Stable xenon nitride at high pressures

Nitrides in many ways are fascinating since they often appear as superconductors, high-energy density, and hard materials. Though there exist a large variety of nitrides, noble gas nitrides are missing in nature. Pursuit of noble gas nitrides has therefore become the subject of topical interests, but remains as a great challenge since molecular nitrogen (${\mathrm{N}}_2$, a major form of nitrogen) and noble gases are both inert systems and do not interact at normal conditions. We show through a first-principles swarm-structure search that high pressure enables a direct interaction of ${\mathrm{N}}_2$ and xenon (Xe) above 146 GPa. The resultant Xe nitride has a peculiar stoichiometry of ${\mathrm{XeN}}_6$, possessing a high-energy density of approximately $2.4{\mathrm{kJ}\mathrm{g}}^{-1}$, rivaling that of the modern explosives. Structurally, ${\mathrm{XeN}}_6$ is intriguing with the appearance of chaired ${\mathrm{N}}_6$ hexagons and unusually high 12-coordination of Xe bonded with N. Our work opens up the possibility of achieving Xe nitride with superior high-energy density whose formation is long sought as impossible.

Figure 1

Phase stabilities of Xe-N compounds. The enthalpies of the formation of various Xe-N compounds under a series of pressures. The dotted lines connect the data points, and the solid lines denote the convex hull. The data show that ${\mathrm{XeN}}_6$ is the only stable stoichiometry.

Figure 2

Crystal structure and bonding properties of ${\mathrm{XeN}}_6$. (a) ${\mathrm{XeN}}_6$ in an $R-3m$ structure without showing any Xe-N bonds at 150 GPa where chaired ${\mathrm{N}}_6$ hexagons are observed. The small green and large purple balls represent N and Xe atoms, respectively. (b) ELF plots at 150 GPa with an isosurface value of 0.83, showing covalent Xe-N bonding polarized towards N. (c) and (d) Two structural units of fourfold bonded N (i.e., N in $s{p}^3$ hybridization) and 12-fold bonded Xe, respectively.

Figure 3

Phonon dispersion curve of ${\mathrm{XeN}}_6$ at 150 GPa.

Figure 4

Electronic PDOS and partial band decomposed charge density of ${\mathrm{XeN}}_6$. (a) PDOS of $\mathrm{Xe}-5p$ states for ${\mathrm{XeN}}_6$ and hypothetical ${\mathrm{XeN}}_0$ at 150 GPa. The dashed line indicates the Fermi energy. (b) PDOS and partial band decomposed charge density of ${\mathrm{XeN}}_6$ are shown in top and bottom panels, respectively. The right and left dashed lines denote the Fermi energy and the energy level of −16 eV, respectively. Partial band decomposed charge densities corresponding to energy windows of −32 to −16 eV and −16 to 0 eV are responsible for N-N and N-Xe covalent bonds, respectively. Hybrid functional Heyd-Scuseria-Ernzerhof (HSE06) for the electron exchange energy is used to better account for energy band gap.

Figure 5

Mechanism of pressure-driven reactivity of Xe with ${\mathrm{N}}_2$. (a) Enthalpies (black lines, left scale) and volumes (blue lines, right scale) of ${\mathrm{XeN}}_6$ with planar, chair, and boat ${\mathrm{N}}_6$ units at 150 GPa. (b) Interatomic distances as a function of external pressures. Blue line presents the sum (2.15 Å) of $\mathrm{Xe}-5p$ and $\mathrm{N}-2p$ covalent radii. (c) Histograms of N-Xe separations in ${\mathrm{XeN}}_6$ with planar, chair, and boat ${\mathrm{N}}_6$ units at 150 GPa. Dotted line presents the sum (2.15 Å) of $\mathrm{Xe}-5p$ and $\mathrm{N}-2p$ covalent radii. (d) Phase diagram of Xe-${\mathrm{N}}_2$ system. The dashed line separates the Xe-${\mathrm{N}}_2$ mixture (left and green region) and the stable ${\mathrm{XeN}}_6$ compound (right and blue region).