Nitrogen is one of the simplest molecules in nature, yet its behavior under extreme conditions continues to surprise scientists. For more than half a century, the crystal structure of γ-N2, a molecular phase of solid nitrogen, remained a subject of debate despite extensive experimental and theoretical investigations.
Recently, 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 collaborators from the University of Edinburgh and other international institutions, have resolved this long-standing puzzle by combining advanced high-pressure experiments with first-principles calculations.
The results were published in Matter and Radiation at Extremes.
Using synchrotron X-ray diffraction, Raman spectroscopy, infrared spectroscopy and density functional theory calculations, the team systematically investigated γ-N2 across a wide pressure-temperature range. Their results provide compelling evidence that γ-N2 adopts a monoclinic P21/c structure containing two nitrogen molecules per unit cell.
The study not only confirms a theoretical prediction proposed more than fifteen years ago, but also clarifies the structural relationship between γ-N2 and θ-N2, two phases that exhibit remarkably similar vibrational signatures despite forming under vastly different conditions.
This work advances the understanding of the nitrogen phase diagram and sheds new light on the structural evolution and vibrational properties of molecular materials under extreme conditions.
This research was supported by the National Natural Science Foundation of China, the Innovation Program for Quantum Science and Technology, the Youth Innovation Promotion Association of CAS, and the HFIPS Director' s Fund of the Chinese Academy of Sciences.
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 LIU Xiaodi)