Calculated Phonon Spectra Research Articles

Prediction of one-dimensional CrN nanostructure as a promising ferromagnetic half-metal

Searching for one-dimensional (1D) nanostructure with ferromagnetic (FM) half-metallicity is of significance for the development of miniature spintronic devices. Here, based on the first-principles calculations, we propose that the 1D CrN nanostructure is a FM half-metal, which can generate the fully spin-polarized current. The ab initio molecular dynamic simulation and the phonon spectrum calculation demonstrate that the 1D CrN nanostructure is thermodynamically stable. The partially occupied Cr-d orbitals endow the nanostructure with FM half-metallicity, in which the half-metallic gap (Δs) reaches up to 1.58 eV. The ferromagnetism in the nanostructure is attributed to the superexchange interaction between the magnetic Cr atoms, and a sizable magnetocrystalline anisotropy energy (MAE) is obtained. Moreover, the transverse stretching of nanostructure can effectively modulate Δs and MAE, accompanied by the preservation of half-metallicity. A nanocable is designed by encapsulating the CrN nanostructure with a BN nanotube, and the intriguing magnetic and electronic properties of the nanostructure are retained. These novel characteristics render the 1D CrN nanostructure as a compelling candidate for exploiting high-performance spintronic devices.

Tuning the electronic properties of molybdenum di-sulphide monolayers via doping using first-principles calculations

In 2D semiconductors, doping offers an effective approach for modulating their structural and electronic properties-owing to the creation of newly formed chemical bonds and bond relaxation. By means of density functional theory (DFT), we systematically explored the electronic properties of monolayer MoS2 doped with X-atoms (X comprises of metals Li, Be, Al; metalloids B, Si; non-metals (NMs) C, N, P, O and the NM atoms belonging to halogen group (F, Cl)). The bonding nature of the host structures with the doped elements have been determined using electron localization function (ELF). Phonon spectra calculations are performed to distinguish between the dynamically stable and unstable systems. The band gap of MoS2 stands divided into smaller values in a variety of magnitude depending on the dopant site and the nature of the substituted atom. The results show that halogen non-metals exhibit n-type conduction in both the (Mo- and S-rich) environments. Thus, substitutional doping of impurity atoms belonging to different groups can successfully tune the band gap of MoS2 to the desired level for its useful applications in semiconducting electronic devices in addition to other interesting information on the nature of doping, which could be adopted to dope other 2D-TMDs to tailor their electronic and optical characteristics for more efficient electronic devices.

Visible light modulation and anomalous thermal transport in two-dimensional <i>X</i>-AlN (<i>X</i> = C, Si, TC) semiconductor

Aluminum nitride (AlN) is of paramount importance in developing electronic devices because of excellent stability and thermal transport performance. However, lack of novel materials which can provide colorful physical and chemical properties seriously hinders further digging out application potential. In this work, we perform an evolutionary structural search based on first-principles calculation and verify the dynamic and thermal dynamic stability of porous buckled AlN and <i>X</i>-AlN (<i>X</i> = C, Si, TC) structural system, which constructs by introducing C, Si atoms and triangular carbon (TC) into the porous vacancy of AlN, by calculating phonon spectra and first-principles molecular dynamic simulations. Structural deformation becomes gradually serious with the increase of structural unit size and significantly influences structural, electronic, and thermal transport properties. Firstly, we point out that a flat energy band appears around the Fermi level in C-AlN and Si-AlN because of weak interatomic interaction between C/Si and the neighbor Al atoms. Unoccupied C-/Si-p<sub><i>z</i></sub> and Al-p<sub><i>z</i></sub> do not form <inline-formula><tex-math id="M1">\begin{document}$ {\rm{\pi }} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20230116_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20230116_M1.png"/></alternatives></inline-formula> bond and only a localized flat band near Fermi level arises, and thus the absorption peaks of structures are enhanced and the red shift occurs. Bonding state of <inline-formula><tex-math id="M2">\begin{document}$ {\rm{\pi }} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20230116_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20230116_M2.png"/></alternatives></inline-formula> bond from hybridized C-p<sub><i>z</i></sub> orbitals in triangular carbon of TC-AlN lowers the energy of conduction band at <i>K</i> point in the first Brillouin zone and the corresponding antibonding state raises the band at <i>Γ</i>, therefore transition from indirect bandgap of AlN to direct bandgap of TC-AlN appears. Secondly, porous buckled AlN shows the lowest thermal conductivity due to asymmetric Al—N bonds around the porous vacancy and vertically stacked N—N bonds. Introduced C and Si atoms both reduce structural anharmonicity, while the former has a relatively small distortion, and so it has a higher thermal conductivity. Triangular carbon in TC-AlN hinders phonon scattering between FA mode and other phonon modes and has the weakest anharmonicity because of the strongest bond strength, and obtains the highest thermal transport performance. Finally, we unveil the physical mechanism of anomalous thermal conductivity in <i>X</i>-AlN system by modulating the biaxial tensile strain. Enhanced vertical N—N bonds dominate thermal transport due to its weaker anharmonicity with a slightly strain, and when tensile strain is above the 4%, soften phonon modes reduce phonon velocity and thus hinders the thermal transport process. Therefore, occurs the anomalous thermal transport behavior, i.e. thermal conductivity first rises and then drops with applied biaxial strain increasing. Our work paves the way for modulating two-dimensional AlN performance and provides a new insight for designing promising novel two-dimensional semiconductors.

