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As a high-energy density material, polymeric nitrogen has attracted considerable attention, while the exceptionally high synthesis pressure hinders its studies and applications. A significant discovery indicates that the insertion of noble gas elements can effectively reduce the synthesis pressure of polymeric nitrogen compounds. This work utilized the particle swarm optimization algorithm and first-principles calculations to extensively explore the stoichiometry of Xe–N compounds under high pressures. Two phases of XeN14, P6mm and P-62m, have been discovered, which are energetically more stable than the basic mixture of Xe and N2. Evidence of charge transfer between Xe and N was found, verifying that Xe plays a crucial role in forming polymeric nitrogen compounds. These two compounds are kinetically stable at pressures ranging from 50 to 200 GPa and exhibit semiconductor properties. A unique channel-like structure was discovered in the P6mm phase. The energy densities of P6mm and P-62m phases are 7.39 and 7.59 kJ/g, respectively, significantly exceeding those of TNT (Trinitrotoluene) and HMX (Octogen), indicating their potential as high-energy density materials. [ABSTRACT FROM AUTHOR]

Potassium azide was investigated by Raman scattering spectroscopy up to a pressure of 55.0 GPa by use of diamond anvil cell at room temperature. A pressure-induced reversible phase transition was revealed. The onset of the phase transition was characterized by the hardening of a previously soft lattice mode at 13.6 GPa. This transition is considered a structural phase transition. Compression induces a symmetry reduction, which is indicated by the splitting of the librational modes, the development of infrared active vibrational modes, and the appearance of other new modes in the external mode region. The new high-pressure phase, with azide ions still in a molecular state, can be preserved down to 1.2 GPa. The Grüneisen parameters for the parent phase were calculated. [ABSTRACT FROM AUTHOR]

Silver azide (AgN3) was compressed up to 51.3 GPa. The results reveal a reversible second-order orthorhombic-to-tetragonal phase transformation starting from ambient pressure and completing at 2.7 GPa. The phase transition is accompanied by a proximity of cell parameters a and b, a 3° rotation of azide anions, and a change of coordination number from 4-4 (four short, four long) to eight fold. The crystal structure of the high pressure phase is determined to be in I4/mcm space group, with Ag at 4a, N1 at 4d, and N2 at 8h Wyckoff positions. Both of the two phases have anisotropic compressibility: the orthorhombic phase exhibits an anomalous expansion under compression along a-axis and is more compressive along b-axis than c-axis; the tetragonal phase is more compressive along the interlayer direction than the intralayer directions. The bulk moduli of the orthorhombic and tetragonal phases are determined to be KOT = 39 ± 5 GPa with KOT' = 10 ± 7 and KOT = 57 ± 2 GPa with KOT' = 6.6 ± 0.2, respectively. [ABSTRACT FROM AUTHOR]

The crystal structure and equation of state (EOS) of aluminum carbide (Al4C3) have been determined directly up to 50.1 GPa at room temperature by the synchrotron x-ray diffraction techniques. The results indicate that Al4C3 remained in rhombohedral structure under all tested pressure-temperature conditions and exhibited anisotropic compressibility, with the c-axis more compressible than the a-axis. Fitting the experimental data to third order Birch–Murnaghan EOS yields a bulk modulus of KOT=233±6 GPa with its pressure derivative KOT′=3.4±0.4, while the second-order EOS yields KOT=223±2 GPa. [ABSTRACT FROM AUTHOR]

Nanocrystalline silicon carbide (SiC) with a diameter of 7 nm was synthesized in a high voltage electrical explosion. It has a larger lattice parameter (0.1%) than its bulk material. Synchrotron x-ray diffraction measurements of nanocrystalline SiC were carried out to 15.8 GPa. The bulk modulus was determined to be K0T=201±10 GPa, which is substantially smaller than those of bulk material and nanocrystalline with large grain sizes. It is considered to be caused by the weaker atomic bonding strength and the imperfections of the nanocrystalline SiC. [ABSTRACT FROM AUTHOR]

