High-pressure Synthesis of Cobalt Polynitrides: Unveiling Intriguing Crystal Structures and Nitridation Behavior.

In this study, we conduct extensive high-pressure experiments to investigate phase stability in the cobalt-nitrogen system. Through a combination of synthesis in a laser-heated diamond anvil cell, first-principles calculations, Raman spectroscopy, and single-crystal X-ray diffraction, we establish the stability fields of known high-pressure phases, hexagonal NiAs-type CoN, and marcasite-type CoN2 within the pressure range of 50-90 GPa. We synthesize and characterize previously unknown nitrides, Co3N2, Pnma-CoN and two polynitrides, CoN3 and CoN5, within the pressure range of 90-120 GPa. Both polynitrides exhibit novel types of polymeric nitrogen chains and networks. CoN3 feature branched-type nitrogen trimers (N3) and CoN5 show π-bonded nitrogen chain. As the nitrogen content in the cobalt nitride increases, the CoN6 polyhedral frameworks transit from face-sharing (in CoN) to edge-sharing (in CoN2 and CoN3), and finally to isolated (in CoN5). Our study provides insights into the intricate interplay between structure evolution, bonding arrangements, and high-pressure synthesis in polynitrides, expanding the knowledge for the development of advanced energy materials.

A MHz X-ray diffraction set-up for dynamic compression experiments in the diamond anvil cell.

An experimental platform for dynamic diamond anvil cell (dDAC) research has been developed at the High Energy Density (HED) Instrument at the European X-ray Free Electron Laser (European XFEL). Advantage was taken of the high repetition rate of the European XFEL (up to 4.5 MHz) to collect pulse-resolved MHz X-ray diffraction data from samples as they are dynamically compressed at intermediate strain rates (≤103 s-1), where up to 352 diffraction images can be collected from a single pulse train. The set-up employs piezo-driven dDACs capable of compressing samples in ≥340 µs, compatible with the maximum length of the pulse train (550 µs). Results from rapid compression experiments on a wide range of sample systems with different X-ray scattering powers are presented. A maximum compression rate of 87 TPa s-1 was observed during the fast compression of Au, while a strain rate of ∼1100 s-1 was achieved during the rapid compression of N2 at 23 TPa s-1.

Direct measurement of thermal conductivity in solid iron at planetary core conditions.

The conduction of heat through minerals and melts at extreme pressures and temperatures is of central importance to the evolution and dynamics of planets. In the cooling Earth's core, the thermal conductivity of iron alloys defines the adiabatic heat flux and therefore the thermal and compositional energy available to support the production of Earth's magnetic field via dynamo action. Attempts to describe thermal transport in Earth's core have been problematic, with predictions of high thermal conductivity at odds with traditional geophysical models and direct evidence for a primordial magnetic field in the rock record. Measurements of core heat transport are needed to resolve this difference. Here we present direct measurements of the thermal conductivity of solid iron at pressure and temperature conditions relevant to the cores of Mercury-sized to Earth-sized planets, using a dynamically laser-heated diamond-anvil cell. Our measurements place the thermal conductivity of Earth's core near the low end of previous estimates, at 18-44 watts per metre per kelvin. The result is in agreement with palaeomagnetic measurements indicating that Earth's geodynamo has persisted since the beginning of Earth's history, and allows for a solid inner core as old as the dynamo.

Intermolecular coupling and fluxional behavior of hydrogen in phase IV.

We performed Raman and infrared (IR) spectroscopy measurements of hydrogen at 295 K up to 280 GPa at an IR synchrotron facility of the Shanghai Synchrotron Radiation Facility (SSRF). To reach the highest pressure, hydrogen was loaded into toroidal diamond anvils with 30-µm central culet. The intermolecular coupling has been determined by concomitant measurements of the IR and Raman vibron modes. In phase IV, we find that the intermolecular coupling is much stronger in the graphenelike layer (G layer) of elongated molecules compared to the Br2-like layer (B layer) of shortened molecules and it increases with pressure much faster in the G layer compared to the B layer. These heterogeneous lattice dynamical properties are unique features of highly fluxional hydrogen phase IV.

Laser heating system at the Extreme Conditions Beamline, P02.2, PETRA III.

