Pressure-stabilized high-energy-density material YN₁₀

Aslandukov A., Aslandukova A., Laniel D., Koemets I., Fedotenko T., Yuan L., Steinle-Neumann G., Glazyrin K., Hanfland M., Dubrovinsky L., Dubrovinskaia N.

Miao J., Lu Z., Peng F., Lu C.

Polymeric nitrogen is a promising candidate for a high-energy-density material. Synthesis of energetic compounds with high chemical stability under ambient conditions is still a challenging problem. Here we report a theoretical study on yttrium nitrides by first principles calculations combined with an effective crystal structure search method. It is found that many yttrium nitrides with high nitrogen content can be formed under relatively moderate pressures. The results indicate that the nitrogen-rich YN5 and YN8 compounds are recoverable as metastable high-energy materials under ambient conditions, and can release enormous energies (2.51 kJ·g−1 and 3.18 kJ·g−1) while decomposing to molecular nitrogen and YN. Our findings enrich the family of transition metal nitrides, and open avenues for design and synthesis of novel high-energy-density materials.

Bykov M., Fedotenko T., Chariton S., Laniel D., Glazyrin K., Hanfland M., Smith J., Prakapenka V., Mahmood M., Goncharov A., Ponomareva A., Tasnádi F., Abrikosov A., Bin Masood T., Hotz I., et. al.

High pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here we report the synthesis at ~90 GPa of novel beryllium polynitrides, monoclinic and triclinic BeN4. The triclinic phase, upon decompression to ambient conditions, transforms into a compound with atomic-thick BeN4 layers interconnected via weak van der Waals bonds consisting of polyacetylene-like nitrogen chains with conjugated {\pi}-systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN4 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 BeN4 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.

Miao M., Sun Y., Zurek E., Lin H.

Thanks to the development of experimental high-pressure techniques and methods for crystal-structure prediction based on quantum mechanics, in the past decade, numerous new compounds, mostly binary, with atypical compositions have been predicted, and some have been synthesized. Differing from conventional solid-state materials, many of these new compounds are comprised of various homonuclear chemical species, such as dimers, trimers, pentagonal and heptagonal rings, polymeric chains, atomic layers and 3D networks. Strikingly, it has been shown that pressure can alter the chemistry of an element by activating its (semi)core electrons, unoccupied orbitals and even the non-atom-centred quantum orbitals located on the interstitial sites, leading to many new surprising phenomena. This Review provides a summary of atypical compounds that result from the effects of high pressure on either the chemical bonds or the local orbitals. We describe various unusual chemical species and motifs, show how the chemical properties of the elements are altered under pressure and illustrate how compound formation is favoured even in situations in which chemical bonds are not formed. An extraordinary new picture of chemistry emerges as we piece together these unexpected high-pressure phenomena. In marked contrast to the previously held beliefs regarding the behaviour of solids under pressure, we are learning that the quantum-mechanical features of electrons, such as those that lead to the formation of directional bonds, inhomogeneous distributions of electrons and atoms, as well as variations in symmetry, might be magnified under pressure. We discuss the influence of these phenomena on future studies that will probe chemistry at higher pressures and explore more complex chemical compositions than those that have been studied to date. High pressure leads to striking new chemistry. Many new compounds with atypical compositions and a plethora of novel chemical species can be stabilized by the formation of homonuclear bonds and the activation of core electrons, non-valence and non-atomic orbitals.

Yi W., Zhao L., Liu X., Chen X., Zheng Y., Miao M.

Stabilizing pentazolate anion in solid compounds is a challenging and important approach toward making environmentally friendly high energy density materials. Using a constrained crystal structure search method and rigorous bond analysis, we predicted the metastable structures of Al (N5)3 and Mg(N5)2. In contrast to common wisdom, our work demonstrates that metals, especially the multivalent metals such as Al and Mg, can stabilize pentazolate anions by forming strong metal-N bonds. Both compounds are metastable at ambient pressure and temperatures as high as 600 K. The multivalent metals not only stabilize the pentazolate anions but also greatly enhance the energy density. The decomposition of Al(N5)3 or Mg(N5)2 can release an energy of 4.61 kJ/g or 3.10 kJ/g, respectively, making them promising candidates as high energy density materials.

Ji C., Adeleke A.A., Yang L., Wan B., Gou H., Yao Y., Li B., Meng Y., Smith J.S., Prakapenka V.B., Liu W., Shen G., Mao W.L., Mao H.

