18 Search Results

Transition metal borides are known due to their attractive mechanical, electronic, refractive, and other properties. A new class of rhenium borides was identified by synchrotron single-crystal X-ray diffraction experiments in laser-heated diamond anvil cells between 26 and 75 GPa. Recoverable to ambient conditions, compounds rhenium triboride (ReB₃) and rhenium tetraboride (ReB₄) consist of close-packed single layers of rhenium atoms alternating with boron networks built from puckered hexagonal layers, which link short bonded (~ 1.7 angstrom) axially oriented B₂ dumbbells. The short and incompressible Re-B and B-B bonds oriented along the hexagonal c-axis contribute to low axial compressibility comparable with themore » linear compressibility of diamond. Sub-millimeter samples of ReB₃ and ReB₄ were synthesized in a large-volume press at pressures as low as 33 GPa and used for material characterization. Crystals of both compounds are metallic and hard (Vickers hardness, H_V = 34(3) GPa). Geometrical, crystal-chemical, and theoretical analysis considerations suggest that potential ReB_x compounds with x > 4 can be based on the same principle of structural organization as in ReB₃ and ReB₄ and possess similar mechanical and electronic properties.« less

In this work, the high-pressure structural and vibrational properties of a layered transition metal dichalcogenide 2H-NbSe₂ were investigated using single-crystal x-ray diffraction and Raman spectroscopy, demonstrating its structural stability up to 35 GPa. The lattice compressibility changes character from being highly anisotropic at low pressures to largely isotropic at high pressures. Concomitantly, the interatomic bonds demonstrate highly anisotropic compression behavior with the Se-Se interlayer bonds compressing by >20%, while the intramolecular Se-Se distance shows a nonmonotonic pressure dependence with a maximum at-12 GPa. The nearest-neighbor central force lattice vibrational model yields pressure dependencies of the interatomic forces in qualitative agreementmore » with bond length compression, providing insight into the vibrational properties of 2H-NbSe₂ at high pressures.« less

Hard stoichiometric WB ₄ is synthesized under high-pressure and high-temperature conditions.

Theoretical modelling predicts very unusual structures and properties of materials at extreme pressure and temperature conditions. Hitherto, their synthesis and investigation above 200 gigapascals have been hindered both by the technical complexity of ultrahigh-pressure experiments and by the absence of relevant in situ methods of materials analysis. Here we report on a methodology developed to enable experiments at static compression in the terapascal regime with laser heating. We apply this method to realize pressures of about 600 and 900 gigapascals in a laser-heated double-stage diamond anvil cell, producing a rhenium–nitrogen alloy and achieving the synthesis of rhenium nitride Re₇N₃—which, asmore » our theoretical analysis shows, is only stable under extreme compression. Full chemical and structural characterization of the materials, realized using synchrotron single-crystal X-ray diffraction on microcrystals in situ, demonstrates the capabilities of the methodology to extend high-pressure crystallography to the terapascal regime.« less

Understanding of recently reported putative close-to-room-temperature superconductivity in C-S-H compounds at 267 GPa demands a reproducible synthesis protocol as well as knowledge of the compounds' structure and composition. Here, we synthesized C-S-H compounds with various carbon compositions at high pressures from elemental carbon C and methane CH₄, sulfur S, and molecular hydrogen H₂. Here, we focus on compounds synthesized using methane as these allow a straightforward determination of their structure and composition by combining single-crystal x-ray diffraction and Raman spectroscopy. We applied a two-stage synthesis of [(CH₄)_x(H₂S)_(1-x)]₂H₂ compounds with various compositions by first reacting sulfur and mixed methane-hydrogen fluids andmore » forming CH₄-doped H₂S crystals at 0.5-3 GPa and then by growing single crystals of the desired hydrogen-rich compound. Raman spectroscopy applied to this material shows the presence of CH₄ molecules incorporated into the lattice and allows the determination of the CH₄ content, while single-crystal x-ray diffraction results suggest that the methane molecules substitute H₂S molecules. The structural behavior of these compounds is very similar to the previously investigated methane-free crystals demonstrating a transition from Al₂Cu type I4/mcm molecular crystal to a modulated molecular structure at 20-30 GPa and back to the same basic I4/mcm structure in an extended modification with greatly modified Raman spectra. This latter phase demonstrates a distortion into a Pnma structure at 132-159 GPa and then transforms into a common Im$$\bar{3}$$m H₃S phase at higher pressures; however, no structural anomaly is detected near 220 GPa, where a sharp upturn in T_c has been reported.« less

Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nanoelectronics. In this work we report the synthesis of nickel diazenide NiN₂ which possesses atomic-thick layers comprised of Ni₂N₃ pentagons forming Cairo-type tessellation. The layers of NiN₂ are weakly bonded with the calculated exfoliation energy of 0.72 J/m², which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN₂ ismore » a direct-band-gap semiconductor, while the bulk material is metallic. Furthermore, this indicates the promise of NiN₂ to be a precursor of a pentagonal 2D material with a tunable direct band gap.« less

Three oxygen-free pentazolate salts Na ₂ N ₅ , NaN ₅ and NaN ₅ ·N ₂ were synthesized from sodium azide NaN ₃ and molecular nitrogen N ₂ at 50 GPa.

Alkaline earth metal peroxides are typical examples of ionic compounds containing polyanions. We herein report a stable BaO₂ phase at high pressure up to 130 GPa found via a first-principles computational structure searches and high-pressure experimental investigations. The identified monoclinic structure (space group C2/m) can be derived by sublattice distortions of Ba atoms and peroxide groups associated with the phonon mode softening of the lower-pressure Cmmm structure. Contrary to the previous expectation of polymerization of the peroxide group at elevated pressure, the new phase retains the peroxide group and interestingly exhibits an insulating behavior demonstrating an increase of the bandmore » gap under compression. Our synchrotron XRD measurements could not distinguish between Cmmm and C2/m BaO₂ definitively because the difference in XRD patterns is very subtle. However, our data do not show any sign of polymerization transition up to 120 GPa. Raman spectra of the O-O peroxide vibration show a small anomaly in frequency at 110 GPa, which is qualitatively similar to that predicted theoretically due to the Cmmm to C2/m phase transition thus supporting the predicted transformation.« less

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. In this paper, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN₄. A triclinic phase of beryllium tetranitride tr-BeN₄ was synthesized from elements at similar to 85 GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN₄ layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated pi systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN₄ layer show that its electronic latticemore » is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN₄ layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions.« less

With the exception of Li, alkali metals do not react with elemental nitrogen neither at ambient conditions nor at elevated temperatures, requiring the search for alternative synthetic routes to their nitrogen-containing compounds. Here using a controlled decomposition of sodium azide NaN3 at high pressure conditions we synthesize two novel compounds Na₃(N₂)₄ and NaN₂ both containing dinitrogen anions. NaN₂ synthesized at 4 GPa might be the common intermediate in high-pressure solid-state metathesis reactions where NaN₃ is used as a source of nitrogen, while Na₃(N₂)₄ opens a new class of compounds, where [N₂] units accommodate a non-integer formal charge of -0.75. Thismore » finding can dramatically extend the expected compositions in other group 1-2 metal-nitrogen systems. Electronic structure calculations show the metallic character for both compounds.« less