Structural Design and Physical Properties of Gallium Nitrides under High Pressures

Zhang Y., Huang X., Yao Y., Zhang Z., Tian F., Chen W., Chen S., Jiang S., Duan D., Cui T.

Topological semimetals exhibit novel quantum states and electronic structures with interesting Fermi surfaces. In contrast to Weyl and Dirac nodal points, nodal lines can have multiple topological configurations in momentum space that lead to new properties, such as long-range Coulomb interaction and flat Landau levels. Herein, we report the discovery of a Dirac nodal-line semimetal polynitride ${\mathrm{ZnN}}_{4}$, synthesized from the elements under high pressure via laser-heated diamond anvil cell technique. The crystal structure of ${\mathrm{ZnN}}_{4}$ has an $Ibam$ space group, and features infinite one-dimensional nitrogen chains in the pressure range of 14--130 GPa. The Dirac cone is observed midway between $S$ and $X$ points in the Brillouin zone and the Dirac nodal-line is found near the $T$ point. The measured electrical resistance of the ${\mathrm{ZnN}}_{4}$ sample shows an anomalous increase with pressure, which is attributed to the localization of valence electrons. The case of ${\mathrm{ZnN}}_{4}$ provides a unique example of Dirac nodal-line semimetal in nitrides.

Peng F., Song X., Liu C., Li Q., Miao M., Chen C., Ma Y.

An enduring geological mystery concerns the missing xenon problem, referring to the abnormally low concentration of xenon compared to other noble gases in Earth’s atmosphere. Identifying mantle minerals that can capture and stabilize xenon has been a great challenge in materials physics and xenon chemistry. Here, using an advanced crystal structure search algorithm in conjunction with first-principles calculations we find reactions of xenon with recently discovered iron peroxide FeO2, forming robust xenon-iron oxides Xe2FeO2 and XeFe3O6 with significant Xe-O bonding in a wide range of pressure-temperature conditions corresponding to vast regions in Earth’s lower mantle. Calculated mass density and sound velocities validate Xe-Fe oxides as viable lower-mantle constituents. Meanwhile, Fe oxides do not react with Kr, Ar and Ne. It means that if Xe exists in the lower mantle at the same pressures as FeO2, xenon-iron oxides are predicted as potential Xe hosts in Earth’s lower mantle and could provide the repository for the atmosphere’s missing Xe. These findings establish robust materials basis, formation mechanism, and geological viability of these Xe-Fe oxides, which advance fundamental knowledge for understanding xenon chemistry and physics mechanisms for the possible deep-Earth Xe reservoir. The abnormally low concentration of xenon compared to other noble gases in Earth’s atmosphere remains debated, as the identification of mantle minerals that can capture and stabilize xenon is challenging. Here, the authors propose that xenon iron oxides could be potential Xe hosts in Earth’s lower mantle.

Zhang D., Xu X., Lu M., Bi T., Tian Y., Zhang S., Yan Y., Du Y., Zhang M., Gao L.

Transition metal nitrides have been long thought to hold a promise for hard materials. In this work, we systematically explored the crystal structures of transition metal nitrides via a model system, namely Ti–N system, to investigate their physical related properties employing a global structure searches within ab initio electronic structure framework. As a result of the advanced crystal structure searches, several stoichiometries, e.g. TiN5, TiN2, Ti3N4, TiN, Ti2N, and Ti3N, were found to become stable at high pressures. A sequence of stable Ti–N compounds identified all exhibit metallic behaviors with the evidence that several bands crossing the Fermi level. The present results do show these predicted compounds are hard materials as expected. The findings of new hard materials here put forward further understanding of the crystal structures and electronic properties of Ti–N compounds at high pressures.

Du Y., Li W., Zurek E., Gao L., Cui X., Zhang M., Liu H., Tian Y., Zhang S., Zhang D.

The structure of I4̄-CsSi, a potential photovoltatic material and precursor of a superconducting silicon allotrope.

Kou C., Tian Y., Zhang M., Zurek E., Qu X., Wang X., Yin K., Yan Y., Gao L., Lu M., Yang W.

Liu Z., Li D., Wei S., Liu Y., Tian F., Duan D., Cui T.

Here, the energetic gallium nitrides with the network and zigzag poly-nitrogen configurations at modest pressures have been predicted. The nitrogen-rich Cmc21-GaN5, high-pressure P21/m-GaN5 and C2/c-GaN6 phases can release higher energy of ∼3.27 kJ g−1, 4.12 kJ g−1, 5.71 kJ g−1, respectively, which are close to or even higher than that of the traditional high energy density materials TNT and possess distinguished detonation performance simulated. The predicted synthesis pressures of GaN5 and GaN6 (25 and 50 GPa) are much lower than that of the famous atomic cg-N. The VSEPR theory and Zintl-Klemm concept are employed to reasonably explain the bonding properties of N-N bonds in locally environments. High pressures modulate the electron transfer between the different orbits and further induce higher energy density. The conjugation effect of π electrons in planar polymeric nitrogen chains is the main reason for the metallization of gallium nitrides.

