Compression of helium-nitrogen mixtures: evidence for loading-path dependent phase separation and structure transition via Raman methods

Nitrogen is one of the most fundamental and abundant elements in the universe, investigating its phase behaviors under high pressure is of great significance in many areas. Vos et al. studies the helium-nitrogen mixtures under isobaric cooling, which aims to further understand the behaviors of nitrogen. In our study, Raman spectra and visual observation are used to investigate the phase transition of nitrogen-helium mixtures with helium concentrations of 95%, 70%, 50%, 40% and 20% along the 295 K isothermal loading pressure 1, unloading pressure and loading pressure 2. Several two-phase separation and phase transition have been observed and futher investigated in different He:N₂ mixtures and the high-quality (N₂)₁₁He single crystal surrounded by helium-rich fluid (F₂) are obtained above 8.8 GPa. The structure, as determined by single-crystal x-ray diffraction at 11.6 GPa, is hexagonal with unit cell parameters a=8.1246 Å, c=9.6145 Å. Furthermore, pressure-induced nitrogen molecule ordering at 32.6 GPa and a structural phase transition at 110 GPa are observed in (N₂)₁₁He. Finally, at 187 GPa, a pressure-induced transition to an amorphous state is identified by the simultaneous disappearance of Raman active modes, as well as a change from transparency to opacity.