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 ScN₅ phases nitrogen is catenated forming previously unknown N₆⁶⁻ and N₈⁶⁻ units and $${\!\,}_{\infty }{\!\,}^{2}({{{{{\rm{N}}}}}}_{5}^{3-})$$∞ 2 ( N 5 3 − ) anionic corrugated 2D-polynitrogen layers consisting of fused N₁₂ rings. Density functional theory calculations, confirming the dynamical stability of the synthesized compounds, show that Sc₂N₆ and Sc₂N₈ possess an anion-driven metallicity, while ScN₅ is an indirect semiconductor. Sc₂N₆, Sc₂N₈, and ScN₅ solids are promising high-energy-density materials with calculated volumetric energy density, detonation velocity, and detonation pressure higher than those of TNT.

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 YH ₃ 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 the following compounds: Y ₃ H ₁₁ , Y ₂ H ₉ , Y ₄ H ₂₃ , Y ₁₃ H ₇₅ , and Y ₄ H ₂₅ . The study also uncovered a carbide (YC ₂ ) 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.

The field of polyhalogen chemistry, specifically polyhalogen anions (polyhalides), is rapidly evolving. Here, we present the synthesis of three sodium halides with unpredicted chemical compositions and structures (tP10-Na₂Cl₃, hP18-Na₄Cl₅, and hP18-Na₄Br₅), a series of isostructural cubic cP8-AX₃ halides (NaCl₃, KCl₃, NaBr₃, and KBr₃), and a trigonal potassium chloride (hP24-KCl₃). The high-pressure syntheses were realized at 41-80 GPa in diamond anvil cells laser-heated at about 2000 K. Single-crystal synchrotron X-ray diffraction (XRD) provided the first accurate structural data for the symmetric trichloride Cl₃^- anion in hP24-KCl₃ and revealed the existence of two different types of infinite linear polyhalogen chains, [Cl]_∞^n- and [Br]_∞^n-, in the structures of cP8-AX₃ compounds and in hP18-Na₄Cl₅ and hP18-Na₄Br₅. In Na₄Cl₅ and Na₄Br₅, we found unusually short, likely pressure-stabilized, contacts between sodium cations. Ab initio calculations support the analysis of structures, bonding, and properties of the studied halogenides.

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. Decompression 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.

Chemical stability of the alkali halides NaCl and KCl has allowed for their use as inert media in high-pressure high-temperature experiments. Here we demonstrate the unexpected reactivity of the halides with metals (Y, Dy, and Re) and iron oxide (FeO) in a laser-heated diamond anvil cell, thus providing a synthetic route for halogen-containing binary and ternary compounds. So far unknown chlorides, Y₂Cl and DyCl, and chloride carbides, Y₂ClC and Dy₂ClC, were synthesized at ~40 GPa and 2000 K and their structures were solved and refined using in situ single-crystal synchrotron X-ray diffraction. Also, FeCl₂ with the HP-PdF₂-type structure, previously reported at 108 GPa, was synthesized at ~160 GPa and 2100 K. The results of our ab initio calculations fully support experimental findings and reveal the electronic structure and chemical bonding in these compounds.