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.

Liu Z., Li D., Liu Y., Cui T., Tian F., Duan D.

High pressure can stimulate numerous novel physical effects which are not observed under ambient conditions, such as the electronic redistribution and delocalization phenomenon in strongly covalently bonded nitrides. Through first principles simulations, we report a new N-rich aluminum nitride AlN5, which crystallizes with the space group P1[combining macron] at 20 GPa and then transforms into the I4[combining macron]2d phase at 60 GPa. We have identified and proved the delocalization effects of π electrons in the strongly covalent Lewis poly-nitrogen structure via the one-dimensional particle in a box mechanism, which contributes to the metallization and stability of the system. This implies that not all strongly covalently bonded systems with highly localized electrons exhibit nonmetallic properties in III-V main group nitrides. Furthermore, pressure results in the hybridization configuration mutation from sp2 in the P1[combining macron] phase to a mixture of sp2 and sp3 hybridization in the I4[combining macron]2d phase, which leads to phase transition from metal to insulator. With increasing pressure, the band gap increases abnormally, exhibiting anti-metallization induced by the strong hybridization. Interestingly, the P1[combining macron] and I4[combining macron]2d structures are simultaneously accompanied by a high energy density and hardness, which enable them to have a greater ability to resist elasticity, plastic deformation and external force destruction in potential applications. Their energy density and hardness are up to 3.29 kJ g-1 and 15.2 GPa in the P1[combining macron] phase but especially 6.14 kJ g-1 and 31.7 GPa in the I4[combining macron]2d phase.

Song X., Yin K., Wang Y., Hermann A., Liu H., Lv J., Li Q., Chen C., Ma Y.

Hydrogen-rich compounds attract significant fundamental and practical interest for their ability to accommodate diverse hydrogen bonding patterns and their promise as superior energy storage materials. Here, we report on an intriguing discovery of exotic hydrogen bonding in compressed ammonia hydrides and identify two novel ionic phases in an unusual stoichiometry NH7. The first is a hexagonal R3̅ m phase containing NH3-H+-NH3, H-, and H2 structural units stabilized above 25 GPa. The exotic NH3-H+-NH3 unit comprises two NH3 molecules bound to a proton donated from a H2 molecule. Above 60 GPa, the structure transforms to a tetragonal P41212 phase comprising NH4+, H-, and H2 units. At elevated temperatures, fascinating superionic phases of NH7 with part-solid and part-liquid structural forms are identified. The present findings advance fundamental knowledge about ammonia hydrides at high pressure with broad implications for studying planetary interiors and superior hydrogen storage materials.

Li X., Yong X., Wu M., Lu S., Liu H., Meng S., Tse J.S., Li Y.

The search for hard superconductive materials has attracted a great deal of attention due to their fundamentally interesting properties and potentially practical applications. Here we predict a new class of materials based on sodalite-like BN frameworks, X(BN)6, where X = Al, Si, Cl, etc. Our simulations reveal that these materials could achieve high superconducting critical temperatures ( Tc) and high hardness. Electron-phonon calculations indicate that Tc of these compounds varies with the doping element. For example, the superconducting Tc of sodalite-like Al(BN)6 is predicted to reach ∼47 K, which is higher than that in the renowned MgB2 (39 K). This phase and a series of other sodalite-based superconductors are predicted to be metastable phases but are dynamically stable as well. These doped sodalite-based structures are likely to become recoverable as potentially useful superconductors with high hardness. Our current results present a new strategy for searching for hard high- Tc materials.

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...

Ma S., Peng F., Zhu S., Li S., Gao T.

Nitrides have attracted great attention due to their outstanding applications as superconductors, high-energy materials, and hard materials. Utilizing particle swarm optimization algorithm on crystal structure prediction in combination with first-principle calculations, a novel stable stoichiometry, AlN4 with space group P31c, is thermodynamically stable above 123 GPa and remains metastable at ambient conditions. Structurally, AlN4 is intriguing with the appearance of N8 with distorted double triangular conical columns and Al atoms sharing the same lattice hP4 with sodium. The metastable of AlN4 stems from the inherent stability of three-dimensional covalent bonded nets from N8 and Al–N8. The new aluminum nitride with hardness of 45.7 GPa by Gao’s model is a potential hard material. Interestingly, the band gap of AlN4 is becoming wider with increasing pressure. Our work reveals a new form of N cluster and also provides a key perspective toward the understanding of novel chemical bonding in nitrogen-rich...

Li Y., Feng X., Liu H., Hao J., Redfern S.A., Lei W., Liu D., Ma Y.

