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

Theoretical Investigation of Two-Dimensional FeC4 Structures with Surface Van Hove Singularity for Electrochemical Nitric Oxide Reduction Reaction

The electrochemical nitric oxide reduction reaction (eNORR) is an efficient method for converting aqueous NO into NH3. The pursuit of innovative electrocatalysts with enhanced activity, selectivity, durability, and cost-effectiveness for NORR remains a research focus. In this study, using particle swarm optimization (PSO) searches, density functional theory (DFT), and the constant-potential method (CPM), we predict two stable two-dimensional FeC4 monolayers, designated as α-FeC4 and β-FeC4, as promising electrocatalysts for the NORR. Our results demonstrate that both α-FeC4 and β-FeC4 monolayers possess intrinsic metallicity with surface Van Hove singularity (SVHS), showing remarkable NORR catalytic performance. Additionally, the substantial disparity in adsorption free energies between NO and H atom at 0 V ensures the high selectivity of these novel FeC4 monolayers toward NORR. These findings not only contribute to the expanding family of two-dimensional transition metal carbides but also provide a new idea for the design of highly efficient NORR electrocatalysts.

N18 ring: A building block for constructing 1D and 2D polymeric nitrogen frameworks

Exploring the fundamental building block is essential for constructing functional materials with extended topological configurations. In the polymeric nitrogen system, no fundamental building blocks have been reported so far. Here, we successfully synthesize the buckled 1-dimensional (1D) band-shaped and 2-dimensional (2D) layered polymeric nitrogen frameworks with N18 ring as a fundamental building block for the first time. Furthermore, the dimensions of the polymeric nitrogen frameworks can be regulated by pressure conditions. Bader charge analyses indicate that the charge transfer from the La atom to the low-order bonded nitrogen atom plays a crucial role in stabilizing these two low-dimensional polymeric frameworks. Both LaN16 and LaN8 are promising high-energy-density materials (HEDMs). This study reveals that the N18 ring can serve as a fundamental building block, analogous to the six-membered ring in carbon-based materials, enabling the construction of novel polymeric nitrogen materials with extended frameworks.

Combination of Ambient and High-Temperature Beryllium Nitride Motifs in W2Be4N5 and W4Be8N9

Compounds of transition metals and beryllium have a wide range of applications, from everyday tools to high tech applications. Remarkably, no single ternary beryllium nitride with a transition metal is known. Here, we report on the synthesis and properties of the first transition metal nitridoberyllates, namely W2Be4N5 and W4Be8N9. Both compounds were synthesized in a high-temperature high-pressure approach from Be3N2 and W, using azide generated N2 as an oxidizing agent. The crystal structures, consisting of alternating layers of WN6 trigonal prisms and BeN4 tetrahedra, were elucidated by single-crystal X-ray diffraction (sc-XRD). The separating nitridoberyllate layers show either ambient temperature (α-Be3N2 type) or high temperature (β-Be3N2 type) motifs. W2Be4N5 was further corroborated by infrared (IR), nuclear magnetic resonance (NMR) and UV/Vis spectroscopy and vibrating sample magnetometry (VSM) measurements. The latter revealed a mixed valence with an intermediate oxidation state of 3.5 for the W atoms. Both, the synthesis of the first transition metal nitridoberyllates and the synthesis approach using elemental W pave the way to a new field of nitride chemistry.

6-Fold-Coordinated Beryllium in Calcite-Type Be[CO3]

The anhydrous beryllium carbonate Be[CO3] with calcite-type crystal structure was obtained by a reaction of BeO with CO2 in a laser-heated diamond anvil cell at pressures between 30 GPa and 80 GPa and elevated temperatures. Its calcite-type crystal structure (R3̅c with Z = 6) is characterized by 6-fold-coordinated beryllium atoms forming [BeO6] octahedra and by trigonal-planar [CO3]2- groups. The crystal structure was determined by synchrotron-based single-crystal X-ray diffraction and confirmed by density-functional-theory-based calculations in combination with experimental Raman spectroscopy. Calcite-type Be[CO3] was synthesized at significantly lower pressures than the other very few compounds hosting 6-fold-coordinated beryllium, and it is the first beryllium carbonate with this coordination.

