The New High-Pressure Phases of Nitrogen-Rich Ag–N Compounds

The high-pressure phase diagram of Ag–N compounds is enriched by proposing three stable high-pressure phases (P4/mmm-AgN₂, P1-AgN₇ and P-1-AgN₇) and two metastable high-pressure phases (P-1-AgN₄ and P-1-AgN₈). The novel N₇ rings and N₂₀ rings are firstly found in the folded layer structure of P-1-AgN₇. The electronic structure properties of predicted five structures are studied by the calculations of the band structure and DOS. The analyses of ELF and Bader charge show that the strong N–N covalent bond interaction and the weak Ag–N ionic bond interaction constitute the stable mechanism of Ag–N compounds. The charge transfer between the Ag and N atoms plays an important role for the structural stability. Moreover, the P-1-AgN₇ and P-1-AgN₈ with the high-energy density and excellent detonation properties are potential candidates for new high-energy density species.

Stabilization of hexazine rings in potassium polynitride at high pressure

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 N₆^2- 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 N₆^2- rings characterized here are likely to be present in other high-energy-density materials stabilized by pressure. Under 30 GPa, an unusual N₂^0.75--containing compound with the formula K₃(N₂)₄ was formed instead.

Stable nitrogen-rich scandium nitrides and their bonding features under ambient conditions

All-nitrogen salts have attracted extensive attention because of their unique chemical, physical properties, and potential applications as high-energy density materials. Using first-principles calculations and a particle swarm optimization structure search method, the pressure versus composition phase diagram of the Sc-N system is established. A new stable phase of C2/m-ScN₅ with the intriguing 2D N₁₀^6- is identified for the first time and we also found ScN₃ with the P1[combining macron] structure which has been reported before. Under ambient conditions, both of them have high kinetic and thermodynamic stability with high energy density (2.40 kJ g^-1 and 4.23 kJ g^-1 relative to ScN and N₂ gas). The analyses of chemical bonding pattern indicate that the nitrogen atoms in the N₆^6- chains are connected by covalent bonds with a combined σ and π bond character. We also give a possible high-pressure experimental route to P1[combining macron]-ScN₃ and C2/m-ScN₅ at modest pressure. We expect that our theoretical research could encourage experimental realization in the future and contribute to the understanding of the bonding features of nitrogen chains.

High-pressure formation of antimony nitrides: a first-principles study

The structural phase transition, electronic properties, and bonding properties of antimony nitrides have been studied by using the first principles projector augmented wave method. The relationship between the formation enthalpy and the composition of the Sb–N system has been explored. The novel Sb₂N₃ with the Cmcm space group is stable in a narrow pressure range from 100 GPa to 120 GPa. Apart from the Sb₂N₃, two nitrogen-rich phases SbN₂ and SbN₄ were predicted. The SbN₂ with the C2/m space group is stable at 12 GPa and then transforms to the high-pressure phase at 23 GPa. The nitrogen-rich SbN₄ appears at 14 GPa then undergoes C2/m → P1̄ → P1̄ phase transitions, and the calculated pressures of the phase transitions are 31 and 60 GPa, respectively. The nitrogen-rich SbN₂ and SbN₄ have similar structural features. Both SbN₂ and SbN₄ can be seen as a sandwich structure composed of the Sb–N layers and N₂ dimers. The pressure-induced phase transitions of SbN₂ and SbN₄ are accompanied by the electron transfer between the Sb–N layers and N₂ dimers. Moreover, the nitrogen-rich SbN₄ has a higher energy density of 2.42 kJ g⁻¹ and is a potentially high energy density material.

The structural phase transition, electronic properties, and bonding properties of antimony nitrides have been studied by using a first principles method.

Route to high-energy density polymeric nitrogen t-N via He-N compounds

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.