Polymeric nitrogen, stabilized by compressing pure molecular nitrogen, has yet to be recovered to ambient conditions, precluding its application as a high-energy density material. Here we suggest a route for synthesis of a tetragonal polymeric nitrogen, denoted t-N, via He-N compounds at high pressures. Using first-principles calculations with structure searching, we predict a class of nitrides with stoichiometry HeN4 that are energetically stable (relative to a mixture of solid He and N2) above 8.5 GPa. At high pressure, HeN4 comprises a polymeric channel-like nitrogen framework filled with linearly arranged helium atoms. The nitrogen framework persists to ambient pressure on decompression after removal of helium, forming pure polymeric nitrogen, t-N. t-N is dynamically and mechanically stable at ambient pressure with an estimated energy density of ~11.31 kJ/g, marking it out as a remarkable high-energy density material. This expands the known polymeric forms of nitrogen and indicates a route to its synthesis.Polymeric nitrogen has yet to be recovered to ambient conditions, precluding its practical application as high-energy density material. Here, the authors highlight a possible route to the formation of a tetragonal polymeric nitrogen via helium-nitrogen compounds at high pressures.

Lu C., Li Q., Ma Y., Chen C.

Transition-metal light-element compounds are a class of designer materials tailored to be a new generation of superhard solids, but indentation strain softening has hitherto limited their intrinsic load-invariant hardness to well below the 40 GPa threshold commonly set for superhard materials. Here we report findings from first-principles calculations that two tungsten nitrides, hP4-WN and hP6-WN_{2}, exhibit extraordinary strain stiffening that produces remarkably enhanced indentation strengths exceeding 40 GPa, raising exciting prospects of realizing the long-sought nontraditional superhard solids. Calculations show that hP4-WN is metallic both at equilibrium and under indentation, marking it as the first known intrinsic superhard metal. An x-ray diffraction pattern analysis indicates the presence of hP4-WN in a recently synthesized specimen. We elucidate the intricate bonding and stress response mechanisms for the identified structural strengthening, and the insights may help advance rational design and discovery of additional novel superhard materials.

Zhang M., Guo Y., Zhu L., Feng X., Redfern S.A., Chen J., Liu H., Tse J.S.

Using global structure searches, we have explored the structural stability of CaB3N3, a compound analogous to CaC6, under pressure. There are two high-pressure phases with space groups R3c and Amm2 that were found to be stable between 29 and 42 GPa, and above 42 GPa, respectively. The two phases show different structural frameworks, analogous to graphitic CaC6. Phonon calculations confirm that both structures are also dynamically stable at high pressures. The electronic structure calculations show that the R3c phase is a semiconductor with a band gap of 2.21 eV and that the Amm2 phase is a semimetal. These findings help advance our understanding of the Ca-B-N ternary system.

Zhang D., Bhullar M., Cui X., Zhang M., Wang H., Yao 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.

Zhang D., Cui Y., Zhang M., Chen X., Wang H.

The discovery of clathrate SrB3C3 under high pressure has inspired the exploration of new B-C clathrates. Here, we studied the structural evolution of B-C framework over B and C contents...

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...

Yuan Q., Xu C., Wang Y., Li Y., Fu J., Yao Z.

A high-pressure study of the CeN5 compounds is performed by the crystal structure search method in combination with the first principle calculations. Four new phases C2-CeN5, P-1-CeN5, P2/c-CeN5, and P21/m-CeN5 were predicted at 0, 20, 50, and 100 GPa, respectively. The analysis of stability shows that C2-CeN5, P-1-CeN5, and P21/m-CeN5 can be quenched to ambient conditions. The stabilization mechanism of Ce nitrides is clarified by the analysis of electronic structures. The phase transformation of CeN5 with the pressure is revealed by the enthalpy difference analysis. P2/c-CeN5 and P21/m-CeN5 with polymeric N-chains not only exhibit high energy densities (8.67 and 9.37 kJ/cm3), but also outstanding detonation velocities (11.43 and 12.10 km/s) and detonation pressures (90.39 and 102.82 GPa).

Feng Q., Xiao X., Dai W., Sun W., Ding K., Lu C.

Abstract The nitrogen-rich transition metal nitrides have attracted considerable attention due to their potential application as high energy density materials. Here, a systematic theoretical study of PtN x compounds has been performed by combining first-principles calculations and particle swarm-optimized structure search method at high pressure. The results indicate that several unconventional stoichiometries of PtN2, PtN4, PtN5, and Pt3N4 compounds are stabilized at moderate pressure of 50 GPa. Moreover, some of these structures are dynamically stable even when the pressure release to ambient pressure. The P 1 ˉ phase of PtN4 and the P 1 ˉ phase of PtN5 can release about 1.23 kJ g−1 and 1.71 kJ g−1, respectively, upon the decomposition into elemental Pt and N2. The electronic structure analysis shows that all crystal structures are indirect band gap semiconductors, except for the metallic Pt3N4 with Pc phase, and the metallic Pt3N4 is a superconductor with estimated critical temperature T c values of 3.6 K at 50 GPa. These findings not only enrich the understanding of transition metal platinum nitrides, but also provide valuable insights for the experimental exploration of multifunctional polynitrogen compounds.