Structural evolution and superconductivity of thorium under high pressure and its modulation

The recent revelation of elemental scandium exhibiting a remarkably high superconducting critical temperature (Tc) of 36 K under 260 GPa [J. Ying et al., Phys. Rev. Lett., 2023, 130, 256002] has sparked considerable scientific intrigue and attention. In distinction to this exciting study, we focus on the superconductivity of thorium (Th) under high pressure and explore its underlying modulation mechanism. Based on the CALYPSO structure search method and first-principles calculations, we have conducted comprehensively a theoretical study on the structural evolution and superconductivity of Th under high pressure, up to 300 GPa. Two novel structures of Th, Fmmm and Immm phases, are uncovered. Our results indicate that the superconductivity of the face-centered cubic (fcc) phase of Th at ambient pressure is just 2.7 K and the Tc gradually decreases with the increase of the pressure. We propose an effective way to enhance the superconductivity of Th, which is the extrinsic doping of light elements without changing the fcc framework. Most importantly, the superconductivity of ThB is enhanced to 12.4 K under ambient pressure, five times higher than that of the Th metal. The present findings establish a good paradigm to regulate the superconductivity of metallic Th under ambient pressure and offer insights into the structures and superconductivity mechanisms of Th based compounds.

All-Single Bonds Fused N18 Macro-Rings and N8 Cagelike Building Blocks Stabilized in Lanthanum Supernitrides

Single-bonded polymeric nitrogen has gained tremendous research interest because of its unique physical properties and great potential applications. Despite much progress in theoretical predictions, it is still challenging to experimentally synthesize polynitrogen compounds with novel all-single-bonded units. Herein, we have synthesized two brand-new lanthanum supernitrides LaN8, through a direct reaction between La and N2 in laser-heated diamond anvil cells at megabar pressures. Our experiments and calculations revealed that two LaN8 phases had the R-3 and P4/n symmetry characterized by a unique 2D network with N18 macro-rings and cagelike N8 building blocks, respectively. Differing from known polynitrogen structures, these two polymers were composed of single-bonded nitrogen atoms belonging to sp3 and sp2 hybridizations. In particular, P4/n LaN8 possessed the longest N-N bond length among all of the experimentally reported metal nitrides, potentially being a high-energy-density material. The present study opens a fresh, promising avenue for the rational design and discovery of new supernitrides with unique nitrogen structures via the high-pressure treatment.

High-Pressure oC16-YBr3 Polymorph Recoverable to Ambient Conditions: From 3D Framework to Layered Material

Exfoliation of graphite and the discovery of the unique properties of graphene─graphite's single layer─have raised significant attention to layered compounds as potential precursors to 2D materials with applications in optoelectronics, spintronics, sensors, and solar cells. In this work, a new orthorhombic polymorph of yttrium bromide, oC16-YBr3 was synthesized from yttrium and CBr4 in a laser-heated diamond anvil cell at 45 GPa and 3000 K. The structure of oC16-YBr3 was solved and refined using in situ synchrotron single-crystal X-ray diffraction. At high pressure, it can be described as a 3D framework of YBr9 polyhedra, but upon decompression below 15 GPa, the structure motif changes to layered, with layers comprising edge-sharing YBr8 polyhedra weakly bonded by van der Waals interactions. The layered oC16-YBr3 material can be recovered to ambient conditions, and according to Perdew-Burke-Ernzerhof-density functional theory calculations, it exhibits semiconductor properties with a band gap that is highly sensitive to pressure. This polymorph possesses a low exfoliation energy of 0.30 J/m2. Our results expand the list of layered trivalent rare-earth metal halides and provide insights into how high pressure alters their structural motifs and physical properties.

Two-dimensional Be2P4 as a promising thermoelectric material and anode for Na/K-ion batteries

Incredibly effective and flexible energy conversion and storage systems hold great promise for portable self-powered electronic devices. Owing to their large surface area, exceptional atomic structures, superior electrical conductivity and good mechanical flexibility, two-dimensional (2D) materials are recognized as an attractive option for energy conversion and storage application. In this work, we examined the stability, electronic, thermoelectric and electrochemical aspects of a novel 2D Be2P4 monolayer by adopting density functional theory (DFT). The Be2P4 monolayer exhibits a direct semiconductor gap of 0.9 eV (HSE06), large Young's modulus (∼198 GPa), high carrier mobility (∼104 cm2 V-1 s-1) and a low excitonic binding energy of 0.11 eV. Our calculated findings suggest that Be2P4 shows a lattice thermal conductivity of 1.02 W m K-1 at 700 K, resulting in moderate thermoelectric performance (ZT ∼ 0.7), encouraging its use in thermoelectric materials. In addition, a higher adsorption energy of -2.28 eV (-2.52 eV) and less diffusion barrier of 0.22 eV (0.17 eV) for Na(K)-ion batteries promote fast ion transport in the Be2P4 monolayer. This material also shows a high specific capacity and superior energy density of 8460 W h kg-1 (8883 W h kg-1) for Na(K)-ion batteries. Thus, our results offer insightful information for investigating potential thermoelectric and flexible anode materials based on the Be2P4 monolayer.

