Novel Stable Compounds in the C-H-O Ternary System at High Pressure

Mixture of hydrogen and methane under planetary interior conditions

We employ first-principles molecular dynamics simulations to provide equation-of-state data, pair distribution functions (PDFs), diffusion coefficients, and band gaps of a mixture of hydrogen and methane under planetary interior conditions as relevant for Uranus, Neptune, and similar icy exoplanets. We test the linear mixing approximation, which is fulfilled within a few percent for the chosen P-T conditions. Evaluation of the PDFs reveals that methane molecules dissociate into carbon clusters and free hydrogen atoms at temperatures greater than 3000 K. At high temperatures, the clusters are found to be short-lived. Furthermore, we calculate the electrical conductivity from which we derive the non-metal-to-metal transition region of the mixture. We also calculate the electrical conductivity along the P-T profile of Uranus [N. Nettelmann et al., Planet. Space Sci., 2013, 77, 143-151] and observe the transition of the mixture from a molecular to an atomic fluid as a function of the radius of the planet. The density and temperature ranges chosen in our study can be achieved using dynamic shock compression experiments and seek to aid such future experiments. Our work also provides a relevant data set for a better understanding of the interior, evolution, luminosity, and magnetic field of the ice giants in our solar system and beyond.

The Crystal Structure of Carbonic Acid

Ubiquitous carbonic acid, H2CO3, a key molecule in biochemistry, geochemistry, and also extraterrestrial chemistry, is known from a plethora of physicochemical studies. Its crystal structure has now been determined from neutron-diffraction data on a deuterated sample in a specially built hybrid clamped cell. At 1.85 GPa, D2CO3 crystallizes in the monoclinic space group P21/c with a = 5.392(2), b = 6.661(4), c = 5.690(1) Å, β = 92.66(3)°, Z = 4, with one symmetry-inequivalent anti-anti shaped D2CO3 molecule forming dimers, as previously predicted. Quantum chemistry evidences π bonding within the CO3 molecular core, very strong hydrogen bonding between the molecules, and a massive influence of the crystal field on all bonds; phonon calculations emphasize the locality of the vibrations, being rather insensitive to the extended structure.

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. On further compression, a unique structural composition, (CH_{4})_{3}(H_{2})_{25}, emerges. This novel structure holds a vast amount of molecular hydrogen and represents the first compound to surpass 50 wt % H_{2}. These compounds, stabilized by nuclear quantum effects, persist over a broad pressure regime, exceeding 160 GPa.

Rules of formation of H-C-N-O compounds at high pressure and the fates of planetary ices

The solar system's outer planets, and many of their moons, are dominated by matter from the H-C-N-O chemical space, based on solar system abundances of hydrogen and the planetary ices [Formula: see text]O, [Formula: see text], and [Formula: see text] In the planetary interiors, these ices will experience extreme pressure conditions, around 5 Mbar at the Neptune mantle-core boundary, and it is expected that they undergo phase transitions, decompose, and form entirely new compounds. While temperature will dictate the formation of compounds, ground-state density functional theory allows us to probe the chemical effects resulting from pressure alone. These structural developments in turn determine the planets' interior structures, thermal evolution, and magnetic field generation, among others. Despite its importance, the H-C-N-O system has not been surveyed systematically to explore which compounds emerge at high-pressure conditions, and what governs their stability. Here, we report on and analyze an unbiased crystal structure search among H-C-N-O compounds between 1 and 5 Mbar. We demonstrate that simple chemical rules drive stability in this composition space, which explains why the simplest possible quaternary mixture HCNO-isoelectronic to diamond-emerges as a stable compound and discuss dominant decomposition products of planetary ice mixtures.

Unusual Chemistry of the C-H-N-O System under Pressure and Implications for Giant Planets

C-H-N-O system is central for organic chemistry and biochemistry and plays a major role in planetary science (dominating the composition of "ice giants" Uranus and Neptune). The inexhaustible chemical diversity of this system at normal conditions explains its role as the basis of all known life, but the chemistry of this system at high pressures and temperatures of planetary interiors is poorly known. Using ab initio evolutionary algorithm USPEX, we performed an extensive study of the phase diagram of the C-H-N-O system at pressures of 50, 200, and 400 GPa and temperatures up to 3000 K. Seven novel thermodynamically stable phases were predicted, including quaternary polymeric crystal C₂H₂N₂O₂ and several new N-O and H-N-O compounds. We describe the main patterns of changes in the chemistry of the C-H-N-O system under pressure and confirm that diamond should be formed at conditions of the middle-ice layers of Uranus and Neptune. We also provide the detailed CH₄-NH₃-H₂O phase diagrams at high pressures, which are important for further improvement of the models of ice giants, and point out that current models are clearly deficient. In particular, in the existing models, Uranus and Neptune are assumed to have identical composition, nearly identical pressure-temperature profiles, and a single convecting middle layer ("mantle") made of a mixture of H₂O/CH₄/NH₃ in the ratio of 56.5:32.5:11. Here, we provide new insights, shedding light into the difference of heat flows from Uranus and Neptune, which require them to have different compositions, pressure-temperature conditions, and a more complex internal structure.

