Cerium-promoted conversion of dinitrogen into high-energy-density material CeN6 under moderate pressure

Cai H., Wang X., Zheng Y., Jiang X., Zeng J., Feng Y., Chen K.

Abstract To explore high-energy-density materials, intense attention has been focused on how to stabilize the N–N bond in nitrogen-rich compounds. Here, we report several stable phases of erbium–nitrogen compounds ErN x as high-energy-density materials. Specifically, the phase diagrams of stable high-pressure structures Immm-ErN2, C2-ErN3, P 1 ˉ -ErN4, and P 1 ˉ -ErN6, are theoretically studied by combining first-principles calculation with particle swarm optimization algorithm. In these erbium–nitrogen compounds, the N–N bonds are stabilized as diatomic quasi-molecule N2, helical-like nitrogen chains, armchair nitrogen chains, and armchair–anti-armchair nitrogen chains, respectively. Among them, the P 1 ˉ -ErN6 harbors excellent stability at high thermal up to 1000 K. More importantly, the P 1 ˉ -ErN6 has outstanding explosive performance with high-energy-density of 1.30 kJ g−1, detonation velocity of 10.87 km s−1, and detonation pressure of 812.98 kbar, which shows its promising application prospect as high-energy-density materials.

Guo Y., Wei S., Liu Z., Sun H., Yin G., Chen S., Yu Z., Chang Q., Sun Y.

Abstract Materials under high pressure usually exhibit unique chemical and physical properties. Polynitrogen compounds have received widespread attention as potential high energy density materials. This paper uses CALYPSO crystal structure prediction method to study the structures of ScN6 and ScN7 in 0–100 GPa. Theoretical calculations show that ScN6 is thermodynamically stable above 80 GPa, while ScN7 is thermodynamically stable from 30 GPa to 90 GPa. Furthermore, ScN7 is metastable under ambient conditions, demonstrating that it can be quenched to ambient conditions after high pressure synthesis. The P 1 ¯ -ScN6 is a three-dimensional extended fold multi-nitrogen network, and the P 1 ¯ -ScN7 contains a five-membered ring and a curved N4 molecular unit. Both P 1 ¯ -ScN6 and P 1 ¯ -ScN7 contain a lot of N–N single bonds and N=N double bonds. The energy densities of P 1 ¯ -ScN6 and P 1 ¯ -ScN7 are 3.97 kJ g−1 and 3.12 kJ g−1, respectively. The detonation velocity and detonation pressure of the P 1 ¯ -ScN6 phase and P 1 ¯ -ScN7 phase are also higher than that of TNT. Excellent energy storage properties and detonation performance show that they can be used as potential high-energy materials. These results opened up a new way for the synthesis of nitrogen-rich compounds.

Wang Y., Bykov M., Chepkasov I., Samtsevich A., Bykova E., Zhang X., Jiang S., Greenberg E., Chariton S., Prakapenka V.B., Oganov A.R., Goncharov A.F.

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 N62− 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 N62− rings characterized here are likely to be present in other high-energy-density materials stabilized by pressure. Under 30 GPa, an unusual N20.75−-containing compound with the formula K3(N2)4 was formed instead. The planar hexazine dianion ring (N62–), which had previously been predicted to exist, has now been synthesized from potassium azide (KN3) under laser heating in a diamond anvil cell above 45 GPa; it remains metastable down to 20 GPa. By contrast, at 30 GPa an unusual N2-containing compound with the formula K3(N2)4 was produced.

Lin J., Wang F., Rui Q., Li J., Wang Q., Wang X.

A structural search leads to the prediction of a novel alkaline earth nitride BeN4 containing a square planar N42− ring. This compound has a particular chemical bonding pattern giving it potential as a high-energy-density material. The P4/ nmm phase of BeN4 may be stable under ambient conditions, with a bandgap of 3.72 eV. It is predicted to have high thermodynamic and kinetic stability due to transfer of the outer-shell s electrons of the Be atom to the N4 cluster, with the outer-shell 2 p orbital accommodating the lone-pair electrons of N42−. The total of six π electrons is the most striking feature, indicating that the square planar N42− exhibits aromaticity. Under ambient conditions, BeN4 has a high energy density (3.924 kJ/g relative to Be3N2 and N2 gas), and its synthesis might be possible at pressures above 31.6 GPa.

