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The ζ-N₂ phase is key for comprehending the pressure-driven molecular to polymeric shift in nitrogen. Here, the authors resolved the crystal structure of ζ-N₂ and identified a gradual delocalization of its electronic density under pressure, culminating in the initiation of nitrogen’s polymerization.

Aromatic polynitrogen units can display both high stability and high energy content. A hexazine anion has now been identified in a complex compound, K₉N₅₆, which is formed at high pressures and temperatures under laser-heating in a diamond anvil cell. The [N₆]⁴⁻ ring is planar and proposed to be aromatic.

The lanthanum-hydrogen system has attracted attention following the observation of superconductivity in LaH₁₀ at near-ambient temperatures and high pressures. Here authors describe the high-pressure syntheses of seven La-H phases; they report crystal structures and remarkable regularities in rare-earth element hydrides.

Polynitrogen compounds are potentially promising high energy density materials, but are difficult to synthesize due to their instability. Here, the authors observe the formation, under high pressure, of a Mg₂N₄ magnesium–tetranitrogen salt which remains stable at ambient conditions.

The authors present nuclear magnetic resonance data on lanthanum superhydrides (LaHx) under high pressure. They find that the hydrogen is in a highly diffusive state which results in a dynamic de-hydrogenation of the sample on a time scale of weeks.

The authors demonstrate that carbides with infinite chains of fused [C6] and [C5] rings are synthesized at deep planetary pressures and temperatures. Hydrolysis of these carbides may lead to polycyclic aromatic hydrocarbons in the Universe.

The chemical stability of alkali halides has caused them to be exploited as inert media in high-pressure, high-temperature experiments. Here, NaCl and KCl are unexpectedly found to react with yttrium, dysprosium and iron oxide in a laser-heated diamond anvil cell, producing Y₂Cl, DyCl, Y₂ClC and Dy₂ClC at ~40 GPa and 2000 K and FeCl₂ at ~160 GPa and 2100 K.

The authors discover high-pressure Sc₂N₆, Sc₂N₈, and ScN₅ polynitrides, synthesized at high pressure and temperature conditions, featuring a unique nitrogen catenation and previously unknown N₆⁶⁻ anions, N₈⁶⁻ anions and anionic corrugated 2D-polynitrogen layers.

Pressures of up to 900 gigapascals (9 million atmospheres) are achieved in a laser-heated double-stage diamond cell, enabling the synthesis of Re₇N₃, and materials characterization is performed in situ using single-crystal X-ray diffraction.

The authors use in-situ high pressure nuclear magnetic resonance spectroscopy in diamond anvil cells to show that at all observed H-bond environments undergo a distinct maximum in hydrogen mobility regardless of the structure of the compounds.

Solid hydrogen has increasingly hindered rotation under high pressure, but the effect on spin isomer populations had not been directly probed. Here the authors measure NMR spectra of solid hydrogen up to the megabar, and observe the crossover to a spin 1/2 dipolar system above 70 GPa where distinction between ortho and para spin isomers is lost.

Polycyclic aromatic hydrocarbons, typically wide-band-gap insulators, may transform into metals under compression, offering potential for interesting electronic properties. Here, a pressure-induced insulator-to-semiconductor transition in dicoronylene was demonstrated, achieving a resistivity drop at 23.0 GPa, and a mechanism was proposed for this transition, paving the way for hydrocarbon-based molecular metals.

Structural studies of pyrene, a polycyclic aromatic hydrocarbon, have so far been limited to below 2 GPa. Here, studying the crystal structure of pyrene up to ~35 GPa using in situ single-crystal synchrotron X-ray diffraction in diamond anvil cells, the authors discover two previously unobserved polymorphs, and find that gradual compression results in continuous compaction of molecular packing, eventually leading to a curvature of the molecules.