New stable structures of HeN₃ predicted using first-principles calculations

Unbiased structure searching methods and first-principles density functional theory (DFT) calculations were employed to explore new stable crystal structures of helium azide under high pressure. Five new phases of HeN₃ were discovered, namely, C2/m-I, P21/c, R-3c, R-3m and C2/m-II. The study of formation enthalpy of these structures reveals that the C2/m-II phase is the most energetically favorable structure with a pressure in the range of 40–98 GPa, while the R-3m phase is the most stable one with a pressure between 98–300 GPa. Electronic structure calculations from DFT demonstrate that all five newly predicted phases of HeN₃ shows semiconducting characteristics. Among them, the R-3m phase under the pressure of 300 GPa has the largest band gap of 5.4 eV. Strong covalent bonding and sp³ hybridization among nitrogen atoms in the structures are revealed by a detailed analysis of electronic localization function distributions and Bader atoms-in-molecules method.