2D BN-biphenylene: structure stability and properties tenability from a DFT perspective.

With state-of-the-art density functional theory and ab initio molecular dynamics, we explored the BN-analog of a recently synthesized 2D biphenylene sheet. Its dynamical, thermal, and mechanical stability has been confirmed with phonon spectrum calculations, ab initio molecular dynamics (AIMD), and Born criteria, and its synthesis feasibility has been ascertained in terms of cohesive energy. The phonon spectrum and AIMD results show all positive frequencies and negligible variations in structure (bond length), kinetic and total energy for 5 ps, respectively. Having found this stable BN-BPh, we have analyzed its electronic, mechanical, optical, and vibration properties. Electronic property results (density of states and band structure) show that the BN-equivalent of metallic BPh has a large band gap of 3.12 eV and 4.48 eV with the PBE and HSE06 DFT functionals, respectively, similar to the graphene BN-equivalent (hBN) with a band gap of 6.1 eV. The mechanical strength of BN-BPh is close to that of BPh with a difference of ∼15 N m-1 and ∼10 N m-1 along the x- and y-directions, and its Young's modulus indicates that BN-BPh is stiffer than BN-graphyne. The optical absorption coefficient of BN-BPh shows a first peak at 4.48 eV (3.12 eV) with the HSE06 (PBE) functional. Infrared and Raman spectrum intensity and irreducible representation are computed, and the first IR (Raman) peak is found at 1300 cm-1 (1000 cm-1) for BN-BPh. Finally, substitutions of external atoms such as P, Al, As, C, Li, Mg, S, and Si for B and N atoms in BN-BPh tune its electronic properties to semiconducting, semi-metallic, and even metallic. Considering its mechanical, optical, and band gap tunability, BN-BPh might be useful in electronic, optical, and spintronics devices, even in high-pressure environments.

Bilayer hexagonal structure MnN2 nanosheets with room-temperature ferromagnetic half-metal behavior and a tunable electronic structure.

Two-dimensional (2D) layered materials have atomically thin thickness and outstanding physical properties, attracting intensive research in the past year. As one of these materials, a 2D magnet is an ideal platform for fundamental physics research and magnetic device development. Recently, a non-MoS2-type geometry was found to be more favorable in 2D transition-metal dinitrides. In this work, driven by this new configuration, we perform a comprehensive first-principles study on the bilayer hexagonal structure of 2D manganese dinitrides. Our results show that 2D MnN2 is a ferromagnetic half-metal at its ground state with 100% spin-polarization ratio at the Fermi energy level. The phonon spectrum calculation and ab initio molecular dynamics simulation show that the 2D MnN2 crystal has a high thermodynamic stability and its 2D lattice can be retained at room-temperature. Monte Carlo simulations based on the Heisenberg model predict a Curie temperature of over 563 K and its electronic properties can be regulated by biaxial strain. The half-metallic states are mainly contributed by Mn d orbitals, and the magnetic exchange of the system mainly comes from the Mn-N-Mn super-exchange. The p-d orbital hybridization will provide a small antiparallel magnetic moment of N atoms, and the p-orbital dangling bond can be eliminated by oxidation to enhance the total magnetic moment of the system. The study of magnetic anisotropy energy indicates that the easy magnetization axis is in-plane and hybridization between Mn dyz and dz2 orbitals gives the largest magnetic anisotropy contribution. In view of these results, we consider that novel 2D MnN2 is one of the most promising two-dimensional materials for nano-spintronic applications.

