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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]