A laser heating system for samples confined in diamond anvil cells paired with in situ X-ray diffraction measurements at the Extreme Conditions Beamline of PETRA III is presented. The system features two independent laser configurations (on-axis and off-axis of the X-ray path) allowing for a broad range of experiments using different designs of diamond anvil cells. The power of the continuous laser source can be modulated for use in various pulsed laser heating or flash heating applications. An example of such an application is illustrated here on the melting curve of iron at megabar pressures. The optical path of the spectroradiometry measurements is simulated with ray-tracing methods in order to assess the level of present aberrations in the system and the results are compared with other systems, that are using simpler lens optics. Based on the ray-tracing the choice of the first achromatic lens and other aspects for accurate temperature measurements are evaluated.

High-Pressure Synthesis of Dirac Materials: Layered van der Waals Bonded BeN_{4} Polymorph.

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN_{4}. A triclinic phase of beryllium tetranitride tr-BeN_{4} was synthesized from elements at ∼85 GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN_{4} layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated π systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN_{4} layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN_{4} layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions.

Ethane and methane at high pressures: Structure and stability.

We have performed a combined experimental and theoretical study of ethane and methane at high pressures of up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopies and the USPEX ab initio evolutionary structural search algorithm, respectively. For ethane, we have determined the crystallization point, for room temperature, at 2.7 GPa and also the low pressure crystal structure (phase A). This crystal structure is orientationally disordered (plastic phase) and deviates from the known crystal structures for ethane at low temperatures. Moreover, a pressure induced phase transition has been identified, for the first time, at 13.6 GPa to a monoclinic phase B, the structure of which is solved based on good agreement with the experimental results and theoretical predictions. For methane, our x-ray diffraction measurements are in agreement with the previously reported high-pressure structures and equation of state (EOS). We have determined the EOSs of ethane and methane, which provides a solid basis for the discussion of their relative stability at high pressures.

Immiscibility in N2-H2O solids up to 140 GPa.

Nitrogen and water are very abundant in nature; however, the way they chemically react at extreme pressure-temperature conditions is unknown. Below 6 GPa, they have been reported to form clathrate compounds. Here, we present Raman spectroscopy and x-ray diffraction studies in the H2O-N2 system at high pressures up to 140 GPa. We find that clathrates, which form locally in our diamond cell experiments above 0.3 GPa, transform into a fine grained state above 6 GPa, while there is no sign of formation of mixed compounds. We point out size effects in fine grained crystallites, which result in peculiar Raman spectra in the molecular regime, but x-ray diffraction shows no additional phase or deviation from the bulk behavior of familiar solid phases. Moreover, we find no sign of ice doping by nitrogen, even in the regimes of stability of nonmolecular nitrogen.

Stabilization of Polynitrogen Anions in Tantalum-Nitrogen Compounds at High Pressure.

The synthesis of polynitrogen compounds is of great importance due to their potential as high-energy-density materials (HEDM), but because of the intrinsic instability of these compounds, their synthesis and stabilization is a fundamental challenge. Polymeric nitrogen units which may be stabilized in compounds with metals at high pressure are now restricted to non-branched chains with an average N-N bond order of 1.25, limiting their HEDM performances. Herein, we demonstrate the synthesis of a novel polynitrogen compound TaN5 via a direct reaction between tantalum and nitrogen in a diamond anvil cell at circa 100 GPa. TaN5 is the first example of a material containing branched all-single-bonded nitrogen chains [N5 5- ]∞ . Apart from that we discover two novel Ta-N compounds: TaN4 with finite N4 4- chains and the incommensurately modulated compound TaN2-x , which is recoverable at ambient conditions.

Enhancement of thermoelectric performance across the topological phase transition in dense lead selenide.