Intrinsic half-metallicity in two-dimensional Cr2TeX2 (X = I, Br, Cl) monolayers.

Two-dimensional (2D) materials with intrinsic half-metallicity at or above room temperature are important in spin nanodevices. Nevertheless, such 2D materials in experiment are still rarely realized. In this work, a new family of 2D Cr2TeX2 (X = I, Br, Cl) monolayers has been predicted using first-principles calculations. The monolayer is made of five atomic sublayers with ABCAB-type stacking along the perpendicular direction. It is found that the energies for all the ferromagnetic (FM) half-metallic states are the lowest. The phonon spectrum calculations and molecular dynamics simulations both demonstrate that the FM states are stable, indicating the possibility of experimentally obtaining the 2D Cr2TeX2 monolayers with half-metallicity. The Curie temperatures from Monte Carlo simulations are 486, 445, and 451 K for Cr2TeI2, Cr2TeBr2, and Cr2TeCl2 monolayers, respectively, and their half-metallic bandgaps are 1.72, 1.86 and 1.90 eV. The corresponding magnetocrystalline anisotropy energies (MAEs) are about 1185, 502, 899 μeV per Cr atom for Cr2TeX2 monolayers, in which the easy axes are along the plane for the Cr2TeBr2 and Cr2TeCl2 monolayers, but being out of the plane in the Cr2TeI2. Our study implies the potential application of the 2D Cr2TeX2 (X = I, Br, Cl) monolayers in spin nanodevices.

Heat capacity and thermal conductivity of CdCr2Se4 ferromagnet: Magnetic field dependence, experiment and calculations

The low-temperature specific heat capacity and thermal conductivity under magnetic field 0–13 T is investigated on polycrystalline sample of the ferromagnetic insulator CdCr2Se4, possessing spinel structure with magnetic Cr3+ ions in the octahedrally coordinated sites. The phonon (lattice) and magnon contributions are separated by their characteristic temperature dependencies and by quenching of the magnon component under high magnetic fields in the isothermal measurements. The results are analyzed with respect to formulas for model systems and confronted with ab-initio calculations of phonon and magnon spectra. The theoretical formula for the field-induced quenching of magnon heat capacity is well reproduced, but there are two kinds of deviations from expected behavior deserving attention: (1) Heat capacity at low fields is by several percent lower than expected and approaches the theoretical curve only above ∼0.5 T. We propose that the zero-field magnon heat capacity is diminished by a presence of magnetic domain walls and the expected field dependence is restored when magnetic field converts the sample into a single domain state. (2) The suppression of magnon heat capacity under high magnetic field is incomplete. While the data follow well the theoretical line for given temperature as regards the curvature, the final saturation seems to tend several percent above the expected value for lattice heat capacity. We propose that this behavior is related to the phonon-magnon coupling arising due to Zeeman gap in the magnon spectra and subsequent formation of intersection points with the phonon spectra. The thermal conductivity has shown substantial deviations with respect to model predictions as concerns temperature dependence of the phonon contribution and suppression of the magnon contribution in markedly lower magnetic fields than predicted by the model. We ascribe this discrepancy to a complex character of the intergrain heat transfer.

Prediction of novel tetravalent metal pentazolate salts with anharmonic effect

In recent decades, pentazolate salts have gained considerable attention as high energy density materials (HEDMs). Using the machine-learning accelerated structure searching method, we predicted four pentazolate salts stabilized with tetravalent metals (Ti-N and Zr-N). Specifically, the ground state MN20 (M = Ti, Zr) adopts the space-group P4/mcc under ambient conditions, transforming into the I-4 phase at higher pressure. Moreover, the I-4-MN20 becomes energetically stable at moderate pressure (46.8 GPa for TiN20, 38.7 GPa for ZrN20). Anharmonic phonon spectrum calculations demonstrate the dynamic stabilities of these MN20 phases. Among them, the P4/mcc phase can be quenched to 0 GPa. Further ab-initio molecular dynamic simulations suggest that the N5 rings within these MN20 systems can still maintain integrity at finite temperatures. Calculations of the projected crystal orbital Hamilton population and reduced density gradient revealed their covalent and noncovalent interactions, respectively. The aromaticity of the N5 ring was investigated by molecular orbital theory. Finally, we predicted that these MN20 compounds have very high energy densities and exhibit good detonation velocities and pressures, compared to the HMX explosive. These calculations enrich the family of pentazolate compounds and may also guide future experiments.

