Diverse Chemistry of Stable Hydronitrogens, and Implications for Planetary and Materials Sciences

Polymeric Hydronitrogen N4H: A Promising High-Energy-Density Material and High-Temperature Superconductor

Solid nitrogen-rich compounds are potential high-energy-density materials (HEDMs). The enormous challenge in this area is to synthesize and stabilize these energetic materials at moderate pressure and better under near-ambient conditions. Here, we perform an extensive theoretical study on hydronitrogens by the reverse design method considering both energies and energy densities. Four hydronitrogens with different stoichiometries, that is, N4H, N3H, N2H, and NH, are found to be stable at pressures of about 80-300 GPa and metastable with pressure releasing to ambient pressure. The energy densities of these hydronitrogens are about 5.6-6.5 kJ/g and 1.3-1.5 times larger than that of trinitrotoluene (TNT). Most importantly, the Pbam phase of the N4H compound is an excellent high-temperature superconductor with a Tc of 37.7 K at 72 GPa. The present findings enrich new phases of hydronitrogens under high pressure and characterize their structural and energetic properties and superconductivity, which offer crucial insights for further design and synthesis of exceptional materials with high energy density and high-temperature superconductivity.

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.

Transformation of Ammonium Azide at High Pressure and Temperature

The compression of ammonium azide (AA) has been considered to be a promising route for producing high energy-density polynitrogen compounds. So far though, there is no experimental evidence that pure AA can be transformed into polynitrogen materials under high pressure at room temperature. We report here on high pressure (P) and temperature (T) experiments on AA embedded in N2 and on pure AA in the range 0-30 GPa, 300-700 K. The decomposition of AA into N2 and NH3 was observed in liquid N2 around 15 GPa-700 K. For pressures above 20 GPa, our results show that AA in N2 transforms into a new crystalline compound and solid ammonia when heated above 620 K. This compound is stable at room temperature and on decompression down to at least 7.0 GPa. Pure AA also transforms into a new compound at similar P-T conditions, but the product is different. The newly observed phases are studied by Raman spectroscopy and X-ray diffraction and compared to nitrogen and hydronitrogen compounds that have been predicted in the literature. While there is no exact match with any of them, similar vibrational features are found between the product that was obtained in AA + N2 with a polymeric compound of N9H formula.

Ab initio study of the miscibility for solid hydrogen-helium mixtures at high pressure

Understanding the behavior of H₂-He binary mixtures at high pressure is of great importance. Two more recent experiments [J. Lim and C. S. Yoo, Phys. Rev. Lett. 120, 165301 (2018) and R. Turnbull et al., ibid. 121, 195702 (2018)] are in conflict, regarding the miscibility between H₂ and He in solids at high pressure. On the basis of first-principles calculations combined with the structure prediction method, we investigate the miscibility for solid H₂-He mixtures at pressures from 0 GPa to 200 GPa. It is found that there is no sign of miscibility and chemical reactivity in H₂-He mixtures with any H:He ratio. Moreover, instead of H₂-He mixtures, the calculated Raman modes of the N-H mixtures can better explain the characteristic peaks observed experimentally, which were claimed to be the H-He vibrational modes. These calculation results are more in line with the experimental findings by Turnbull et al. [Phys. Rev. Lett. 121, 195702 (2018)].

Exotic Hydrogen Bonding in Compressed Ammonia Hydrides

Hydrogen-rich compounds attract significant fundamental and practical interest for their ability to accommodate diverse hydrogen bonding patterns and their promise as superior energy storage materials. Here, we report on an intriguing discovery of exotic hydrogen bonding in compressed ammonia hydrides and identify two novel ionic phases in an unusual stoichiometry NH₇. The first is a hexagonal R3̅ m phase containing NH₃-H⁺-NH₃, H^-, and H₂ structural units stabilized above 25 GPa. The exotic NH₃-H⁺-NH₃ unit comprises two NH₃ molecules bound to a proton donated from a H₂ molecule. Above 60 GPa, the structure transforms to a tetragonal P4₁2₁2 phase comprising NH₄⁺, H^-, and H₂ units. At elevated temperatures, fascinating superionic phases of NH₇ with part-solid and part-liquid structural forms are identified. The present findings advance fundamental knowledge about ammonia hydrides at high pressure with broad implications for studying planetary interiors and superior hydrogen storage materials.

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.

High Pressure and High Temperature Synthesis of the Iron Pernitride FeN2

The high pressure chemistry of transition metals and nitrogen was recently discovered to be richer than previously thought, due to the synthesis of several transition metal pernitrides. Here, we explore the pressure-temperature domain of iron with an excess of nitrogen up to 91 GPa and 2200 K. Above 72 GPa and 2200 K, the iron pernitride FeN₂ is produced in a laser-heated diamond anvil cell. This iron-nitrogen compound is the first with a N/Fe ratio greater than 1. The FeN₂ samples were characterized from the maximum observed pressure down to ambient conditions by powder X-ray diffraction and Raman spectroscopy measurements. The crystal structure of FeN₂ is resolved to be a Pnnm marcasite structure, analogously to other transition metal pernitrides. On the basis of the lattice's axial ratios and the recorded N-N vibrational modes of FeN₂, a bond order of 1.5 for the nitrogen dimer is suggested. The bulk modulus of the iron pernitride is determined to be of K₀ = 344(13) GPa, corresponding to an astounding increase of about 208% from pure iron. Upon decompression to ambient conditions, a partial structural phase transition to the theoretically predicted R3̅ m FeN₂ is detected.

