Title: Structure determination of ζ-N₂ from single-crystal X-ray diffraction and theoretical suggestion for the...

The allotropy of solid molecular nitrogen is the consequence of a complex interplay between fundamental intermolecular as well as intramolecular interactions. Understanding the underlying physical mechanisms hinges on knowledge of the crystal structures of these molecular phases. That is especially true for ζ-N₂, key to shed light on nitrogen's polymerization. Here, we perform single-crystal X-ray diffraction on laser-heated N₂ samples at 54, 63, 70 and 86 GPa and solve and refine the hitherto unknown structure of ζ-N₂. In its monoclinic unit cell (space group C2/c), 16 N₂ molecules are arranged in a configuration similar to that of ε-N₂. The structure model provides an explanation for the previously identified Raman and infrared lattice and vibrational modes of ζ-N₂. Density functional theory calculations give an insight into the gradual delocalization of electronic density from intramolecular bonds to intermolecular space and suggest a possible pathway towards nitrogen's polymerization.