Niu S., Xu D., Li H., Yao Z., Liu S., Zhai C., Hu K., Shi X., Wang P., Liu B.

Two stable high-pressure phases (C2/m-MnN4 and P1̄-MnN4) and four metastable phases (P4/mmm-MnN4, P1̄-MnN5, C2/m-MnN6 and P1̄-MnN8) are proposed by using ab initio evolutionary simulations. Besides the reported quasi-diatomic molecule N2, the armchair chain and S-like chain, the N4 ring and N22 ring are firstly reported in the P4/mmm-MnN4 and P1̄-MnN5 phases. A detailed study is performed on the energetic properties, mechanical properties and stability of these polynitrogen structures. Ab initio molecular dynamics simulations show that P1̄-MnN4 and P1̄-MnN5 can be quenched down to ambient conditions, and large decomposition energy barriers result in the high decomposition temperatures of P1̄-MnN4 (2000 K) and P1̄-MnN5 (3000 K). Interestingly, P4/mmm-MnN4 with the N4 ring exhibits outstanding mechanical properties, including high incompressibility, high hardness, uniform strength in the 2-D direction and excellent ductility. Strong N-N covalent bond and weak Mn-N ionic bond interactions are observed in the predicted Mn-N compounds, and the charge transfer between the Mn and N atoms provides an important contribution to the stabilization of polymeric N-structures. All the proposed structures are metallic phases. Our results provide a deep understanding of the chemistry of transition metal polynitrides under pressure and encourage experimental synthesis of these new manganese polynitrides in future.

Lu W., Hao K., Liu S., Lv J., Zhou M., Gao P.

Abstract 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 YN10 adopting I4/m symmetry was discovered at the pressure of 35 GPa and showed metallic character from the analysis of electronic properties. In YN10, all the nitrogen atoms were sp 2-hybridized in the form of N5 ring. Furthermore, the gravimetric and volumetric energy densities were estimated to be 3.05 kJ g−1 and 9.27 kJ cm−1 respectively. 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.

Zhang X., Xie X., Dong H., Zhang X., Wu F., Mu Z., Wen M.

The search for high-energy-density materials (HEDMs) using polymeric nitrogen has attracted widespread attention in recent years. However, it is very difficult to synthesize polymeric nitrogen due ...

Zhang J., Li X., Dong X., Dong H., Oganov A.R., McMahon J.M.

Stable compounds in the V-N binary system are systematically investigated and four new phases are found: $Pbam\text{\ensuremath{-}}{\mathrm{V}}_{5}{\mathrm{N}}_{2}, Pnma\text{\ensuremath{-}}{\mathrm{V}}_{2}\mathrm{N}, P\overline{3}m1\text{\ensuremath{-}}{\mathrm{V}}_{2}{\mathrm{N}}_{3}$, and $I4/mcm\text{\ensuremath{-}}{\mathrm{VN}}_{2}$. All the predicted high-pressure vanadium nitrides are dynamically stable at ambient pressure. Moreover, the thermodynamic stability of vanadium nitrides in the temperature range of 0--1500 K at different pressures (0, 20, 40, 60, and 120 GPa) was also evaluated within the harmonic approximation. The sequence of phases of ${\mathrm{V}}_{2}\mathrm{N}$ under pressure is $\ensuremath{\varepsilon}{\text{-Fe}}_{2}\text{N}\phantom{\rule{0.16em}{0ex}}\text{type}\ensuremath{\rightarrow}\ensuremath{\zeta}{\text{-Fe}}_{2}\text{N}\phantom{\rule{0.16em}{0ex}}\text{type}\ensuremath{\rightarrow}{\text{Fe}}_{2}\text{C}\phantom{\rule{0.16em}{0ex}}\text{type}\ensuremath{\rightarrow}Pnma{\text{-V}}_{2}\text{N}$. In addition, relative stability and lattice dynamics properties of several vanadium mononitrides are systematically calculated and discussed. Structural features, mechanical properties, electronic structures, and chemical bonding of all the V-N compounds are analyzed at 0 GPa. Among these vanadium nitrides, WC-type VN has the highest Vickers hardness ($\ensuremath{\sim}37\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$) and superior fracture toughness (4.3--6.1 MPa ${\mathrm{m}}^{1/2}$), which mainly originate from its strong V-N bonding as well as its strong three-dimensional V-N covalent bond network. The configuration of the strong and short N-N covalent bonds enables the new phase $I4/mcm\text{\ensuremath{-}}{\mathrm{VN}}_{2}$ to exhibit good mechanical properties. Our results also reveal that the formation of a strong covalent-bond network topology in a crystal is a fundamental principle for designing a hard or superhard structure.

