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, P4(2)2(1)2 N2O4 and P2(1)/m N2O5, are determined under high pressure through an ab initio evolutionary algorithm. For N2O4, the Im3 phase transforms to the P4(2)2(1)2 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, P4(2)2(1)2 N2O4, and P2(1)/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.