We synthesized a new allotrope of nitrogen, providing prospects for nitrogen-based high-energy-density materials and 2D materials. Group V elements in crystal structure isostructural to black phosphorus with unique puckered two-dimensional layers exhibit exciting physical and chemical phenomena. However, as the first element of group V, nitrogen has never been found in the black phosphorus structure. Here, we report the synthesis of the black phosphorus–structured nitrogen at 146 GPa and 2200 K. Metastable black phosphorus–structured nitrogen was retained after quenching it to room temperature under compression and characterized in situ during decompression to 48 GPa, using synchrotron x-ray diffraction and Raman spectroscopy. We show that the original molecular nitrogen is transformed into extended single-bonded structure through gauche and trans conformations. Raman spectroscopy shows that black phosphorus–structured nitrogen is strongly anisotropic and exhibits high Raman intensities in two Ag normal modes. Synthesis of black phosphorus–structured nitrogen provides a firm base for exploring new type of high-energy-density nitrogen and a new direction of two-dimensional nitrogen.

Yi W., Zhao K., Wang Z., Yang B., Liu Z., Liu X.

A series of excellent works have demonstrated that high-nitrogen-content metal pentazolate (cyclo-N5-) compounds could be stabilized by high pressure. However, under ambient conditions, low stability precludes their synthesis and application in the field of high-energy-density material. In this work, by using a constrained structure search method, we predicted two new structures as P212121-CuN5 and P21/c-CuN5 containing cyclo-N5- with strong N-N and Cu-N bonds. In both structures, cyclo-N5- form four coordination with the Cu+ ligand, which increases the structural stability by lowering the disturbance to the aromaticity of cyclo-N5-. The calculated results show that the P212121-CuN5 and P21/c-CuN5 structures exhibit high dynamic and thermal stability up to 400 K, indicating that they can be stabilized under ambient conditions. The decomposing energy of P212121-CuN5 and P21/c-CuN5 can reach up to 2.40 and 2.42 kJ/g, respectively. Strikingly, the detonation velocity and the pressure of P212121-CuN5 is predicted to be up to 10.42 km/s and 617.46 kbar, respectively, indicating that they are promising high-energy candidates in the field of explosive combustion.

Wei S., Lian L., Liu Y., Li D., Liu Z., Cui T.

We predicted several N-rich structures under high pressure. C2/c-SrN4 can make the ambient-pressure recovery possible. The energy densities for C2/m-SrN3 and P1̄-SrN5 are 1.08 and 1.09 kJ g−1, respectively, similar to that of common energy materials.

Xia K., Yuan J., Zheng X., Liu C., Gao H., Wu Q., Sun J.

High-energy-density materials (HEDMs) have been intensively studied for their significance in fundamental sciences and practical applications. Here, using the molecular crystal structure search method based on first-principles calculations, we have predicted a series of metastable energetic trivalent metal pentazolate salts MN15 (M= Al, Ga, Sc, and Y). These compounds have high energy densities, with the highest nitrogen content among the studied nitrides so far. Pentazolate N5- molecules stack up face-to-face and form wave-like patterns in the C2221 and Cc symmetries. The strong covalent bonding and very weak noncovalent interactions with nonbonded overlaps coexist in these ionic-like structures. We find MN15 molecular structures are mechanically stable up to high temperature (∼1000 K) and ambient pressure. More importantly, these trivalent metal pentazolate salts have high detonation pressure (∼80 GPa) and velocity (∼12 km/s). Their detonation pressures exceeding that of TNT and HMX make them good candidates for high-brisance green energetic materials.

Kurban M.

In this study, the electronic structure and structural transition of the GaAs clusters with different compositions were examined by quantum chemical calculations for the first time. The GaAs clusters exhibit highly interesting structural and electronic properties as a function of composition, temperature, and pressure. The phase transitions were observed from the zinc blende structure ( P 4 ¯ 3 m ) to the triclinic ( P 1 ¯ ) and the tetragonal ( P 4 ¯ ) structure where there are two and four intermediate phases, respectively. The As-rich clusters are generally more stable than that of the Ga-rich. The HOMO, LUMO and gap energies, Fermi levels, dipole moments and density of states were analyzed. The gap energy for the Ga8As32 cluster was predicted as about 1.22 eV wide, i.e., about 0.29 eV smaller than the measured band gap of bulk Ga0.5As0.5 1.51 eV at T = 0 K, while the gap energy for the Ga32As8 is found to be 0.15 eV. The Ga8As32 and Ga32As8 clusters show semiconductor characters with high and low band energy at different pressures, while the Ga32As8 cluster shows metallic character under heat treatment. The trend of energy gap of the clusters is also compatible with available experimental findings.

