Crystal structures and properties of nitrogen oxides under high pressure

First-principles calculations were performed to investigate the structural, electronic, and elastic properties of N2O4 and N2O5. Two new phases, namely, P42212 N2O4 and P21/m N2O5, are determined under high pressure through an ab initio evolutionary algorithm. For N2O4, the Im3 phase transforms to the P42212 structure at 35 GPa. The pressure–volume curves of N2O4 and N2O5 show that this transition has a first-order nature. The calculated phonon dispersion and elastic constants of Im3 N2O4, P42212 N2O4, and P21/m N2O5 demonstrate that the dynamic and mechanical stable pressure ranges are 2–35, 35–80, and 29–120 GPa, respectively. The electronic properties and projected density of states imply that these three structures are insulators. Furthermore, the N–N and N–O bond length of nitrogen oxides under high pressure are discussed.