Pressure stabilized polymeric nitrogen in N2F and N10F compounds

Polymeric nitrogen possessing ambient quench recoverability is considered to be the most powerful and green high-energy density material, which has triggered extensive interest. Herein, we predicted two hitherto unknown nitrogen-rich nitrogen fluorine compounds with stoichiometries N2F and N10F using structure searching method and first-principles calculations. N2F and N10F are thermodynamically stable relative to ε-N and fluorine at 67–110 and 87–110 GPa, respectively. Both N2F and N10F encompass N6 rings, forming one-dimensional N∞ ribbons by edge-sharing and three-dimensional channel-like open framework by edge-sharing and two N atoms-bridging, respectively. Phonon spectra calculations and molecular dynamics simulations demonstrate the ambient-pressure quench recoverability of the two predicted phases. Interestingly, after the removal of F atoms from open channel in N10F, the polymeric nitrogen framework mC10-N is retained at ambient conditions with the energy density of 11.20 kJ/g, rendering it a remarkable high-energy density material.

Janus Type Monolayers of S-MoSiN2 Family and Van Der Waals Heterostructures with Graphene: DFT-Based Study.

Novel representative 2D materials of the Janus type family X-M-ZN2 are studied. These materials are hybrids of a transition metal dichalcogenide and a material from the MoSi2N4 family, and they were constructed and optimized from the MoSi2N4 monolayer by the substitution of SiN2 group on one side by chalcogen atoms (sulfur, selenium, or tellurium), and possibly replacing molybdenum (Mo) to tungsten (W) and/or silicon (Si) to germanium (Ge). The stability of novel materials is evaluated by calculating phonon spectra and binding energies. Mechanical, electronic, and optical characteristics are calculated by methods based on the density functional theory. All considered 2D materials are semiconductors with a substantial bandgap (>1 eV). The mirror symmetry breaking is the cause of a significant built-in electric field and intrinsic dipole moment. The spin-orbit coupling (SOC) is estimated by calculations of SOC polarized bandstructures for four most stable X-M-ZN2 structures. The possible van der Waals heterostructures of considered Janus type monolayers with graphene are constructed and optimized. It is demonstrated that monolayers can serve as outer plates in conducting layers (with graphene) for shielding a constant external electric field.

Electronic structures and stability investigation of large band gap topological insulators MTl4Te3 ( M = Cd, Hg)

By means of ternary chemical potential phase diagram and phonon spectrum calculations, we propose that $M$Tl$_4$Te$_3$ ($M$ = Cd, Hg), the derivatives of Tl$_5$Te$_3$, are thermodynamically and dynamically stable in the body centered tetragonal crystal structure with $I$4/$mcm$ symmetry. Our electronic structures calculations confirm that a robust $s$-$p$ band inversion occurs near the Fermi level in $M$Tl$_4$Te$_3$, and a topological band gap about 0.13 eV in CdTl$_4$Te$_3$ is induced by the spin-orbit coupling. These results suggest that $M$Tl$_4$Te$_3$ are a new class of large band gap 3D strong topological insulators that are stable and synthesizable in experiments, which could be used to design efficient spin torque equipment and spin device.

