Tungsten Hexanitride with Single-Bonded Armchairlike Hexazine Structure at High Pressure

${WN}_{6}$ phase discovered at 126–165 GPa after heating of W in nitrogen. XRD refinements reveal a unit cell in space group $R ¯ 3 m$ which is consistent with the ${WN}_{6}$ structure with armchairlike hexazine ( $N_{6}$ ) rings, while strong $A_{1 g}$ Raman mode confirms its $N ─ N$ single bonds. Density functional theory (DFT) calculations reveal balanced contributions of attractive interactions between W and covalent $N_{6}$ rings, and repulsions between $N_{6}$ rings that make ${WN}_{6}$ ultrastiff and tough. The ${WN}_{6}$ phase displays long bond lengths in the nearest N-N and pressure-enhanced electronic band gap, which pave the way for finding novel nitrides.

Salke, Nilesh P. and Xia, Kang and Fu, Suyu and Zhang, Youjun and Greenberg, Eran and Prakapenka, Vitali B. and Liu, Jin and Sun, Jian and Lin, Jung-Fu, Tungsten Hexanitride with Single-Bonded Armchairlike Hexazine Structure at High Pressure, Phys. Rev. Lett.126, (6), 2021, 065702, DOI : 10.1103/PhysRevLett.126.065702

abstract

Crystal structure of the ${WN}_{6}$ phase with the space group $R ¯ 3 m$ determined by high-pressure XRD refinement. (a) Le Bail refinement of the ${WN}_{6}$ phase at 165 GPa ( $λ = 0.3344 Å$ ) (red vertical ticks). Blue vertical ticks represent unreacted W in the bcc phase that coexists with the ${WN}_{6}$ . Red open circles, black line and green line represents the experimental data, the fit and the residual difference respectively. (b) Crystal structure of the ${WN}_{6}$ phase with the novel armchairlike $N_{6}$ rings and W atoms in between the rings along the $c$ axis. See a representative XRD image at 165 GPa shown in Fig. S5 in Supplemental Material

Click to Enlarge