Laniel D., Aslandukova A.A., Aslandukov A.N., Fedotenko T., Chariton S., Glazyrin K., Prakapenka V.B., Dubrovinsky L.S., Dubrovinskaia N.

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 N3- 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 Ia3) 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.

Qi J., Zhou S., Xie K., Lin S.

Catalytic role of assembled Ce Lewis acid sites over ceria for electrocatalytic conversion of dinitrogen to ammonia. • We reveal the important role of oxygen vacancy in oxide catalysis for NRR. • We establish two descriptors for assessing the NRR activity on ceria. • This work provides insights for the design oxide catalyst that avoids the use of scarce metals. CeO 2 -based catalysts are emerging as novel candidates for catalyzing nitrogen reduction reaction (NRR). However, despite the increasing amount of experimental and theoretical research, the design of more efficient ceria catalysts for NRR remains a challenge due to the poor knowledge of the catalytic mechanism, particularly the nature of the active sites and how they catalyze NRR. Here, using first-principle calculations, we investigated the NRR catalysis process involving adjacent Ce Lewis acid clusters formed on (111), (110), and (100) facets of CeO 2 as active sites. Our results revealed that the assembled structures of the Ce Lewis acid as active centers after the oxygen vacancies (O v s) were opened. The exposed Ce sites on CeO 2 (111), CeO 2 (110), and CeO 2 (100) can cause N 2 to be adsorbed in a “lying-down” manner, which facilitates the N 2 activation and thus leads to much higher NRR activity. Furthermore, from the perspective of electronic structure, we establish two useful descriptors for assessing the NRR activity on ceria with O v s: The N–N bond strength of the adsorbed N 2 and the adsorption energy of the *N 2 H intermediate. This work thus provides direct guidance for the design of more-effective oxide catalysts without the use of scarce metals.

Liu S., Liu R., Li H., Yao Z., Shi X., Wang P., Liu B.

The high-pressure phase diagram of Co-N compounds is enriched by proposing five stable phases (Pnnm-Co2N, Pmn21-Co2N, Pmna-CoN, Pnnm-CoN2, and P1-CoN4) and two metastable phases (P31c-CoN8 and P1-CoN10). A systematic study has been performed for revealing the novel polymeric nitrogen structure and the outstanding properties of predicted polynitrides, such as structural characterization, energy analysis, stability analysis, and electronic analysis. P31c-CoN8 with the novel layer-shaped N-structure and P1-CoN10 with the novel band-shaped N-structure are first reported in this work. Moreover, P31c-CoN8 (6.14 kJ/g) and P1-CoN10 (5.18 kJ/g) with high energy density can be quenched down to ambient conditions. The proposed seven high-pressure phases are all metallic phases. A weak ionic bond interaction is observed between the Co and N atoms, while a strong N-N covalent bond interaction is observed in the Pnnm-CoN2, P1-CoN4, P31c-CoN8, and P1-CoN10 phases. The N atoms in the polynitrides hybridize in the sp2 state, for which the hybrid orbitals are constructed by the σ bond or lone electronic pair. The charge transfer between the Co and N atoms plays an important role to the structural stability. Moreover, the vibrational analysis of P31c-CoN8 and P1-CoN10 phases is performed to guide the future experimental study.