Synthesis of LaCN3, TbCN3, CeCN5, and TbCN5 Polycarbonitrides at Megabar Pressures

Inorganic ternary metal-C-N compounds with covalently bonded C-N anions encompass important classes of solids such as cyanides and carbodiimides, well known at ambient conditions and composed of [CN]- and [CN2]2- anions, as well as the high-pressure formed guanidinates featuring [CN3]5- anion. At still higher pressures, carbon is expected to be 4-fold coordinated by nitrogen atoms, but hitherto, such CN4-built anions are missing. In this study, four polycarbonitride compounds (LaCN3, TbCN3, CeCN5, and TbCN5) are synthesized in laser-heated diamond anvil cells at pressures between 90 and 111 GPa. Synchrotron single-crystal X-ray diffraction (SCXRD) reveals that their crystal structures are built of a previously unobserved anionic single-bonded carbon-nitrogen three-dimensional (3D) framework consisting of CN4 tetrahedra connected via di- or oligo-nitrogen linkers. A crystal-chemical analysis demonstrates that these polycarbonitride compounds have similarities to lanthanide silicon phosphides. Decompression experiments reveal the existence of LaCN3 and CeCN5 compounds over a very large pressure range. Density functional theory (DFT) supports these discoveries and provides further insight into the stability and physical properties of the synthesized compounds.

One-Dimensional Non-coplanar Nitrogen Chains in Manganese Tetranitride under High Pressure

Transition metal nitrides have great potential applications as incompressible and high energy density materials. Various polymeric nitrogen structures significantly affect their properties, contributing to their complex bonding modes and coordination conditions. Herein, we first report a new manganese polynitride MnN4 with bifacial trans-cis [N4]n chains by treating with high-pressure and high-temperature conditions in a diamond anvil cell. Our experiments reveal that MnN4 has a P-1 symmetry and could stabilize in the pressure range of 56-127 GPa. Detailed pressure-volume data and calculations of this phase indicate that MnN4 is a potential hard (255 GPa) and high energy density (2.97 kJ/g) material. The asymmetric interactions impel N1 and N4 atoms to hybridize to sp2-3, which causes distortions of [N4]n chains. This work discovers a new polynitride material, fills the gap for the study of manganese polynitride under high pressure, and offers some new insights into the formation of polymeric nitrogen structures.

Nitrogen-rich Ce-N compounds under high pressure

Four high-pressure N-rich compounds (Pmn21-CeN7, Amm2-CeN9, P1̄-CeN10, and P1̄-II-CeN10) are proposed using first-principles calculations. Novel polymeric units (a heart shaped layered structure, chain-like N8 rings, and two new banded structures) in four cerium nitrides are reported for the first time in this study. The analyses of electronic structures and bonding properties show that the charge transfer between Ce and N atoms promotes the formation of the Ce-N ionic bond and N-N covalent bond, which play an important role in stabilizing the nitrogen skeleton. Four new phases possess high energy densities (3.24-3.86 kJ g-1), indicating that they are favorable high-energy density materials. Moreover, P1̄-CeN10 possesses ultra-incompressibility along the [1 0 0] direction. Finally, infrared and Raman spectra are analyzed to provide guidance for experimental synthesis.

Stabilization of N6 and N8 anionic units and 2D polynitrogen layers in high-pressure scandium polynitrides

Nitrogen catenation under high pressure leads to the formation of polynitrogen compounds with potentially unique properties. The exploration of the entire spectrum of poly- and oligo-nitrogen moieties is still in its earliest stages. Here, we report on four novel scandium nitrides, Sc2N6, Sc2N8, ScN5, and Sc4N3, synthesized by direct reaction between yttrium and nitrogen at 78-125 GPa and 2500 K in laser-heated diamond anvil cells. High-pressure synchrotron single-crystal X-ray diffraction reveals that in the crystal structures of the nitrogen-rich Sc2N6, Sc2N8, and ScN5 phases nitrogen is catenated forming previously unknown N66- and N86- units and ∞ 2 ( N 5 3 - ) anionic corrugated 2D-polynitrogen layers consisting of fused N12 rings. Density functional theory calculations, confirming the dynamical stability of the synthesized compounds, show that Sc2N6 and Sc2N8 possess an anion-driven metallicity, while ScN5 is an indirect semiconductor. Sc2N6, Sc2N8, and ScN5 solids are promising high-energy-density materials with calculated volumetric energy density, detonation velocity, and detonation pressure higher than those of TNT.