The site preference and doping effect on mechanical properties of Ni3Al-based γ′ phase in superalloys by combing first-principles calculations and thermodynamic model

The fundamental aspects of site preference of alloying elements on sublattice of the strengthen γ′ phase with L12 structure have not been well understood, which hinders the optimized design of advanced Ni-based high-temperature alloys. In this contribution, the temperature- and composition-dependent site occupying preferences of the binary, ternary, and quaternary of Ni3Al-based γ′ phase alloyed with Mi where Mi represents the additional transitional metals Co, Cr, Cu, Fe, Mn, Mo, Re, Ta, Ti, V, or W atoms (arranged in alphabetical order) chosen frequently, were studied using a two-sublattice thermodynamic model (Ni, Al, Mi)1a(Ni, Al, Mi)3c. The site occupying fractions (SOFs) were calculated based on a thermodynamic database established in this work, where the thermodynamic data of the end-members involved were obtained using first-principles calculations and phonon spectrum calculations. The calculated SOFs results show that there is an obvious site preference for stoichiometry binary Ni3Al, and its site configuration changes from (Al)1a(Ni)3c at room temperature to (Al0.9984Ni0.0015)1a (Al0Ni0.9994)3c at 1273 K. For the γ′ phase with the composition 78Ni-26Al-4Mi (atom ratio and xNi/xAl = 3:1), Mo atoms always preferred to occupy the 1a sublattice (Al site), Co, Mn, and Ti atoms always prefer the 3c sublattice (Ni site) in the whole temperature range, while the site preference of Cr, Cu, Fe, Re, Ta, V, or W atom is affected by temperature. For example, when the heat treatment temperature is lower than 700 K, Cr, Cu, Fe, Ta, V, and W atoms occupy the 1a and 3c sublattice randomly, and Re atoms prefer to 3c sublattice, while when the heat treatment temperature is higher than 1273 K, Cr, Cu, and W atoms prefer 3c sublattice, Fe and Ta atoms prefer to 1a sublattice, while all Re atoms occupy the 3c sublattice exclusively, and all V atoms occupy the 1a sublattice exclusively, respectively. Likewise, the site preference of the quaternary system with selective compositions 78Ni-26Al-2 M1-2 M2 was also predicted. Based on calculated SOFs results, the mechanical and thermodynamic properties were studied at the ground state. It has been revealed that Cr, Re, and V doping can improve the microhardness of Ni3Al alloys; in particular, the effect of Cr is extraordinary; and all elements, except Mn, Mo, and Ti, would enhance the bulk modulus of Ni3Al-based γ′ phase, in which Mn have the greatest influence on reducing the bulk and shear modulus, respectively. Furthermore, all the B/G ratios of the computed Ni3Al-based γ′ phase are >1.75, showing inherent ductility. Only Cr doping significantly enhances the Debye temperature of the Ni3Al-based γ′ phase.

Tunable Electronic Properties of Novel 2D Janus MSiGeN4 (M = Ti, Zr, Hf) Monolayers by Strain and External Electric Field

AbstractSince the recent successful experimental synthesis of MoSi2N4 [Science, 369 (2020), 670], the “MA2Z4 family” has been of particular interest to the scientists in the field of materials science due to its outstanding physical properties. In this paper, the first‐principles calculations are performed to study the structural, elastic, and electronic properties of novel two‐dimensional (2D) Janus MSiGeN4 monolayers (M = Ti, Zr, Hf). The calculations of phonon spectra indicate that monolayers MSiGeN4 are dynamically stable and can be experimentally synthesized. The obtained Young's modulus and Poisson's ratio of the Janus structures MSiGeN4 are much larger than that of other binary 2D materials and meet the mechanical stability criteria suggested by Born and Huang. In the calculations using either PBE or HSE06 functionals, the Janus MSiGeN4 structures exhibit indirect semiconductor characteristics with larger band gaps than that of similar septuple‐atomic‐layer materials, such as MoSiGeN4 and WSiGeN4. In addition, the influences of biaxial strain and external electric field on the electronic structure of MSiGeN4 are investigated. It is found that the biaxial strain tunes the electronic characteristics more significantly than the external electric field. The obtained results can provide insights into novel Janus monolayers with potential applications in electronic devices.