Liu L., Wang D., Zhang S., Zhang H.

Pressure-induced GdN6 with armchair–antiarmchair polynitrogen has a high energy density of 1.62 kJ g−1, with excellent explosive performance comparable to that of TNT, becoming the first high energy density material among lanthanide nitrides.

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.

Niu S., Li Z., Li H., Shi X., Yao Z., Liu B.

A systematic high-pressure study of the CdNx (x = 2, 3, 4, 5, and 6) system is performed by using the first-principles calculation method in combination with the particle swarm optimization algorit...

Lin J., Peng D., Wang Q., Li J., Zhu H., Wang X.

Enthalpies of formation of P1̄-ScN3 and C2/m-ScN5 are predicted relative to ScN and N2 with CALYPSO structural search.

Yang Y., Song J., Zhang H., Li Z., Liu S., Wang Y., Su X.

Pressure-induced nitrogen-rich compounds hold significant application prospects in high-energy-density materials. Utilizing first-principles calculations and swarm-intelligence structure search methods, we have identified ten new types of Gd-N compounds with different configurations, such as one-dimensional N-chains composed of N6 rings or N8 rings, and two-dimensional N-layers constructed of N14 rings, N18 rings, or N18 + N6 rings. Moreover, the predicted Gd-N compounds exhibit different magnetic properties, and a magnetic phase diagram is constructed in the pressure range of 0 to 200 GPa. Remarkably, the volumetric energy density (11.58–17.79 kJ/cm3) of Gd polynitrides with high nitrogen content, including P-1(I)-GdN6, P-1(II)-GdN6, R-3-GdN8, C2mm-GdN9, and P1-GdN10, surpassed that of TNT (7.05 kJ/cm3), making them promising candidates for energetic materials. The discovery of diverse chain-like and layered structures in the GdNx compounds highlights the role of gadolinium in inducing the diversity and complexity of nitrogen arrangements.

Liu Y., Li Z., Jin B., Liu S., Lu S., Yao Z.

Niu S., Liu Y., Mao Y., Zhang W., Yang Z., Zhai C., Liu S., Wang H.

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

Meng X., Wang Y., Wang Y., Li A., Fang Y., Li L., wang K., Li Q.

Developing efficient, single-phase white-light phosphors remains a formidable challenge in optoelectronics. Herein, high pressure initially induces trimeric contraction and distortion in (C9NH20)9(ZnCl4)2[Pb3Cl11], regulating the transition equilibrium of self-trapped excitons (STEs) with varied emission colors. Then, considerable structural distortion and destruction lead to intense white-light emission of an amorphous phase. A narrowed bandgap with promoted excitation efficiency, as well as strengthened electron–phonon coupling effect with increased binding energy of STEs, together result in the significant emission enhancement. This work provides valuable insights into white-light luminescent materials and offers new strategies for designing white-light-emitting devices.

Mao H., Chen B., Gou H., Li K., Liu J., Xiao H., Yang W.

Xu Y., Chen G., Du F., Li M., Wu L., Yao D., Liu X., Ding J., Zeng Z., Liu R., Lin H., Wang X.

