Synthesis of NaWH and NaReH Ternary Hydrides at High Pressures.

The Na-W-H and Na-Re-H ternary systems were studied in a diamond anvil cell through X-ray diffraction and Raman spectroscopy, supported by density functional theory and molecular dynamics calculations. NaWH can be synthesized above 7.8 GPa and 1400 K, remaining stable between at least 0.

High pressure study of sodium trihydride.

The reactivity between NaH and H has been investigated through a series of high-temperature experiments up to pressures of 78 GPa in diamond anvil cells combined with calculations. Powder X-ray diffraction measurements show that heating NaH in an excess of H to temperatures around 2000 K above 27 GPa yields sodium trihydride (NaH), which adopts an orthorhombic structure (space group ). Raman spectroscopy measurements indicate that NaH hosts quasi-molecular hydrogen () within a NaH lattice, with the stretching mode downshifted compared to pure H (Δ ∼-120 cm at 50 GPa).

Pressure-Induced Metallization of BaH and the Effect of Hydrogenation.

Using optical spectroscopy, X-ray diffraction, and electrical transport measurements, we have studied the pressure-induced metallization in BaH and BaH. Our combined measurements suggest a structural phase transition from BaH-II to BaH-III accompanied by band gap closure and transformation to a metallic state at 57 GPa. The metallization is confirmed by resistance measurements as a function of the pressure and temperature.

Synthesis and characterization of XeAr2 under high pressure.

The binary Xe-Ar system has been studied in a series of high pressure diamond anvil cell experiments up to 60 GPa at 300 K. In-situ x-ray powder diffraction and Raman spectroscopy indicate the formation of a van der Waals compound, XeAr2, at above 3.5 GPa.

H Chemical Bond in a High-Pressure Crystalline Environment.

We show that the hydrogen in metal superhydride compounds can adopt two distinct states-atomic and molecular. At low pressures, the maximum number of atomic hydrogens is typically equal to the valency of the cation; additional hydrogens pair to form molecules with electronic states far below the Fermi energy causing low-symmetry structures with large unit cells. At high pressures, molecules become unstable, and all hydrogens become atomic.

Chemically Assisted Precompression of Hydrogen Molecules in Alkaline-Earth Tetrahydrides.

Through a series of high pressure diamond anvil experiments, we report the synthesis of alkaline earth (Ca, Sr, Ba) tetrahydrides, and investigate their properties through Raman spectroscopy, X-ray diffraction, and density functional theory calculations. The tetrahydrides incorporate both atomic and quasi-molecular hydrogen, and we find that the frequency of the intramolecular stretching mode of the units downshifts from Ca to Sr and to Ba upon compression. The experimental results indicate that the larger the host cation, the longer the bond.

Formation and Stability of Dense Methane-Hydrogen Compounds.

Through a series of x-ray diffraction, optical spectroscopy diamond anvil cell experiments, combined with density functional theory calculations, we explore the dense CH_{4}-H_{2} system. We find that pressures as low as 4.8 GPa can stabilize CH_{4}(H_{2})_{2} and (CH_{4})_{2}H_{2}, with the latter exhibiting extreme hardening of the intramolecular vibrational mode of H_{2} units within the structure.

In-situ abiogenic methane synthesis from diamond and graphite under geologically relevant conditions.

Diamond and graphite are fundamental sources of carbon in the upper mantle, and their reactivity with H-rich fluids present at these depths may represent the key to unravelling deep abiotic hydrocarbon formation. We demonstrate an unexpected high reactivity between carbons' most common allotropes, diamond and graphite, with hydrogen at conditions comparable with those in the Earth's upper mantle along subduction zone thermal gradients. Between 0.

Superionicity, disorder, and bandgap closure in dense hydrogen chloride.

Hydrogen bond networks play a crucial role in biomolecules and molecular materials such as ices. How these networks react to pressure directs their properties at extreme conditions. We have studied one of the simplest hydrogen bond formers, hydrogen chloride, from crystallization to metallization, covering a pressure range of more than 2.

High-temperature phase transitions in dense germanium.

Through a series of high-pressure x-ray diffraction experiments combined with in situ laser heating, we explore the pressure-temperature phase diagram of germanium (Ge) at pressures up to 110 GPa and temperatures exceeding 3000 K. In the pressure range of 64-90 GPa, we observe orthorhombic Ge-IV transforming above 1500 K to a previously unobserved high-temperature phase, which we denote as Ge-VIII. This high-temperature phase is characterized by a tetragonal crystal structure, space group I4/mmm.

Synthesis of Weaire-Phelan Barium Polyhydride.

By combining pressures up to 50 GPa and temperatures of 1200 K, we synthesize the novel barium hydride, BaH, stable down to 27 GPa. We use Raman spectroscopy, X-ray diffraction, and first-principles calculations to determine that this compound adopts a highly symmetric structure with an unusual 5:1 hydrogen-to-barium ratio. This singular stoichiometry corresponds to the well-defined type-I clathrate geometry.

High-Pressure Insertion of Dense H into a Model Zeolite.

Our combined high-pressure synchrotron X-ray diffraction and Monte Carlo modeling studies show super-filling of the zeolite, and computational results suggest an occupancy by a maximum of nearly two inserted H molecules per framework unit, which is about twice that observed in gas hydrates. Super-filling prevents amorphization of the host material up to at least 60 GPa, which is a record pressure for zeolites and also for any group IV element being in full 4-fold coordination, except for carbon. We find that the inserted H forms an exotic topologically constrained glassy-like form, otherwise unattainable in pure hydrogen.