Purely single-bonded spiral nitrogen chains stabilized by trivalent lanthanum ions

Inspired by the single-bonded nitrogen chains stabilized by tetravalent cerium, pentavalent tantalum, and hexavalent tungsten atoms, we explored the possibility of single-bonded nitrogen polymorphs stabilized by trivalent lanthanum ions. To achieve this, we utilized the crystal structure search method on the phase diagram of binary La-N compounds. We identified three novel thermodynamically stable phases, the C2/c LaN3, P-1 LaN4, and P-1 LaN8. Among them, the C2/c phase with infinite helical poly-N6 chains becomes thermodynamically stable above 50 GPa. Each nitrogen atom in the poly-N6 chain acquires one extra electron, and the spiral chain is purely single-bonded. The C2/c phase has an indirect band gap of ∼1.6 eV at 60 GPa. Notably, the band gap exhibits non-monotonic behavior, decreases first and then increases with increasing pressure. This abnormal behavior is attributed to the significant bonding of two La-N bonds at around 35 GPa. Phonon spectrum calculations and AIMD simulations have confirmed that the C2/c phase can be quenched to ambient conditions with slight distortion, and it exhibits excellent detonation properties. Additionally, we also discovered armchair-like nitrogen chains in LaN4 and the armchair and zigzag-like mixed nitrogen chains in LaN8. These results provide valuable insights into the electronic and bonding properties of nitrides under high pressure and may have important implications for the design and development of novel functional materials.

Pressure-induced novel ZrN4 semiconductor materials with high dielectric constants: a first-principles study

In addition to Zr3N4 and ZrN2 compounds, zirconium nitrides with a rich family of phases always exhibit metal phases. By employing an evolutionary algorithm approach and first-principles calculations, we predicted seven novel semiconductor phases for the ZrN4 system at 0-150 GPa. Through calculating phonon dispersions, we identified four dynamically stable semiconductor structures under ambient pressure, namely, α-P1̄, β-P1̄, γ-P1̄, and β-P1 (with bandgaps of 1.03 eV, 1.10 eV, 2.33 eV, and 1.49 eV calculated using the HSE06 hybrid density functional, respectively). The calculated work functions and dielectric functions show that the four dynamically stable semiconductor structures are all high dielectric constant (high-k) materials, among which the β-P1̄ phase has the largest static dielectric constant (3.9 times that of SiO2). Furthermore, we explored band structures using the HSE06 functional and density of states (DOS) and the response of bandgaps to pressure using the PBE functional for the four new semiconductor configurations. The results show that the bandgap responses of the four structures exhibit significant differences when hydrostatic pressure is applied from 0 to 150 GPa.

Carrier mobility of two-dimensional Dirac materials: the influence of optical phonon scattering

We developed an analytical formula to calculate the influence of optical phonons on the mobility of two-dimensional Dirac materials at arbitrary temperature and arbitrary doping concentration. The method was combined with first-principles calculations to show that the effect of optical phonons on mobility is not negligible for typical Dirac materials such as graphene even though the occupation number of optical phonons is relatively small. Unlike the treatment of electron-acoustic phonon coupling, the energy change of electrons in the scattering process with optical phonons is crucial, which leads to a non-power temperature dependence of mobility under weak doping. The formalism was applied to calculate and analyze the mobility of two well-known Dirac materials, α-graphyne and the VCl3 monolayer, which differs by one to two orders of magnitude.

Polysilyne chains bridged with beryllium lead to flat 2D Dirac materials

Polysilyne with repeating disilyne units, a silicon analogue of polyacetylene, has a high potential for application to various novel silicon-based electronic devices because of the unique properties of Si=Si units with a smaller HOMO-LUMO energy gap than that of C=C units. However, one-dimensional (1D) polysilyne has not been synthesized yet. Here we propose a planar and air-stable two-dimensional (2D) silicon-based material with one-atom thickness consisting of beryllium-bridged 1D all-trans polysilyne, based on the first-principles calculations. The flat structure of 1D polysilyne, which is essential for the air stability of silicon π-electron conjugated systems, is realized by embedding polysilyne in a planar sheet. It was found that the 2D crystal optimized at the rhombus unit cell with the D2h group symmetry is a silicon-based Dirac semimetal with linear dispersion at the Fermi energy and hosts anisotropic Dirac fermions.

High-pressure synthesis of fully sp2-hybridized polymeric nitrogen layer in potassium supernitride

Searching for fully sp²-hybridized layered structures is of fundamental importance because of their fascinating physical properties and potential to host topologically non-trivial electronic states. However, the synthesis of fully sp²-hybridized layered polymeric nitrogen structures remains a challenging work because of their low stability. Here, we report the synthesis of a fully sp²-hybridized layered polymeric nitrogen structure featuring fused 18-membered rings in potassium supernitride (K₂N₁₆) under high-pressure and high-temperature conditions. Bader charge analysis reveals that the potassium atomic layer stabilizes the unique sp²-hybridized polymeric nitrogen layers through the charge transfer effect in K₂N₁₆. The calculation of electronic structure indicates that K₂N₁₆ is a topological semimetal with multiple Dirac points and hosts higher-order Dirac fermions with cubic dispersion, which are contributed by the sp²-hybridized polymeric nitrogen layers arranged in P6/mcc symmetry. The high-pressure synthesis of the fully sp²-hybridized polymeric nitrogen layered structure provides promising prospects for exploring novel topological materials with effective stabilization routes.