Kurban M., Kürkçü C., Yamçıçıer Ç., Göktaş F.

In this study, the structural phase transition and optoelectronic properties of perovskite-hydride MgFeH3 under high pressure have been performed by ab initio calculations based on GGA-PBE functional. The phase transitions were observed from the cubic structure ([Formula: see text]) to the orthorhombic [Formula: see text] and [Formula: see text] structure. During the phase transition, the [Formula: see text] and [Formula: see text] intermediate phases were predicted. The energy-volume (E-V) relationships show that the most stable phase is [Formula: see text]. The lattice parameters and volume increased as based on the phase transforms. From the electronic band analysis, the MgFeH3 shows a metallic character from the cubic to orthorhombic structure. The MgFeH3 indicates the peaks at 2.67 eV (464 nm) for [Formula: see text] phase, 5.21 eV (238 nm) for [Formula: see text] phase and 2.63 eV (471 nm) for [Formula: see text] phase. [Formula: see text] and [Formula: see text] phases correspond to the visible region. The absorption peaks are getting wider and have higher magnitude from [Formula: see text] to [Formula: see text] phase. The optical conductivity for the cubic structure with [Formula: see text] phase was found to be higher than orthorhombic structures with [Formula: see text], and [Formula: see text] phases. The reflectivity maxima decrease from [Formula: see text] to [Formula: see text].

Kürkçü C., Yamçıçıer Ç., Kurban M.

CaS crystallizes in cubic NaCl (B1) type structure with symmetry Fm 3 ¯ m . In this work, the structural and electronic properties of CaS were investigated by considering the Density Functional calculations within the framework of Generalized Gradient Approximation (GGA) under high pressure. The structural change was found at the B1 type structure of CaS. B1 type structure transformed into another cubic CsCl (B2) type structure with symmetry Pm 3 ¯ m at 36.6 GPa. An intermediate state with symmetry R 3 ¯ m was predicted during this transition. Besides, the effects of the pressure on the electronic properties of CaS were also studied. Both the B1 and B2 type structures exhibited semiconducting behaviors with direct band gaps at the Γ -point and R -point, respectively. Intermediate state was searched during this phase change first time in detail. The obtained results were compared with experimental and theoretical ones in the literature.

Xia K., Zheng X., Yuan J., Liu C., Gao H., Wu Q., Sun J.

Polynitrogen compounds especially pentazolate anion complexes recently have attracted substantial attention due to their promising potential as high-energy-density materials. Here, using a machine-learning-accelerated crystal structure search method and first-principles calculations, we predict a new hybrid compound by inserting a large fraction of nitrogen into alkaline-earth metals. It is a new stoichiometric type MN10 (M = Be, Mg), which possesses a metal-centering octahedral pentazolate framework with the space group Fdd2. This type of ionic-like molecular crystal is found to be energetically more favorable than the mixtures of M3N2 or MN4 compounds and pure nitrogen and is possibly synthesized at relatively low pressures (around 12 GPa for MgN10). The ab initio molecular dynamics simulations show that they are metastable and can be quenched to ambient conditions once synthesized at high pressure. Moreover, decomposition of this polymeric MN10 structure can release a large amount of energy and shows h...

Gao B., Gao P., Lu S., Lv J., Wang Y., Ma Y.

Binns J., Donnelly M., Peña-Alvarez M., Wang M., Gregoryanz E., Hermann A., Dalladay-Simpson P., Howie R.T.

Transition-metal nitrides have applications in a range of technological fields. Recent experiments have shown that new nitrogen-bearing compounds can be accessed through a combination of high temperatures and pressures, revealing a richer chemistry than was previously assumed. Here, we show that at pressures above 50 GPa and temperatures greater than 1500 K elemental copper reacts with nitrogen, forming copper diazenide (CuN2). Through a combination of synchrotron X-ray diffraction and first-principles calculations we have explored the stability and electronic structure of CuN2. We find that the novel compound remains stable down to 25 GPa before decomposing to its constituent elements. Electronic structure calculations show that CuN2 is metallic and exhibits partially filled N2 antibonding orbitals, leading to an ambiguous electronic structure between Cu+/Cu2+. This leads to weak Cu-N bonds and the lowest bulk modulus observed for any transition-metal nitride.