All-nitrogen anions have attracted extensive attention because of their unique chemical and physical properties, and potential applications as high-energy density materials. Here, we discovered two N44- anions with planar zigzag and tripodal structures in BeN2 compound using first-principle calculations with structural search. At ambient conditions, both of them have high kinetic and thermodynamic stability with high energy density (2.48 kJ/g, and 2.95 kJ/g relative to Be3N2 and N2 gas). The zigzag N44− anion has P21/c space group in BeN2 compound and is energetically favorite in the pressure range of 0−48 GPa. While tripodal N44− anion has Cm space group and is energetically favorite in the pressure range of 48−100 GPa. Further analysis of chemical bonding pattern and electronic properties show that the Be atoms provide 2s electrons to alter the bonding state of N44− as well as use empty outer shell 2p orbital to accommodate lone pair electrons of N atoms, forming coordinate bonds to stabilize the compounds. More important, the calculated formation of enthalpies indicate that the zigzag and tripodal N44− can be synthesized via compressing Be3N2 and N2 at a modest pressure. Our results provide a scheme to synthesis N44- anions and stabilize all-nitrogen anions by introducing beryllium atoms. [ABSTRACT FROM AUTHOR]

Recently, a novel two-dimensional (2D) metal-free organic material, the C4N3H monolayer, has been proposed and predicted to be a 2D Dirac material with high Fermi velocities. Herein, we investigated its mechanical properties and tensile strain effects on its electronic properties based on first-principles calculations. We demonstrated that this material is quite soft with small stiffness constants and can sustain large strains. Compared to many other 2D materials, this material presents a remarkable elastic anisotropy and a large Poisson's ratio, which are very important for strain engineering. We also found that the Dirac cone of this material is very robust against the tensile strains and the Fermi velocity is high. The small stiffness constant, large Poisson's ratio, robust Dirac cone, and high Fermi velocity make the C4N3H monolayer a promising material in high-speed flexible electronic devices. [ABSTRACT FROM AUTHOR]

Additional electrons can drastically change the bonding trend of light elements. For example, N atoms in alkali metal azides form the linear N3- anions instead of N2 molecules with the introduction of additional electrons. The effect of the additional electrons on the polymerization of N under pressure is important and thus far unclear. Using first principles density functional methods and the particle swarm optimization structure search algorithm, we systematically study the evolution of LiN3 structures under pressures up to 600 GPa. A stable structure featuring polymerized N under pressures higher than 375 GPa is identified for the first time. It consists of zig-zag N polymer chains that are formed by N5- five-member rings sharing N-N pairs. Throughout the stable pressure range, the structure is insulating and consists of N atoms in sp3 hybridizations. Comparing with the atomic and electronic structures of previous phases, our study completes the structural evolution of LiN3 under pressure and reveals the structural changes which are accompanied and driven by the change of atomic orbital hybridization, first from sp to sp2 and then from sp2 to sp3. [ABSTRACT FROM AUTHOR]

The phase analysis of sodium azide (NaN3) has been investigated by in situ synchrotron X-ray diffraction measurements in a diamond anvil cell up to 52.0 GPa at room temperature. Three pressure-induced phase transitions were observed. The phase transition pressures were determined to be 0.3, 17.3, and 28.7 GPa verified by three different pressure transmitting media. The first high pressure phase, α-NaN3 (0.3 ∼ 17.3 GPa), was identified to be monoclinic with a C2/m space group. The β-NaN3 to α-NaN3 transition is a second-order phase transition, accompanied by the shearing of the Na-layers and the tilting of the azide chains. The second high pressure phase, γ-NaN3 (18.4 ∼ 28.7 GPa), has a lower symmetry than the α-NaN3. A further phase transition of γ-NaN3 to δ-NaN3 at 28.7 GPa was observed. [ABSTRACT FROM AUTHOR]

Ammonium azide (NH4N3) has been studied by in situ high-pressure X-ray diffraction and Raman scattering at room temperature. NH4N3 exhibits strong hydrogen bonding features with compression. The hydrogen bond weaken with increasing pressure due to the bending of N-H...N bond, leading to the increase of N-H stretch frequency and rotation of azide anions at 2b and 4h Wyckoff positions up to 2.9 GPa. The orientation of azide anions obviously influences the compressibility properties of NH4N3. The phase transition involves rotation of azide anions and a proximity of a and c, temporally assigned as a reversible second-order orthorhombic-to-tetragonal transition. [ABSTRACT FROM AUTHOR]