Tunable magnetic and electronic properties of armchair BeN4 nanoribbons

Recently, layered BeN₄ as a novel Dirac semimetal has been fabricated (M. Bykov, T. Fedotenko, S. Chariton et al. Phys. Rev. Lett., 2021, 126, 175501). Motivated by the experiment, we perform first-principles calculations to predict the stability, magnetic configurations, and electronic structures of unsaturated BeN₄ nanoribbons with an armchair-terminated edge. The magnetic interactions and electronic properties of BeN₄ nanoribbons are sensitively influenced by the edge morphology. The BeN₄ nanoribbons with both edges occupied by Be atoms undergo a transition from a ferromagnetic (FM) metal to an antiferromagnetic (AFM) semiconductor with the increase of ribbon width. The configurations with edges situated by Be and N atoms are FM/ferrimagnetic (FIM) metals or nearly half-metals, and the spin polarizability is as high as 85% when the ribbon width is N = 5. The nanoribbons with both edge sites occupied by pentagonal N atoms are nonmagnetic (NM), while the nanoribbons terminated by N atoms in a hexagonal ring are FM metals. We also explore the magnetic properties and band structures of BeN₄ nanoribbons with hydrogen passivation. Our results open up a versatile edge engineering avenue to design BeN₄-based spintronic and nanoelectronic devices.

Novel high-pressure phases of nitrogen-rich Y-N compounds

Five new high-pressure phases (I4̄3d-Y₄N₃, R3c-Y₂N₃, P1̄-II-YN₄, P1̄-YN₆, and P31c-YN₈) are proposed by the crystal structure prediction. A series of polynitrogen forms were achieved in the nitrogen-rich Y-N compounds, including diatomic N₂, an isolated N₈ chain, an infinite N chain with an N₆ unit, and an infinite N layer with bent N₁₈ rings. The high energy densities of P1̄-II-YN₄ (1.98 kJ g^-1), P1̄-YN₆ (2.35 kJ g^-1), and P31c-YN₈ (3.77 kJ g^-1) make them potential high energy density materials. More importantly, P1̄-II-YN₄, P1̄-YN₆, and P31c-YN₈ exhibit excellent explosive performance, with detonation pressures 4-8 times that of TNT (19 GPa) and detonation velocities 1-2 times that of TNT (6.90 km s^-1). The electronic structure and bonding properties show that the high stability of Y-N compounds originates from the strong N-N covalent bond and the weak Y-N ionic bond interaction. The increase in the transferred charge quantity as the pressure decreased is more conducive to stabilizing the polymeric nitrogen structure, which leads to the metastable properties of P1̄-II-YN₄ and P1̄-YN₆ under ambient conditions. Finally, the infrared (IR) spectra of P1̄-II-YN₄, P1̄-YN₆, and P31c-YN₈ are calculated to provide a reference in experimental synthesis.

Revealing Phosphorus Nitrides up to the Megabar Regime: Synthesis of α'-P3 N5, δ-P3 N5 and PN2

Non-metal nitrides are an exciting field of chemistry, featuring a significant number of compounds that can possess outstanding material properties. These properties mainly rely on maximizing the number of strong covalent bonds, with crosslinked XN6 octahedra frameworks being particularly attractive. In this study, the phosphorus-nitrogen system was studied up to 137 GPa in laser-heated diamond anvil cells, and three previously unobserved phases were synthesized and characterized by single-crystal X-ray diffraction, Raman spectroscopy measurements and density functional theory calculations. δ-P3 N5 and PN2 were found to form at 72 and 134 GPa, respectively, and both feature dense 3D networks of the so far elusive PN6 units. The two compounds are ultra-incompressible, having a bulk modulus of K0 =322 GPa for δ-P3 N5 and 339 GPa for PN2 . Upon decompression below 7 GPa, δ-P3 N5 undergoes a transformation into a novel α'-P3 N5 solid, stable at ambient conditions, that has a unique structure type based on PN4 tetrahedra. The formation of α'-P3 N5 underlines that a phase space otherwise inaccessible can be explored through materials formed under high pressure.