Alternative technologies are required in order to meet a worldwide demand for clean non-polluting energy sources. Thermoelectric generators, which generate electricity from heat in a compact and reliable manner, are potential devices for waste heat recovery. However, thermoelectric performance, as encapsulated by the figure of merit ZT, has remained at around 1.0 at room temperature, which has limited practical applications. Here, we study the effects of pressure on ZT in Cr-doped PbSe, which has a maximum ZT of less than 1.0 at a temperature of about 700 K. By applying external pressure using a diamond anvil cell, we obtained a room-temperature ZT value of about 1.7. From thermoelectric, magnetoresistance and Raman measurements, as well as density functional theory calculations, a pressure-driven topological phase transition is found to enable this enhancement. Experiments also support the appearance of a topological crystalline insulator after the transition. These findings point to the possibility of using compression to increase not just ZT in existing thermoelectric materials, but also the possibility of realizing topological crystalline insulators.

Novel Hydrogen Clathrate Hydrate.

We report a new hydrogen clathrate hydrate synthesized at 1.2 GPa and 298 K documented by single-crystal x-ray diffraction, Raman spectroscopy, and first-principles calculations. The oxygen sublattice of the new clathrate hydrate matches that of ice II, while hydrogen molecules are in the ring cavities, which results in the trigonal R3c or R3[over ¯]c space group (proton ordered or disordered, respectively) and the composition of (H_{2}O)_{6}H_{2}. Raman spectroscopy and theoretical calculations reveal a hydrogen disordered nature of the new phase C_{1}^{'}, distinct from the well-known ordered C_{1} clathrate, to which this new structure transforms upon compression and/or cooling. This new clathrate phase can be viewed as a realization of a disordered ice II, unobserved before, in contrast to all other ordered ice structures.

Dinitrogen as a Universal Electron Acceptor in Solid-State Chemistry: An Example of Uncommon Metallic Compounds Na3(N2)4 and NaN2.

With the exception of lithium, alkali metals do not react with elemental nitrogen either at ambient conditions or at elevated temperatures, requiring the search for alternative synthetic routes to their nitrogen-containing compounds. Here using a controlled decomposition of sodium azide (NaN3) at high pressure conditions, we synthesize two novel compounds, Na3(N2)4 and NaN2, both containing dinitrogen anions. NaN2 synthesized at 4 GPa might be the common intermediate in high-pressure solid-state metathesis reactions, where NaN3 is used as a source of nitrogen, while Na3(N2)4 opens a new class of compounds, where [N2] units accommodate a noninteger formal charge of 0.75-. This finding can dramatically extend the expected compositions in other group 1 and 2 metal-nitrogen systems. Electronic structure calculations show the metallic character for both compounds.

Polymorphism of polymeric nitrogen at high pressures.

Polymeric nitrogen at 120 GPa-180 GPa is known in two monatomic crystalline cubic gauche (cg-N) and layered polymeric (LP-N) phases and one amorphous modification (η-N), and all these high-pressure phases attract considerable attention for their potential application as a high energy density material. Here, we investigated the stability of these modifications at high pressures in the laser heated diamond anvil cell upon decompression from 161 GPa. Pure LP-N was synthesized above 152 GPa upon laser heating of η-N to 2500 K, while cg-N forms below 150 GPa. Upon laser heating at 129 GPa and 123 GPa, the LP-N clearly diminished, indicating that the synthesis of cg-N becomes more favorable in a mixed phase region below 129 GPa. Upon unloading, cg-N and LP-N were metastable to at least 71 GPa at up to 2500 K and at room temperature, respectively. These observations clarified a complicated polymorphism of monatomic nitrogen at high pressures and large hysteretic phenomena related to a transition to nonmolecular nitrogen.

Stability of Ar(H2)2 to 358 GPa.

"Chemical precompression" through introducing impurity atoms into hydrogen has been proposed as a method to facilitate metallization of hydrogen under external pressure. Here we selected Ar(H2)2, a hydrogen-rich compound with molecular hydrogen, to explore the effect of "doping" on the intermolecular interaction of H2 molecules and metallization at ultrahigh pressure. Ar(H2)2 was studied experimentally by synchrotron X-ray diffraction to 265 GPa, by Raman and optical absorption spectroscopy to 358 GPa, and theoretically using the density-functional theory. Our measurements of the optical bandgap and the vibron frequency show that Ar(H2)2 retains 2-eV bandgap and H2 molecular units up to 358 GPa. This is attributed to reduced intermolecular interactions between H2 molecules in Ar(H2)2 compared with that in solid H2 A splitting of the molecular vibron mode above 216 GPa suggests an orientational ordering transition, which is not accompanied by a change in lattice symmetry. The experimental and theoretical equations of state of Ar(H2)2 provide direct insight into the structure and bonding of this hydrogen-rich system, suggesting a negative chemical pressure on H2 molecules brought about by doping of Ar.