Coexistence of plastic and partially diffusive phases in a helium-methane compound

Helium and methane are major components of giant icy planets and are abundant in the universe. However, helium is the most inert element in the periodic table and methane is one of the most hydrophobic molecules, thus whether they can react with each other is of fundamental importance. Here, our crystal structure searches and first-principles calculations predict that a He3CH4 compound is stable over a wide range of pressures from 55 to 155 GPa and a HeCH4 compound becomes stable around 105 GPa. As nice examples of pure van der Waals crystals, the insertion of helium atoms changes the original packing of pure methane molecules and also largely hinders the polymerization of methane at higher pressures. After analyzing the diffusive properties during the melting of He3CH4 at high pressure and high temperature, in addition to a plastic methane phase, we have discovered an unusual phase which exhibits coexistence of diffusive helium and plastic methane. In addition, the range of the diffusive behavior within the helium-methane phase diagram is found to be much narrower compared to that of previously predicted helium-water compounds. This may be due to the weaker van der Waals interactions between methane molecules compared to those in helium-water compounds, and that the helium-methane compound melts more easily.

Optical and electronic solutions for power stabilization of CO2 lasers

High pressure-temperature conditions can be readily achieved through the laser-heated diamond anvil cell (LH-DAC). A stable laser source is required for reliable in situ measurements of the sample, as the sample is small with a thermal time constant of the order of microseconds. Here, we show that the power instabilities typical of CO₂ gas lasers used in LH-DAC's are ±5% at the second timescale and ∼±50% at the microsecond timescale. We also demonstrate that the pointing instability of the laser requires either a diffuser or an integrating sphere for reliable total power measurements with small sized detectors. We present a simple solution for stabilizing the power of a CO₂ gas laser on the second timescale by the direct modulation of the current across the tube and another solution that stabilizes the power to the microsecond timescale by externally modulating the CO₂ laser beam. Both solutions can achieve a ±0.3% power stability.

High Pressure Hydrocarbons Revisited: From van der Waals Compounds to Diamond

Methane and other hydrocarbons are major components of the mantle regions of icy planets. Several recent computational studies have investigated the high-pressure behaviour of specific hydrocarbons. To develop a global picture of hydrocarbon stability, to identify relevant decomposition reactions, and probe eventual formation of diamond, a complete study of all hydrocarbons is needed. Using density functional theory calculations we survey here all known C-H crystal structures augmented by targeted crystal structure searches to build hydrocarbon phase diagrams in the ground state and at elevated temperatures. We find that an updated pressure-temperature phase diagram for methane is dominated at intermediate pressures by CH 4 :H 2 van der Waals inclusion compounds. We discuss the P-T phase diagram for CH and CH 2 (i.e., polystyrene and polyethylene) to illustrate that diamond formation conditions are strongly composition dependent. Finally, crystal structure searches uncover a new CH 4 (H 2 ) 2 van der Waals compound, the most hydrogen-rich hydrocarbon, stable between 170 and 220 GPa.

Water-carbon dioxide solid phase equilibria at pressures above 4 GPa

A solid phase in the mixed water-carbon dioxide system, previously identified as carbonic acid, was observed in the high-pressure diamond-anvil cell. The pressure-temperature paths of both its melting and peritectic curves were measured, beginning at 4.4 GPa and 165 °C (where it exists in a quadruple equilibrium, together with an aqueous fluid and the ices H₂O(VII) and CO₂(I)) and proceeding to higher pressures and temperatures. Single-crystal X-ray diffraction revealed a triclinic crystal with unit cell parameters (at 6.5 GPa and 20 °C) of a = 5.88 Å, b = 6.59 Å, c = 6.99 Å, α = 88.7°, β = 79.7°, and γ = 67.7°. Raman spectra exhibit a major line at ~1080 cm⁻¹ and lattice modes below 300 cm⁻¹.

Prediction of superconducting ternary hydride MgGeH6: from divergent high-pressure formation routes

Invigorated by the high temperature superconductivity in some binary hydrogen-dominated compounds, we systematically explored high-pressure phase diagrams and superconductivity of a ternary Mg–Ge–H system usingab initiomethods.