Revisiting the structural and optical properties of γ -N2

For decades, γ-N2 has been known to exist at very low temperatures and pressures, located in a tiny area of the nitrogen phase diagram. Recently, it was shown that γ-N2 occupies most of the P–T space usually associated with molecular phases such as δ, ɛ, and ζ, and that it plays a pivotal role in shaping nitrogen’s phase diagram. Using powder synchrotron X-ray diffraction, Raman and infrared spectroscopy, and density function theory calculations, we have investigated the structural and optical properties of γ-N2 in a wide P–T range. The combined X-ray diffraction and infrared spectroscopy results unequivocally demonstrate that γ-N2 adopts the monoclinic (P21/c space group) configuration with two N2 molecules per unit cell. It appears that the γ-N2 is structurally closely related to θ-N2, leading to both phases having very similar Raman signatures. Additionally, the Raman spectroscopy reveals a vibrational mode intensity resonance effect in both phases, caused by a strong vibrational coupling between the isotopic 15N14N and 14N2 vibrational excitations.