Prediction of Two-Dimensional Group IV Nitrides AxNy (A = Sn, Ge, or Si): Diverse Stoichiometric Ratios, Ferromagnetism, and Auxetic Mechanical Property

In this work, we unveiled a new class of two-dimensional (2D) group IV nitride A_xN_y (A = Sn, Ge, or Si) prototypes, C2/m A₄N, P3̅m1 A₃N, P3m1 A₂N, P3̅m1 A₃N₂, P6̅m2 AN, P3̅m1 AN, P6̅2m A₃N₄, P3m1 A₂N₃, P4̅2₁m AN₂, and P3̅m1 AN₃, by using evolutionary algorithms combined with first-principles calculations. Using HSE06 functional calculations, a wide range of band gaps from metal to semiconductor (0.405-5.050 eV) and ultrahigh carrier mobilities (1-24 × 10³ cm² V^-1 s^-1) were evidenced in these 2D structures. We found that 2D P3m1 Sn₂N₃, Ge₂N₃, and Si₂N₃ are intrinsic ferromagnetic semiconductors with gaps of 0.677, 1.285, and 2.321 eV, respectively. The lattice symmetry and Si-to-N₂ charge transfer upon strain lead to large anisotropic negative Poisson's ratios (-0.281 to -0.146) along whole in-plane directions in 2D P4̅2₁m SiN₂. Our findings not only enrich the family of 2D nitrides but also highlight the promising optoelectronic and nanoauxetic applications of 2D group IV nitrides.

Synthesis of rare-earth metal compounds through enhanced reactivity of alkali halides at high pressures

Chemical stability of the alkali halides NaCl and KCl has allowed for their use as inert media in high-pressure high-temperature experiments. Here we demonstrate the unexpected reactivity of the halides with metals (Y, Dy, and Re) and iron oxide (FeO) in a laser-heated diamond anvil cell, thus providing a synthetic route for halogen-containing binary and ternary compounds. So far unknown chlorides, Y2Cl and DyCl, and chloride carbides, Y2ClC and Dy2ClC, were synthesized at ~40 GPa and 2000 K and their structures were solved and refined using in situ single-crystal synchrotron X-ray diffraction. Also, FeCl2 with the HP-PdF2-type structure, previously reported at 108 GPa, was synthesized at ~160 GPa and 2100 K. The results of our ab initio calculations fully support experimental findings and reveal the electronic structure and chemical bonding in these compounds.

Domain Auto Finder (DAFi) program: the analysis of single-crystal X-ray diffraction data from polycrystalline samples

This paper presents the Domain Auto Finder (DAFi) program and its application to the analysis of single-crystal X-ray diffraction (SC-XRD) data from multiphase mixtures of microcrystalline solids and powders. Superposition of numerous reflections originating from a large number of single-crystal domains of the same and/or different (especially unknown) phases usually precludes the sorting of reflections coming from individual domains, making their automatic indexing impossible. The DAFi algorithm is designed to quickly find subsets of reflections from individual domains in a whole set of SC-XRD data. Further indexing of all found subsets can be easily performed using widely accessible crystallographic packages. As the algorithm neither requires a priori crystallographic information nor is limited by the number of phases or individual domains, DAFi is powerful software to be used for studies of multiphase polycrystalline and microcrystalline (powder) materials. The algorithm is validated by testing on X-ray diffraction data sets obtained from real samples: a multi-mineral basalt rock at ambient conditions and products of the chemical reaction of yttrium and nitro-gen in a laser-heated diamond anvil cell at 50 GPa. The high performance of the DAFi algorithm means it can be used for processing SC-XRD data online during experiments at synchrotron facilities.

High switching ratio and inorganic gas sensing performance in BeN4based nanodevice: a first-principles study

Recently, Dirac material BeN₄has been synthesized by using laser-heated diamond anvil-groove technology (Bykovet al2021Phys. Rev. Lett.126175501). BeN₄layer, i.e. beryllonitrene, represents a qualitatively class of two-dimensional (2D) materials that can be built of a metal atom and polymeric nitrogen chains, and hosts anisotropic Dirac fermions. Enlighten by this discovered material, we study the electronic structure, anisotropic transport properties and gas sensitivity of 2D BeN₄using the density functional theory combined with non-equilibrium Green's function method. The results manifest that the 2D BeN₄shows a typical semi-metallic property. The electronic transport properties of the intrinsic BeN₄devices show a strong anisotropic behavior since electrons transmitting along the armchair direction is much easier than that along the zigzag direction. It directly results in an obvious switching characteristic with the switching ratio up to 10⁵. Then the adsorption characteristics indicate that H₂S, CO, CO₂and H₂molecules are physisorption, while the NH₃, NO, NO₂, SO₂molecules are chemisorption. Among these chemisorption cases, the 2D gas sensor devices show an extremely high response for SO₂recognition, and the high anisotropy of the original 2D BeN₄device still maintains after adsorbing gas molecules. Finally, high switching ratio and inorganic gas sensing performance of BeN₄monolayer could be clearly understood with local density of states, bias-dependent spectra, scattered state distribution. In general, the results indicate that the designed BeN₄devices have potential practical application in high-ratio switching devices and high gas-sensing molecular devices.