Cubic gauche nitrogen (cg-N) has received wide attention for its exceptionally high energy density and environmental friendliness. However, traditional synthesis methods for cg-N predominantly rely on high-pressure techniques or the utilization of nanoconfined effects using highly toxic and sensitive sodium azide as precursor, which substantially restrict its practical application. On the basis of the first-principles simulations, we found that adsorption of potassium on the cg-N surface exhibits superior stabilization compared to sodium. Then, we chose safer potassium azide as precursor for synthesizing cg-N. Through plasma-enhanced chemical vapor deposition treatment, the free-standing cg-N was successfully synthesized without the need for high-pressure and nanoconfined effects. It demonstrated excellent thermal stability up to 760 K, and then rapid and intense thermal decomposition occurred, exhibiting typical thermal decomposition behaviors of high-energy-density materials. The explosion parameters were also measured using laser-induced plasma spectroscopy. Our work has substantially promoted the practical application of cg-N as HEDMs.

Zhang H., Zhang Y., Wang Y., Sui M., Yue L., Liu S., Li Q., Liu Z., Yao Z., Wang P., Liu B.

AbstractMetal polynitrides have raised significant interest for their applications as potential high‐energy‐density materials (HEDMs). Despite extensive research on energetic polynitrogen species, reducing synthesis pressures and realizing their recovery at ambient conditions remain challenging. Here, for the first time, the zigzag N4 chains are successfully stabilized in the new polynitride Ce2N6 and recovered to ambient pressure and temperature by introducing cerium nitride as a precursor. The stable mechanism of the recoverable N4 chain originates from the large quantity of charge captured, the favorable bonding environment, and low structural enthalpy. This study reveals the crucial role of precursors in stabilizing and recovering polynitrogen species, providing an alternative route to design and prepare novel HEDMs.

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.

Chen Y., Niu S., Li Y., Dou W., Yang X., Shan C., Shen G.

AbstractSensitive, flexible, and low false alarm rate X‐ray detector is crucial for medical diagnosis, industrial inspection, and scientific research. However, most semiconductors for X‐ray detectors are susceptible to interference from ambient light, and their high thickness hinders their application in wearable electronics. Herein, a flexible visible‐blind and ultraviolet‐blind X‐ray detector based on Indium‐doped Gallium oxide (Ga2O3:In) single microwire is prepared. Joint experiment−theory characterizations reveal that the Ga2O3:In microwire possess a high crystal quality, large band gap, and satisfactory stability, and reliability. On this basis, an extraordinary sensitivity of 5.9 × 105 µC Gyair−1 cm−2 and a low detection limit of 67.4 nGyair s−1 are achieved based on the prepared Ag/Ga2O3:In/Ag device, which has outstanding operation stability and excellent high temperature stability. Taking advantage of the flexible properties of the single microwire, a portable X‐ray detection system is demonstrated that shows the potential to adapt to flexible and lightweight formats. The proposed X‐ray detection system enables real‐time monitor for X‐rays, which can be displayed on the user interface. More importantly, it has excellent resistance to natural light interference, showing a low false alarm rate. This work provides a feasible method for exploring high‐performance flexible integrated micro/nano X‐ray detection devices.

Zhang G., Yi W., Cao Y., Zhang S., Liu X.

The P21c-C(N5)4 can build nitrogen cycle between N≡N triple bonds and N–N single bonds to achieve energy storage and release, receive considerable interest in green energy and sustainable materials.

Wang Y., Liu S., Lu S., Li Y., Yao Z.

Four high-pressure N-rich compounds (Pmn21-CeN7, Amm2-CeN9, P1 ̅-CeN10, and P1 ̅-Ⅱ-CeN10) are proposed by the first-principles calculation. The novel polymeric units (“heart” shaped layered structure, chain-like N8 rings, and two...

Li X., Bergara A., Zhang X., Li F., Liu Y., Yang G.

The discovery of novel structural units in compounds is of great interest in condense matter and material science. This not only leads to interesting properties, especially in superconductivity, as seen...

Lin S., Chen J., Zhang B., Hao J., Xu M., Li Y.

The high-pressure phase diagrams of the La-N binary system were systematically constructed using the CALYPSO method and first-principles calculations. In addition to the pressure-induced La-N compounds reported previously, we have...

Ding C., Yuan J., Han Y., Zhang Z., Jia Q., Wang J., Sun J.

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.