Pressure-induced chemical reactions in the N2(H2)2 compound: from the N2 and H2 species to ammonia and back down into hydrazine

Theory predicts a very rich high pressure chemistry of hydronitrogens with the existence of many N_xH_y compounds. The stability of these phases under pressure is being investigated by the compression of N₂-H₂ mixtures of various compositions. A previous study had disclosed a eutectic-type N₂-H₂ phase diagram with two stoichiometric van der Waals compounds: (N₂)₆(H₂)₇ and N₂(H₂)₂. The structure and pressure induced chemistry of the (N₂)₆(H₂)₇ compound have already been investigated. Here, we determine the structure of the N₂(H₂)₂ compound and characterize using Raman spectroscopy measurements the chemical changes under a pressure cycle up to 60 GPa and back to ambient conditions. A N₂(H₂)₂ single crystal was grown from a 1 : 2 N₂-H₂ mixture and its crystalline structure was solved using synchrotron X-ray diffraction. Similar to the (N₂)₆(H₂)₇ solid, N₂(H₂)₂ has a remarkable host-guest structure containing N₂ molecules orientationally disordered with spherical, ellipsoidal and planar shapes. Above 50 GPa, N₂(H₂)₂ was found to undergo a chemical reaction. The reaction products were determined to be of the azane family, with NH₃ as the main constituent, along with molecular nitrogen. Upon pressure decrease, the reaction products are found to react in such a way that below 10 GPa, hydrazine is the sole azane detected. Observed down to the opening of the diamond anvil cell, the formation of metastable hydrazine instead of the energetically favorable ammonia is puzzling and remains to be elucidated. That could change the current view of Jovian planets' atmospheres in which ammonia is assumed the only stable hydronitrogen molecule.

Stabilization of ammonia-rich hydrate inside icy planets

The interior structure of the giant ice planets Uranus and Neptune, but also of newly discovered exoplanets, is loosely constrained, because limited observational data can be satisfied with various interior models. Although it is known that their mantles comprise large amounts of water, ammonia, and methane ices, it is unclear how these organize themselves within the planets-as homogeneous mixtures, with continuous concentration gradients, or as well-separated layers of specific composition. While individual ices have been studied in great detail under pressure, the properties of their mixtures are much less explored. We show here, using first-principles calculations, that the 2:1 ammonia hydrate, (H₂O)(NH₃)₂, is stabilized at icy planet mantle conditions due to a remarkable structural evolution. Above 65 GPa, we predict it will transform from a hydrogen-bonded molecular solid into a fully ionic phase O^2-([Formula: see text])₂, where all water molecules are completely deprotonated, an unexpected bonding phenomenon not seen before. Ammonia hemihydrate is stable in a sequence of ionic phases up to 500 GPa, pressures found deep within Neptune-like planets, and thus at higher pressures than any other ammonia-water mixture. This suggests it precipitates out of any ammonia-water mixture at sufficiently high pressures and thus forms an important component of icy planets.

Modeling of Extended N-H Solids at High Pressures

The formation of nitrogen-hydrogen networked compounds is a promising approach for obtaining high energy density materials. Multiple experimental reports indicate that the synthesis pressure and temperature of high-energy nitrogen networked compounds significantly decrease when adding hydrogen to nitrogen. One- and two-dimensional structures of nitrogen-hydrogen mixtures are reported to form during synthesis and have also been observed with simulations; however, the structures are not thoroughly established or well understood. Here, we present results of calculations of nitrogen-hydrogen mixtures at pressures up to 50 GPa and predict their structural transformations upon applying and releasing pressure using density functional theory and evolutionary algorithms. Improvements in the computational procedure resulted in efficient on-the-fly elimination of slowly converging structures during the geometry optimization process. This enabled the continuation of long evolution simulations of the nitrogen-hydrogen structures with N/H ratios of 3:1, 4:1, and 9:1 at high pressures (10-50 GPa). New stable crystalline structures with high symmetry and covalent bonds are predicted that have (i) infinite chains and (ii) two-dimensional sheets of nitrogen-hydrogens. The structure with N/H ratio of 4:1 is found to be metallic at 50 GPa. Some crystalline phases stabilized by high pressure may exist as metastable structures with high symmetry and high mass density after lowering the pressure from 50 GPa down to 10 GPa. Vibration modes of calculated Raman and IR spectra are in agreement with published experimental data.

Hexacoordinated nitrogen(V) stabilized by high pressure

In all of its known connections nitrogen retains a valence shell electron count of eight therefore satisfying the golden rule of chemistry - the octet rule. Despite the diversity of nitrogen chemistry (with oxidation states ranging from + 5 to −3), and despite numerous efforts, compounds containing nitrogen with a higher electron count (hypervalent nitrogen) remain elusive and are yet to be synthesized. One possible route leading to nitrogen’s hypervalency is the formation of a chemical moiety containing pentavalent nitrogen atoms coordinated by more than four substituents. Here, we present theoretical evidence that a salt containing hexacoordinated nitrogen(V), in the form of an NF₆⁻ anion, could be synthesized at a modest pressure of 40 GPa (=400 kbar) via spontaneous oxidation of NF₃ by F₂. Our results indicate that the synthesis of a new class of compounds containing hypervalent nitrogen is within reach of current high-pressure experimental techniques.