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Metal carbides are known to contain small carbon units similar to those found in the molecules of methane, acetylene, and allene. However, for numerous binary systems ab initio calculations predict the formation of unusual metal carbides with exotic polycarbon units, [C₆] rings, and graphitic carbon sheets at high pressure (HP). Here we report the synthesis and structural characterization of a HP-CaC₂ polymorph and a Ca₃C7 compound featuring deprotonated polyacene-like and para-poly(indenoindene)-like nanoribbons, respectively. We also demonstrate that carbides with infinite chains of fused [C6] rings can exist even at conditions of deep planetary interiors (~140 GPa and ~3300 K). Hydrolysismore » of high-pressure carbides may provide a possible abiotic route to polycyclic aromatic hydrocarbons in Universe.« less

The yttrium-hydrogen system has gained attention because of near-ambient temperature superconductivity reports in yttrium hydrides at high pressures. We conducted a study using synchrotron single-crystal x-ray diffraction (SCXRD) at 87 to 171 GPa, resulting in the discovery of known (two YH3 phases) and five previously unknown yttrium hydrides. These were synthesized in diamond anvil cells by laser heating yttrium with hydrogen-rich precursors—ammonia borane or paraffin oil. The arrangements of yttrium atoms in the crystal structures of new phases were determined on the basis of SCXRD, and the hydrogen content estimations based on empirical relations and ab initio calculations revealed themore » following compounds: Y₃H₁₁, Y₂H₉, Y₄H₂₃, Y₁₃H₇₅, and Y₄H₂₅. The study also uncovered a carbide (YC₂) and two yttrium allotropes. Complex phase diversity, variable hydrogen content in yttrium hydrides, and their metallic nature, as revealed by ab initio calculations, underline the challenges in identifying superconducting phases and understanding electronic transitions in high-pressure synthesized materials.« less

Abstract Nitrogen catenation under high pressure leads to the formation of polynitrogen compounds with potentially unique properties. The exploration of the entire spectrum of poly- and oligo-nitrogen moieties is still in its earliest stages. Here, we report on four novel scandium nitrides, Sc₂N₆, Sc₂N₈, ScN_5,and Sc₄N₃, synthesized by direct reaction between yttrium and nitrogen at 78-125 GPa and 2500 K in laser-heated diamond anvil cells. High-pressure synchrotron single-crystal X-ray diffraction reveals that in the crystal structures of the nitrogen-rich Sc₂N₆, Sc₂N_8,and ScN₅phases nitrogen is catenated forming previously unknown N₆⁶⁻and N₈⁶⁻units and$$$${\!\,}_{\infty }{\!\,}^{2}({{{{{\rm{N}}}}}}_{5}^{3-})$$$$ ${}_{\infty }more »$ 2 ( N 5 3 − ) anionic corrugated 2D-polynitrogen layers consisting of fused N₁₂rings. Density functional theory calculations, confirming the dynamical stability of the synthesized compounds, show that Sc₂N₆and Sc₂N₈possess an anion-driven metallicity, while ScN₅is an indirect semiconductor. Sc₂N₆, Sc₂N₈, and ScN₅solids are promising high-energy-density materials with calculated volumetric energy density, detonation velocity, and detonation pressure higher than those of TNT.« less

Abstract A series of isostructural Ln₃O₂(CN₃) (Ln=La, Eu, Gd, Tb, Ho, Yb) oxoguanidinates was synthesized under high‐pressure (25–54 GPa) high‐temperature (2000–3000 K) conditions in laser‐heated diamond anvil cells. The crystal structure of this novel class of compounds was determined via synchrotron single‐crystal X‐ray diffraction (SCXRD) as well as corroborated by X‐ray absorption near edge structure (XANES) measurements and density functional theory (DFT) calculations. The Ln₃O₂(CN₃) solids are composed of the hitherto unknown CN₃⁵⁻guanidinate anion—deprotonated guanidine. Changes in unit cell volumes and compressibility of Ln₃O₂(CN₃) (Ln=La, Eu, Gd, Tb, Ho, Yb) compounds are found to be dictated by the lanthanide contraction phenomenon. Decompressionmore » experiments show that Ln₃O₂(CN₃) compounds are recoverable to ambient conditions. The stabilization of the CN₃⁵⁻guanidinate anion at ambient conditions provides new opportunities in inorganic and organic synthetic chemistry.« less