Express: Extensible, high-level workflows for swifter ab initio materials modeling

In this work, we introduce an open-source Julia project, express, an extensible, lightweight, high-throughput, high-level workflow framework that aims to automate ab initio calculations for the materials science community. express is shipped with well-tested workflow templates, including structure optimization, equation of state (EOS) fitting, phonon spectrum (lattice dynamics) calculation, and thermodynamic property calculation in the framework of the quasi-harmonic approximation (QHA). It is designed to be highly modularized so that its components can be reused across various occasions, and customized workflows can be built on top of that. Users can also track the status of workflows in real-time, and rerun failed jobs thanks to the data lineage feature express provides. Two working examples, i.e., all workflows applied to lime and akimotoite, are also presented in the code and this paper. Program summaryProgram Title:expressCPC Library link to program files:https://doi.org/10.17632/bbzzrxsm4b.1Developer's repository link:https://github.com/MineralsCloud/Express.jlLicensing provisions: GNU General Public License v3.0Programming language:JuliaNature of problem: High-performance ab initio calculation is gaining more and more popularity when investigating the physical and chemical properties of materials in the scientific community. There are several ab initio programs in the market, but they are often not user-friendly to new users because of their intrinsic complexity. Even for familiar users, dealing with daily preparation and post-analysis could be trivial and fallible. Therefore, plenty of workflow frameworks have been developed to solve this problem. However, most of them cannot meet our expectations for several reasons. They are either too complex to read and modify, or too heavy for simple tasks. Plus, they put little focus on addressing the real pain points of everyday calculations, such as data preparation and output parsing.Solution method: We developed a workflow framework that can simplify this process, i.e., most of the mundane work can be replaced by writing a few lines of configurations. We also automated the three most-used work procedures into configurable workflows: equations of state fitting (structural optimization), phonon spectrum calculation, and thermodynamics calculation.

Stress-driven structural and bond reconstruction in 2D ferromagnetic semiconductor VSe2

Two-dimensional (2D) semiconducting transition metal dichalcogenides can be used to make high-performance electronic, spintronic, and optoelectronic devices. Recently, room-temperature ferromagnetism and semiconduction in 2D VSe2 nanoflakes were attributed to the stable 2H-phase of VSe2 in the 2D limit. Here, our first-principles investigation shows that a metastable semiconducting H′ phase can be formed from the H VSe2 monolayer through uniaxial stress or uniaxial strain. The calculated phonon spectra indicate the dynamical stability of the metastable H′ VSe2 and the path of phase switching between the H and H′ VSe2 phases is calculated. For the uniaxial stress (or strain) scheme, the H′ phase can become lower in total energy than the H phase at a transition point. The H′ phase has stronger ferromagnetism and its Curier temperature can be enhanced by applying uniaxial stress or strain. Applying uniaxial stress or strain can substantially change spin-resolved electronic structures, energy band edges, and effective carrier masses for both of the H and H′ phases, and can cause some flat bands near the band edges in the strained H′ phase. Further analysis indicates that one of the Se–Se bonds in the H′ phase can be shortened by 19% and the related Se–V–Se bond angles are reduced by 23% with respect to those of the H phase, which is believed to increase the Se–Se covalence feature and reduce the valence of the nearby V atoms. Therefore, structural and bond reconstruction can be realized by applying uniaxial stress in such 2D ferromagnetic semiconductors for potential spintronic and optoelectronic applications.

A new 2D metallic K3Cl2 nanosheet as a promising candidate of NO2 gas sensor and capturer

Based on first-principles computations and global search method, we identify a new two-dimensional K3Cl2 nanosheet with unique structural and electrical properties. Formation energy and phonon spectrum calculations and ab initio molecular dynamics (AIMD) simulations confirm the stability of this predicted structure. The metallic character of the K3Cl2 can be maintained under tensile uniaxial and biaxial strains. The calculation demonstrates that K3Cl2 nanosheet can effectively detect NO2 molecules from the mixture of H2, H2O, NH3, CO, NO, NO2 and CO2. Thereason of the conversion from metal to semiconductor of K3Cl2 adsorbing different number of NO2 molecules mightlie in the change of bonding/antibonding state of K-N, K-O and NO bonds. Our calculated results show that K3Cl2 nanosheet is a promising NO2 gas sensor and capturer with high selectivity and sensitivity.

