Nitrogen oxides under pressure: stability, ionization, polymerization, and superconductivity

Nitrogen oxides are textbook class of molecular compounds, with extensive industrial applications. Nitrogen and oxygen are also among the most abundant elements in the universe. We explore the N-O system at 0 K and up to 500 GPa though ab initio evolutionary simulations. Results show that two phase transformations of stable molecular NO(2) occur at 7 and 64 GPa, and followed by decomposition of NO(2) at 91 GPa. All of the NO(+)NO(3)(−) structures are found to be metastable at T = 0 K, so experimentally reported ionic NO(+)NO(3)(−) is either metastable or stabilized by temperature. N(2)O(5) becomes stable at 9 GPa, and transforms from P-1 to C2/c structure at 51 GPa. NO becomes thermodynamically stable at 198 GPa. This polymeric phase is superconducting (T(c) = 2.0 K) and contains a -N-N- backbone.