All-nitrogen anions have attracted extensive attention because of their unique chemical and physical properties, and potential applications as high-energy density materials. Here, we discovered two N44- anions with planar zigzag and tripodal structures in BeN2 compound using first-principle calculations with structural search. At ambient conditions, both of them have high kinetic and thermodynamic stability with high energy density (2.48 kJ/g, and 2.95 kJ/g relative to Be3N2 and N2 gas). The zigzag N44− anion has P21/c space group in BeN2 compound and is energetically favorite in the pressure range of 0−48 GPa. While tripodal N44− anion has Cm space group and is energetically favorite in the pressure range of 48−100 GPa. Further analysis of chemical bonding pattern and electronic properties show that the Be atoms provide 2s electrons to alter the bonding state of N44− as well as use empty outer shell 2p orbital to accommodate lone pair electrons of N atoms, forming coordinate bonds to stabilize the compounds. More important, the calculated formation of enthalpies indicate that the zigzag and tripodal N44− can be synthesized via compressing Be3N2 and N2 at a modest pressure. Our results provide a scheme to synthesis N44- anions and stabilize all-nitrogen anions by introducing beryllium atoms. [ABSTRACT FROM AUTHOR]

The effect of high pressure on the crystal structure of thiourea dioxide has been investigated by Raman spectroscopy and angle-dispersive X-ray diffraction (ADXRD) in a diamond anvil cell up to 10.3 GPa. The marked changes in the Raman spectra at 3.7 GPa strongly indicated a structural phase transition associated with the distortions of hydrogen bonding. There were no further changes up to the maximum pressure of 10.3 GPa and the observed transition was completely reversible when the system was brought back to ambient pressure. This transition was further confirmed by the changes of ADXRD spectra. The high-pressure phase was indexed and refined to an orthorhombic structure with a possible space group Pbam. The results from the first-principles calculations suggested that this phase transition was mainly related to the changes of hydrogen-bonded networks in thiourea dioxide.

Alkali metal azides can be used as starting materials in the synthesis of polymeric nitrogen, a potential high-energy-density material. The structural evolutionary behaviors of nitrogen in CsN3 have been studied up to 200 GPa using particle swarm optimization structure search combining with density functional theory. Three stable new phases with C2/m, P21/m, and P-1 structure at pressure of 6, 13, and 51 GPa are identified for the first time. The phase transition to chain like structure (P-1 phase) occurs at a modest pressure 51 GPa, the azide ions N3 (-) (linear chains of three N atoms with covalent bonds and interact weakly with each other) begin to show remarkable polymeric N properties in the CsN3 system. Throughout the stable pressure range, the structure is metallic and consists of N atoms in sp(2) hybridizations. Our study completes the structural evolution of CsN3 under pressure and reveals that the introduced Cs atoms are responsible for the decreased synthesis pressure comparing to pure molecular nitrogen under compression.

In this study, high pressure infrared (IR) absorption and Raman scattering studies for ammonium azide (NH4N3) were carried out at room temperature up to 20 GPa and 22 GPa, respectively. For comparison and further assignment, the vibrational spectra at ambient conditions were calculated using CASTEP code, particularly for the far- and mid-IR modes. The recorded vibrational data consistently indicated a pressure-induced phase transition at 2.9 GPa. All observed vibrational modes maintained their identities at the high pressure phase, indicating that NH4N3 was still presented in the form of ammonium cations and azide anions linked by the hydrogen bond (N-H⋯N). Above 2.9 GPa, the relative magnitude of the torsional mode weakened and the N-H symmetric stretch displayed a redshift, indicating strengthened hydrogen bonding energy. The opposite effects were observed above 12 GPa, where the relative magnitude of the torsional mode strengthened and the N-H symmetric stretch reverted to a blueshift, indicating weakened hydrogen bonding energy. It can be concluded that the hydrogen bonding energy exhibited a weakening (0-2.9 GPa), strengthening (2.9-12 GPa), and then again weakening (12-22 GPa) phenomena with the increasing of compression. The hydrogen bonding energy changing with the increase of pressure can be ascribed to a phase transition at 2.9 GPa and a rotational or bending behavior of azide ions at 12 GPa.