High-Pressure Synthesis of Metal-Inorganic Frameworks Hf4 N20 ⋠N2 , WN8 ⋠N2 , and Os5 N28 ⋠3 N2 with Polymeric Nitrogen Linkers.

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one-step synthesis of metal-inorganic frameworks Hf4 N20 ⋅N2 , WN8 ⋅N2 , and Os5 N28 ⋅3 N2 via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf4 N20 , WN8 , and Os5 N28 ) are built from transition-metal atoms linked either by polymeric polydiazenediyl (polyacetylene-like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high-pressure reaction between Hf and N2 also leads to a non-centrosymmetric polynitride Hf2 N11 that features double-helix catena-poly[tetraz-1-ene-1,4-diyl] nitrogen chains [-N-N-N=N-]∞ .

Comment on "High-Pressure Behavior of Hydrogen and Deuterium at Low Temperatures".

X.-D. Liu et al. [Phys. Rev. Lett. 119, 065301 (2017)PRLTAO0031-900710.1103/PhysRevLett.119.065301] report on the existence of a new unique solid phase of D_{2}, which makes the high-pressure low-temperature behavior distinct from H_{2}. Here, based on the analysis of their Raman data and phase transition theory, we show that the presented data do not support this claim.

Helium-hydrogen immiscibility at high pressures.

Hydrogen and helium are the most abundant elements in the universe, and they constitute the interiors of gas giant planets. Thus, their equations of states, phase, chemical state, and chemical reactivity at extreme conditions are of great interest. Applying Raman spectroscopy, visual observation, and synchrotron X-ray diffraction in diamond anvil cells, we performed experiments on H2-He 1:1 and D2-He 1:10 compressed gas mixtures up to 100 GPa at 300 K. By comparing with the available data on pure bulk materials, we find no sign of miscibility, chemical reactivity, and new compound formation. This result establishes a new baseline for future investigations of miscibility in the He-H2 system at extreme P-T conditions.

Synthesis of Xenon and Iron-Nickel Intermetallic Compounds at Earth's Core Thermodynamic Conditions.

Using in situ synchrotron x-ray diffraction and Raman spectroscopy in concert with first principles calculations we demonstrate the synthesis of stable Xe(Fe,Fe/Ni)_{3} and XeNi_{3} compounds at thermodynamic conditions representative of Earth's core. Surprisingly, in the case of both the Xe-Fe and Xe-Ni systems Fe and Ni become highly electronegative and can act as oxidants. The results indicate the changing chemical properties of elements under extreme conditions by documenting that electropositive at ambient pressure elements could gain electrons and form anions.

Iodine in Metal-Organic Frameworks at High Pressure.

Capture of highly volatile radioactive iodine is a promising application of metal-organic frameworks (MOFs), thanks to their high porosity with flexible chemical architecture. Specifically, strong charge-transfer binding of iodine to the framework enables efficient and selective iodine uptake as well as its long-term storage. As such, precise knowledge of the electronic structure of iodine is essential for a detailed modeling of the iodine sorption process, which will allow for rational design of iodophilic MOFs in the future. Here we probe the electronic structure of iodine in MOFs at variable iodine···framework interaction by Raman and optical absorption spectroscopy at high pressure ( P). The electronic structure of iodine in the straight channels of SBMOF-1 (Ca- sdb, sdb = 4,4'-sulfonyldibenzoate) is modified irreversibly at P > 3.4 GPa by charge transfer, marking a polymerization of iodine molecules into a 1D polyiodide chain. In contrast, iodine in the sinusoidal channels of SBMOF-3 (Cd- sdb) retains its molecular (I2) character up to at least 8.4 GPa. Such divergent high-pressure behavior of iodine in the MOFs with similar port size and chemistry illustrates adaptations of the electronic structure of iodine to channel topology and strength of the iodine···framework interaction, which can be used to tailor iodine-immobilizing MOFs.