Phase diagram of dense oxygen and nitrogen binary systems

Through a series of high-pressure Raman scattering and X-ray diffraction experiments on N2–O2 binary mixtures with oxygen concentrations varying from 10% to 77%, we systematically explore the molecular interactions of the mixtures. Our study greatly extends the room-temperature phase diagram for the N2–O2 system from 12 to ∼60 GPa for all concentrations and up to ∼160 GPa for 28% of oxygen. A complex phase coexistence is observed below 17 GPa in O2-rich mixtures, while in N2-rich mixtures, the δ-to-ɛ transition of O2 is shifted to higher pressures. Above 17 GPa, the mixture appears structurally as a composite of the ɛ–O2 + ɛ–N2 phase with the volume of oxygen being different from that of the pure element. Upon further compression, both elements broadly follow the phase transition sequence of the pure species, implying an absence of interactions and miscibility. Furthermore, oxygen metallization in the mixture occurs within a pressure range analogous to that of the pure sample, while above 130 GPa, amorphization and dissociation of nitrogen dominates, leading to the vibrational peaks of both O2 and N2 becoming weak.