Anisotropic Phononic and Electronic Thermal Transport in BeN4

Beryllium polynitride (BeN₄) has been recently synthesized under high-pressure conditions [Bykov et al. Phys. Rev. Lett. 2021, 126, 175501]. Its anisotropic lattice structure dependent on the applied pressure motivates exploration of its thermal transport properties with a theoretical framework that combines the Boltzmann transport equation with ab initio calculations. The bonding anisotropy (impacting the phonon and electron group velocities) and bonding anharmonicity (captured through three- and four-phonon scatterings) are reflected in the strong anisotropy of both phononic and electronic components of the thermal conductivity. Moreover, the pressure-driven evolution of the interlayer Be-N bonding, from partially covalent (under high-pressure synthesis conditions) to van der Waals (under ambient pressure), drives a largely interlayer thermal conductivity. These findings highlight an alternative strategy for achieving directional control of the thermal transport in synthetic materials.

Stabilization of hexazine rings in potassium polynitride at high pressure

Polynitrogen molecules are attractive for high-energy-density materials due to energy stored in nitrogen-nitrogen bonds; however, it remains challenging to find energy-efficient synthetic routes and stabilization mechanisms for these compounds. Direct synthesis from molecular dinitrogen requires overcoming large activation barriers and the reaction products are prone to inherent inhomogeneity. Here we report the synthesis of planar N₆^2- hexazine dianions, stabilized in K₂N₆, from potassium azide (KN₃) on laser heating in a diamond anvil cell at pressures above 45 GPa. The resulting K₂N₆, which exhibits a metallic lustre, remains metastable down to 20 GPa. Synchrotron X-ray diffraction and Raman spectroscopy were used to identify this material, through good agreement with the theoretically predicted structural, vibrational and electronic properties for K₂N₆. The N₆^2- rings characterized here are likely to be present in other high-energy-density materials stabilized by pressure. Under 30 GPa, an unusual N₂^0.75--containing compound with the formula K₃(N₂)₄ was formed instead.

Thermal Properties of 2D Dirac Materials MN4 (M = Be and Mg): A First-Principles Study

Recently, a novel two-dimensional (2D) Dirac material BeN4 monolayer has been fabricated experimentally through high-pressure synthesis. In this work, we investigate the thermal properties of a new class of 2D materials with a chemical formula of MN4 (M = Be and Mg) using first-principles calculations. First, the cohesive energy and phonon dispersion curve confirm the dynamical stability of BeN4 and MgN4 monolayers. Besides, BeN4 and MgN4 monolayers have the anisotropic lattice thermal conductivities of 842.75 (615.97) W m-1 K-1 and 52.66 (21.76) W m-1 K-1 along the armchair (zigzag) direction, respectively. The main contribution of the lattice thermal conductivities of BeN4 and MgN4 monolayers are from the low frequency phonon branches. Moreover, the average phonon heat capacity, phonon group velocity, and phonon lifetime of BeN4 monolayer are 3.54 × 105 J K-1 m-3, 3.61 km s-1, and 13.64 ps, which are larger than those of MgN4 monolayer (3.42 × 105 J K-1 m-3, 3.27 km s-1, and 1.70 ps), indicating the larger lattice thermal conductivities of BeN4 monolayer. Furthermore, the mode weighted accumulative Grüneisen parameters (MWGPs) of BeN4 and MgN4 monolayers are 2.84 and 5.62, which proves that MgN4 monolayer has stronger phonon scattering. This investigation will enhance an understanding of thermal properties of MN4 monolayers and drive the applications of MN4 monolayers in nanoelectronic devices.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

Single-Bonded Cubic AsN from High-Pressure and High-Temperature Chemical Reactivity of Arsenic and Nitrogen

Chemical reactivity between As and N2 , leading to the synthesis of crystalline arsenic nitride, is here reported under high pressure and high temperature conditions generated by laser heating in a diamond anvil cell. Single-crystal synchrotron X-ray diffraction at different pressures between 30 and 40 GPa provides evidence for the synthesis of a covalent compound of AsN stoichiometry, crystallizing in a cubic P21 3 space group, in which each of the two elements is single-bonded to three atoms of the other and hosts an electron lone pair, in a tetrahedral anisotropic coordination. The identification of characteristic structural motifs highlights the key role played by the directional repulsive interactions between non-bonding electron lone pairs in the formation of the AsN structure. Additional data indicate the existence of AsN at room temperature from 9.8 up to 50 GPa. Implications concern fundamental aspects of pnictogens chemistry and the synthesis of innovative advanced materials.