Zhang B., Xu M., Wang Z., Hao J., Li Y.

Wang J., Kong P., Jiang Z., Zhang D., Jing Y., Dai W.

Chen M., Huang S., Fu P., Chen B., Chen C., Bi J., Ding K., Lu C.

Abstract Clusters represent intermediate states between isolated atoms and bulk solids. They serve as model systems to elucidate the physical properties of compounds from the atomic or molecular scale to the macroscopic bulk phase. Here, we perform thorough structure searches of neutral boron doped nitrogen clusters by crystal structural analysis by particle swarm optimization cluster structural prediction and density functional theory calculations. The calculated results indicate that the ground state structures of BN n (n= 4–16) clusters are evolutional from one-dimensional chains to two dimensional rings, and finally to three-dimensional (3D) geometries. Interestingly, the intriguing BN12 cluster, characterized by a 3D configuration with a central boron atom connecting four N3 chains in distinct directions, exhibits exceptional stability. The chemical bonding analysis reveals that the outstanding stability of BN12 cluster is attributed to the strong σ and π interactions between the 2p orbitals of the boron atom and the surrounding nitrogen atoms, as well as the robust σ bonds along the four nitrogen chains. The present findings offer important insights for understanding the geometries and electronic properties of neutral boron doped nitrogen clusters and provide an avenue for the design and synthesis of nitrogen-rich compounds.

Gao X., Wei S., Liu X., Zhang S., Li S., Chang Q., Sun Y.

Mikhailov Oleg V.

The review integrates and systematizes data published in the last 15 years on the physicochemical characteristics of specific chemical compounds formed by metal elements with nitrogen atoms containing three or more nitrogen atoms per metal atom. Most often, the total number of nitrogen atoms exceeds their greatest number allowed by the formal higher oxidation state of the metal atom present in the compound. The conceptual possibility of practical application of these compounds now and in the future is also discussed.The bibliography includes 230 references.

Gao X., Wei S., Guo Y., Yin G., Chang Q., Sun Y.

As shown in recent years, introducing exogenous elements (M) into nitrogen materials under pressure can form new multi-nitrogen structures (MNx) which are more stable and have milder synthesis conditions than pure nitrogen structures. We used the structure prediction software to search for the optimized components of transition metal yttrium nitrogen compounds, as well as for the stable crystal structures, by changing the chemical ratio and pressure conditions. We have predicted the six structures of yttrium nitrogen compounds: P1¯-YN4, C2/m-YN5, YN6 (C2/c phase, C2/m phase, P1¯ phase) and P1¯-YN7 under high pressure conditions, among them, the C2/c-YN6 and P1¯-YN7 are newly discovered in this paper. It was found that C2/c-YN6 and P1¯-YN7 can be synthesized by YN + N2 at high pressure. Molecular dynamics calculations show that the two structures are stable at high temperature. The structure analysis shows that the nitrogen atoms in the two structures are connected alternately by nitrogen single bond and nitrogen double bond, which is beneficial to energy storage of the structure. The energy density of these two structures is comparable to that of TATB and RDX, and the explosive velocity and explosive pressure are higher than those of TNT and HMX, so these two structures could be used as potential high energy density materials. The predicted structures of C2/c-YN6 and P1¯-YN7 may provide new clues for synthesis and exploration of novel stable polymeric nitrogen.

Yan H., Chen L., Yin R., Zhang Y., Zhang M., Wei Q.

The recently proposed transition metal polynitride, tetragonal TiN8 featured infinitely armchair-like N chains, has attracted much attention for its larger high-energy density value than the well-known TNT explosive. However, as a typical transition metal nitrogen-rich compounds with outstanding mechanical properties, the electronic properties and structure-mechanical relations remain to be further explored. Here, we performed first principles calculations that investigate the thermodynamic stabilities of tetragonal TMN8 (TM = Ti, Zr, Hf), and fully studied the mechanical and elastic anisotropic behavior of these materials. In particular, the stress-strain relations and the related evolutions of chemical bonding were systematically studied. The uniaxial ultra-incompressible nature of these TMN8 has been demonstrated by the calculated elastic moduli, originated from the strong N–N covalent bonds along the c-axis. Under (110) [11¯0] shear direction, the TMN8 exhibits an abnormally large breaking shear strain that produces a strong shear strength exceeding 40 GPa. This unusual behavior stems from the strongly and stably 3D structure framework consisted of strong N–N covalent bonds in infinite N chains connected by Ti atom through Ti–N covalent bonds in Ti–N hexahedrons. These obtained findings explicate the crystal structural configurations and chemical bonding characters that are responsible for the mechanical properties of TMN8 and provide insights for understanding other transition metal polynitrides.

