At room temperature, the onset of the cubic–orthorhombic transition was observed at ∼3.5 GPa with a ∼3.4% volume reduction. At an elevated temperature of 1000 K, the reversed orthorhombic-to-cubic transition was observed at 6.12 GPa, indicating a positive Clapeyron slope for the phase boundary. Interestingly, phase-transition induced elastic softening in PbSe was also observed, which can be mainly attributed to the loosely bonded trigonal prisms along the <i>b</i>-axis in the <i>Cmcm</i> structure. In a comparison with the cubic phase, orthorhombic PbSe exhibits a large negative pressure dependence of electrical resistivity. In addition, thermoelastic properties of orthorhombic PbSe have been derived from isothermal compression data, such as the temperature derivative of bulk modulus and thermally induced pressure

Tuning of Interlayer Interaction in MoS₂–WS₂ van der Waals Heterostructures Using Hydrostatic Pressure

Van der Waals heterostructures have recently attracted great interest of the scientific community due to their rich exotic physical properties and extensive application prospects. Therefore, we conducted pressure-dependent Raman and photoluminescence spectroscopic studies on MoS2–WS2 heterostructures in different twist angles (24.5 and 54°). Thus, it was confirmed that as the interlayer interaction increases under pressure, an electronic phase transition and a structural phase transition due to layer sliding are observed at ∼1.8 and ∼3.8 GPa in the HS-24.5° structures, while no phase transition is observed in the HS-54° structures. As a result of a larger tunable interlayer space in HS-24.5° structures, optical properties of HS-24.5° structures are more pressure-sensitive than those of the HS-54° structure. It is expected that this work will help comprehensively establish the correlation between the interlayer interactions and optical properties of vdW HSs at the atomic level. Understanding this correlation is crucial for the development of new excitonic devices

Synthesis, Crystal Structure, and Elastic Properties of Novel Tungsten Nitrides

Among transition metal nitrides, tungsten nitrides possess unique and/or superior chemical, mechanical, and thermal properties. Preparation of these nitrides, however, is challenging because the incorporation of nitrogen into tungsten lattice is thermodynamically unfavorable at atmospheric pressure. To date, most materials in the W–N system are in the form of thin films produced by nonequilibrium processes and are often poorly crystallized, which severely limits their use in diverse technological applications. Here we report synthesis of tungsten nitrides through new approaches involving solid-state ion exchange and nitrogen degassing under pressure. We unveil a number of novel nitrides including hexagonal and rhombohedral W₂N₃. The final products are phase-pure and well-crystallized in bulk forms. For hexagonal W₂N₃, hexagonal WN, and cubic W₃N₄, they exhibit elastic properties rivaling or even exceeding cubic-BN. All four nitrides are prepared at a moderate pressure of 5 GPa, the lowest among high-pressure synthesis of transition metal nitrides, making it practically feasible for massive and industrial-scale production