Nitrogen is one of the simplest molecules in nature, yet its behavior under extreme conditions continues to puzzle scientists. For more than 50 years, the crystal structure of γ-N2, a molecular phase of solid nitrogen, remained unresolved despite extensive studies.
A research team led by Prof. LIU Xiaodi from the Hefei Institute of Solid State Physics, Hefei Institutes of Physical Science of the Chinese Academy of Sciences, together with international collaborators from the University of Edinburgh and other institutions, has determined the structure of γ-N2 using high-pressure experiments and theoretical calculations.
The study was published in Matter and Radiation at Extremes.
In this research, the team found that γ-N2 is stable over a much wider pressure–temperature range than previously thought, overlapping with several other molecular phases of nitrogen.
Using synchrotron X-ray diffraction, Raman and infrared spectroscopy under high-pressure conditions as experimental techniques, together with density functional theory-based first-principles calculations, the researchers showed that γ-N₂ has a monoclinic structure with two nitrogen molecules per unit cell.
They also found that γ-N2 is closely related to another phase, θ-N2, which explains why the two show very similar Raman signals. In both phases, a special vibrational resonance effect was observed, linked to interactions between different isotopic nitrogen molecules.
These results offer new insight into the structure of γ-N₂ and contribute to a better understanding of how nitrogen behaves under extreme conditions, according to the team.
Schematic illustration of the pressure-induced structural distortion in γ-N₂, from a body-centered-cubic-like molecular arrangement to a monoclinic P2₁/c structure. (Image by YAN Jinwei)