Ding K., Wang P., Zhou W., Xu H., Ge Z., Zheng W., Lu C.

Abstract Polynitrogen clusters are of great interest as potential high-energy-density materials (HEDMs). However, it is still a challenge to effectively manufacture pure nitrogen clusters. Here, we present a method to produce all-nitrogen clusters ranging in size from 3 to 10 using a self-developed apparatus consisting of a laser vaporization cluster source and a mass spectrometer. The results indicate that the mass peak of the N4 + cluster is dominant, and the cooling of the cluster source with liquid nitrogen effectively enhanced the intensities of larger nitrogen clusters. The trace amounts of water in the carrier gas affects the relative signal intensities of nitrogen clusters, suggesting that water molecules can affect their stabilities. The geometrical structures of the N n + / 0 (n = 3–10) clusters are determined by CALYPSO cluster structural search method and density functional theory calculations. A series of promising polynitrogen clusters, such as Z-shaped N 4 + / 0 clusters and bicyclic N 8 + / 0 clusters, are identified. The molecular dynamics simulations indicate the decomposition energy barrier of (N4–H2O)+ cluster is lower than that of N4 + cluster by 2.3 kcal mol−1, while the decomposition energy barrier of (N8–H2O)+ cluster is higher than that of N8 + cluster by 1.4 kcal mol−1. These findings provide useful information for the generation and stabilization of polynitrogen clusters, which are valuable for the design and synthesis of HEDMs.

Zhang X., Li S., Kang J., Su J., Deng K.

Abstract Polymer bonded explosives (PBXs) are kind of composite materials consisting of multi-layers structures, where the interfacial interactions can significantly affect their structures, properties and performance. To investigate the determinant factors affecting the interfacial interactions, in this work, the adhesion works at different interfaces are studied by molecular dynamics simulations. A key observation is that the hydrogen bonds are found to be a decisive factor that directly affects the interfacial interactions. When the fluoropolymers change from F2321 to F2319, the adhesion works with the HMX and coupling agent layer present a monotonous decrease and increase, respectively, corresponding to the changes in the number of weak hydrogen bonds. Thus the hydrogen bonds can be utilized to benchmark the nonvalent interfacial interactions. Moreover, the coupling agent layer as an intermediary enhances the adsorption between the explosive crystal and the binder, whose thickness significantly impacts the interfacial interactions. Its interactions with the HMX and fluoropolymers both show a similar increase with respect to its thickness and then stabilize at the thickness above 2.5 nm, corresponding to a surface density of six KH550 chains per nm2. This study provides a basic understanding of the nonbonding adhesion mechanisms in the PBXs and is helpful for the material selection and structure design.

Wang Y., Li Z., Niu S., Yi W., Liu S., Yao Z., Liu B.

Synthesis pressure and structural stability are two crucial factors for highly energetic materials, and recent investigations have indicated that cerium is an efficient catalyst for N2 reduction reactions. Here, we systematically explore Ce–N compounds through first-principles calculations, demonstrating that the cerium atom can weaken the strength of the N≡N bond and that a rich variety of cerium polynitrides can be formed under moderate pressure. Significantly, P1̄-CeN6 possesses the lowest synthesis pressure of 32 GPa among layered metal polynitrides owing to the strong ligand effect of cerium. The layered structure of P1̄-CeN6 proposed here consists of novel N14 ring. To clarify the formation mechanism of P1̄-CeN6, the reaction path Ce + 3N2 → trans-CeN6 → P1̄-CeN6 is proposed. In addition, P1̄-CeN6 possesses high hardness (20.73 GPa) and can be quenched to ambient conditions. Charge transfer between cerium atoms and N14 rings plays a crucial role in structural stability. Furthermore, the volumetric energy density (11.20 kJ/cm3) of P1̄-CeN6 is much larger than that of TNT (7.05 kJ/cm3), and its detonation pressure (128.95 GPa) and detonation velocity (13.60 km/s) are respectively about seven times and twice those of TNT, and it is therefore a promising high-energy-density material.