Lithium decorated C3N as high capacity reversible hydrogen storage material: Insights from density functional theory

AbstractLithium‐decorated (Li‐decorated) C3N has been investigated as a potential material for high capacity reversible hydrogen storage. The energetic stability, dynamical stability, and thermal stability were studied. The cohesive energy of C3N (7.03 eV/atom) denotes the energetic stability, imaginary frequencies are not found from the result of phonon spectrum calculation, and the free energy vibrates slightly around −64.63 eV during the 5000 fs period and no structure reconstruction. Electronic properties showed the band gaps are 0.39 and 1.12 eV, via PBE and HSE calculations, respectively. The four probable Li‐adsorbed sites were calculated, indicating that the hollow site above the center of a hexagon ring HC site is the most likely site to absorb Li atom. Hydrogen molecules were added one by one to research the maximum hydrogen gravimetric density. Each Li atom can attach 10 hydrogen molecules within the range of physical adsorption processes (−0.1 to −0.4 eV/H2) and the hydrogen storage capacity can reach as high as 8.81 wt%. Li‐decorated C3N has great potential for on‐board reversible solid‐state hydrogen molecule storage application.

Metallic three-dimensional porous siligraphene as a superior anode material for Li/Na/K-ion batteries

Due to the high specific capacity and low open circuit voltage (OCV), 2D siligraphene has been widely considered as a promising anode material for metal ion batteries (MIBs). Nonetheless, its electrochemical performance is greatly impeded by low mechanical stiffness, poor hopping dynamics and small pore size. Motivated by the great success of 3D carbon materials, we propose a metallic porous 3D-SiC anode using the corresponding 2D tetragonal SiC as a structural unit. By first principles molecular dynamics, mechanical property and phonon spectrum calculations, it is found that 3D-SiC possesses good thermal, mechanical and dynamical stability. The maximum Young’s and bulk moduli of 3D-SiC are 217.16, 400.90 GPa, respectively, exhibiting a moderate mechanical stiffness. More importantly, the intrinsically high electrical conductivity, unique porous structure and low mass density make the 3D-SiC a promising anode candidate for Li/Na/K-ion batteries with small volume changes (6.43 %, 3.75 % and 8.66 %), low diffusion barriers (0.17, 0.19 and 0.017 eV), high storage capacities (947, 947 and 724 mA h/g) and low average OCVs (0.56, 0.34 and 0.11 V). The encouraging results show that siligraphene-based porous 3D anodes are worthy of further investigation for MIBs.

Structure and stability of van der Waals layered group-IV monochalcogenides

As the quest for versatile and multifunctional 2D materials has expanded beyond graphene, hexagonal boron nitride, and transition metal dichalcogenides, van der Waals (vdW) layered monochalcogenides have gathered significant attention due to their attractive (opto)electronic, thermoelectric, and topological properties. These quasi-2D (q2D) materials are also valuable precursors for high-quality 2D materials, thus enlarging the range of materials’ properties and associated functionalities for novel applications. Using density functional theory calculations, we report on the stability of vdW-layered phases of group-IV AX monochalcogenides (where A and X belong, respectively to the sets {C, Si, Ge, Sn, Pb} and {S, Se, Te}) in six potential structural types, some of which not heretofore synthesized. We report phonon spectrum calculations and evaluate their thermodynamic stability using the formation enthalpy. Based on these results on dynamic stability and formation enthalpy of a total of 90 q2D monochalcogenide structures, we suggest that some of the new materials reported here would be synthesizable in current laboratory conditions. Our results, thus, provide guidance for future experimental synthesis and characterization studies and would enable subsequent implementation of novel AX q2D monochalcogenides in various nanoelectronic devices.

Predicted Pressure-Induced High-Energy-Density Iron Pentazolate Salts

Metal-pentazolate compounds as candidates for novel high-energy-density materials have attracted extensive attention in recent years. However, dehydrated pentazolate salts of transition metal iron are rarely reported. We predict two new iron pentazolate salts Fdd2-FeN10 and (No.1)-FeN10 using a constrained crystal search method based on first-principles calculations. We propose that the stable Fdd2-FeN10 crystal may be synthesized from FeN and N2 above 20 GPa, and its formation enthalpy is lower than the reported iron pentazolate salt (marked as (No.2)-FeN10). Crystal (No.1)-FeN10 is composed of iron bispentazole molecules. Formation enthalpy, phonon spectrum and ab initio molecular dynamics calculations are performed to show their thermodynamic, mechanical and dynamic properties. Moreover, the high energy density (3.709 kJ/g, 6.349 kJ/g) and good explosive performance indicate their potential applications as high-energy-density materials.