Dynamic Covalent Properties of a Novel Indolo[3,2-b]carbazole Diradical.

This work describes the synthesis and properties of a dicyanomethylene-substituted indolo[3,2-b]carbazole diradical ICz-CN. This quinoidal system dimerises almost completely to (ICz-CN) , which contains two long C(sp )-C(sp ) σ-bonds between the dicyanomethylene units. The minor open-shell ICz-CN component in the solid-state mixture was identified by EPR spectroscopy.

Nuclear spin coupling crossover in dense molecular hydrogen.

One of the most striking properties of molecular hydrogen is the coupling between molecular rotational properties and nuclear spin orientations, giving rise to the spin isomers ortho- and para-hydrogen. At high pressure, as intermolecular interactions increase significantly, the free rotation of H molecules is increasingly hindered, and consequently a modification of the coupling between molecular rotational properties and the nuclear spin system can be anticipated. To date, high-pressure experimental methods have not been able to observe nuclear spin states at pressures approaching 100 GPa (Meier, Annu.

Quantitative Rotational to Librational Transition in Dense H and D.

Raman spectroscopy demonstrates that the rotational spectrum of solid hydrogen, and its isotope deuterium, undergoes profound transformations upon compression while still remaining in phase I. We show that these changes are associated with a loss of quantum character in the rotational modes and that the angular momentum gradually ceases to be a good quantum rotational number. Through isotopic comparisons of the rotational Raman contributions, we reveal that hydrogen and deuterium evolve from a quantum rotor to a harmonic oscillator.

Synthesis of Superconducting Cobalt Trihydride.

The Co-H system has been investigated through high-pressure, high-temperature X-ray diffraction experiments combined with first-principles calculations. On compression of elemental cobalt in a hydrogen medium, we observe face-centered cubic cobalt hydride (CoH) and cobalt dihydride (CoH) above 33 GPa. Laser heating CoH in a hydrogen matrix at 75 GPa to temperatures in excess of ∼800 K produces cobalt trihydride (CoH) which adopts a primitive structure.

A Chichibabin's Hydrocarbon-Based Molecular Cage: The Impact of Structural Rigidity on Dynamics, Stability, and Electronic Properties.

A three-dimensional π-conjugated polyradicaloid molecular cage , consisting of three Chichibabin's hydrocarbon motifs connected by two benzene-1,3,5-triyl bridgeheads, was synthesized. Compared with its linear model compound , the prism-like has a more rigid structure, which shows significant impact on the molecular dynamics, stability, and electronic properties. A higher rotation energy barrier for the quinoidal biphenyl units was determined in (15.

Complex Hydrogen Substructure in Semimetallic RuH.

When compressed in a matrix of solid hydrogen, many metals form compounds with increasingly high hydrogen contents. At high density, hydrogenic sublattices can emerge, which may act as low-dimensional analogues of atomic hydrogen. We show that at high pressures and temperatures, ruthenium forms polyhydride species that exhibit intriguing hydrogen substructures with counterintuitive electronic properties.

High-pressure polymorphism in pyridine.

Single crystals of the high-pressure phases II and III of pyridine have been obtained by crystallization at 1.09 and 1.69 GPa, revealing the crystal structure of phase III for the first time using X-ray diffraction.

Linear, Non-Conjugated Cyclic and Conjugated Cyclic Paraphenylene under Pressure.

The -paraphenylene family comprises chains of phenylene units linked together by C-C bonds that are between single- and double-bonded, and where corresponds to the number of phenylene units. In this work, we compare the response of the optical properties of different phenylene arrangements. We study linear chains (LPP), cyclic systems (CPPs), and non-conjugated cyclic systems with two hydrogenated phenylenes (H[n]CPP).

Post-tilleyite, a dense calcium silicate-carbonate phase.

Calcium carbonate is a relevant constituent of the Earth's crust that is transferred into the deep Earth through the subduction process. Its chemical interaction with calcium-rich silicates at high temperatures give rise to the formation of mixed silicate-carbonate minerals, but the structural behavior of these phases under compression is not known. Here we report the existence of a dense polymorph of Ca(SiO)(CO) tilleyite above 8 GPa.

Author Correction: Band gap closure, incommensurability and molecular dissociation of dense chlorine.

The original version of this Article omitted references to previous experimental reports on solid hydrogen that are relevant for a full understanding of the context of the previous work. The added references are: 47. Akahama, Y.

Band gap closure, incommensurability and molecular dissociation of dense chlorine.

Diatomic elemental solids are highly compressible due to the weak interactions between molecules. However, as the density increases the intra- and intermolecular distances become comparable, leading to a range of phenomena, such as structural transformation, molecular dissociation, amorphization, and metallisation. Here we report, following the crystallization of chlorine at 1.

Direct Reaction between Copper and Nitrogen at High Pressures and Temperatures.

Transition-metal nitrides have applications in a range of technological fields. Recent experiments have shown that new nitrogen-bearing compounds can be accessed through a combination of high temperatures and pressures, revealing a richer chemistry than was previously assumed. Here, we show that at pressures above 50 GPa and temperatures greater than 1500 K  elemental copper reacts with nitrogen, forming copper diazenide (CuN).