Pressure-stabilized high-energy-density material YN10

Polynitrogen compounds have been intensively studied for potential applications as high energy density materials, especially in energy and military fields. Here, using the swarm intelligence algorithm in combination with first-principles calculations, we systematically explored the variable stoichiometries of yttrium-nitrogen compounds on the nitrogen-rich regime at high pressure, where a new stable phase of YN₁₀adoptingI4/msymmetry was discovered at the pressure of 35 GPa and showed metallic character from the analysis of electronic properties. In YN₁₀, all the nitrogen atoms weresp²-hybridized in the form of N₅ring. Furthermore, the gravimetric and volumetric energy densities were estimated to be 3.05 kJ g^-1and 9.27 kJ cm^-1respectively. Particularly, the calculated detonation velocity and pressure of YN₁₀(12.0 km s^-1, 82.7 GPa) was higher than that of TNT (6.9 km s^-1, 19.0 GPa) and HMX (9.1 km s^-1, 39.3 GPa), making it a potential candidate as a high-energy-density material.

Li-doped beryllonitrene for enhanced carbon dioxide capture

In recent years, the scientific community has given more and more attention to the issue of climate change and global warming, which is largely attributed to the massive quantity of carbon dioxide emissions. Thus, the demand for a carbon dioxide capture material is massive and continuously increasing. In this study, we perform first-principle calculations based on density functional theory to investigate the carbon dioxide capture ability of pristine and doped beryllonitrene. Our results show that carbon dioxide had an adsorption energy of -0.046 eV on pristine beryllonitrene, so it appears that beryllonitrene has extremely weak carbon dioxide adsorption ability. Pristine beryllonitrene could be effectively doped with lithium atoms, and the resulting Li-doped beryllonitrene had much stronger interactions with carbon dioxide than pristine beryllonitrene. The adsorption energy for carbon dioxide on Li-doped beryllonitrene was -0.408 eV. The adsorption of carbon dioxide on Li-doped beryllonitrene greatly changed the charge density, projected density of states, and band structure of the material, demonstrating that it was strongly adsorbed. This suggests that Li-doping is a viable way to enhance the carbon dioxide capture ability of beryllonitrene and makes it a possible candidate for an effective CO2 capture material.

High-Pressure Synthesis of the β-Zn3N2 Nitride and the α-ZnN4 and β-ZnN4 Polynitrogen Compounds

High-pressure nitrogen chemistry has expanded at a formidable rate over the past decade, unveiling the chemical richness of nitrogen. Here, the Zn-N system is investigated in laser-heated diamond anvil cells by synchrotron powder and single-crystal X-ray diffraction, revealing three hitherto unobserved nitrogen compounds: β-Zn₃N₂, α-ZnN₄, and β-ZnN₄, formed at 35.0, 63.5, and 81.7 GPa, respectively. Whereas β-Zn₃N₂ contains the N^3- nitride, both ZnN₄ solids are found to be composed of polyacetylene-like [N₄]_∞^2- chains. Upon the decompression of β-ZnN₄ below 72.7 GPa, a first-order displacive phase transition is observed from β-ZnN₄ to α-ZnN₄. The α-ZnN₄ phase is detected down to 11.0 GPa, at lower pressures decomposing into the known α-Zn₃N₂ (space group Ia3̅) and N₂. The equations of states of β-ZnN₄ and α-ZnN₄ are also determined, and their bulk moduli are found to be K₀ = 126(9) GPa and K₀ = 76(12) GPa, respectively. Density functional theory calculations were also performed and provide further insight into the Zn-N system. Moreover, comparing the Mg-N and Zn-N systems underlines the importance of minute chemical differences between metal cations in the resulting synthesized phases.

Realization of an Ideal Cairo Tessellation in Nickel Diazenide NiN2: High-Pressure Route to Pentagonal 2D Materials

Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nanoelectronics. In this work we report the synthesis of nickel diazenide NiN₂ which possesses atomic-thick layers comprised of Ni₂N₃ pentagons forming Cairo-type tessellation. The layers of NiN₂ are weakly bonded with the calculated exfoliation energy of 0.72 J/m², which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN₂ is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN₂ to be a precursor of a pentagonal 2D material with a tunable direct band gap.

Novel Thermoelectric Character of Rhenium Carbonitride, ReCN

Nowadays, it is very important to study and propose new mechanisms for generating electricity that are environmentally friendly, in addition to using renewable resources. Thermoelectric (TE) devices are fabricated with materials that can convert a temperature difference into electricity, without the need for rotating parts. In this work, we report the TE properties of rhenium carbonitride (ReCN) as an important feature of a hard and thermodynamically stable material of band gap Δ_g = 0.626 eV. We use the electronic band structure behavior near the Fermi energy with the Seebeck coefficient to estimate the figure of merit ZT based on Boltzmann transport theory to characterize this property. Our results show that this compound has interesting TE properties among 300 and 1200 K for p- and n-type doping.