Unusual vibron behavior and Clapeyron slope reversal in cold dense nitrogen

Nitrogen exhibits a rich and complex pressure–temperature (P–T) phase diagram. However, the low-temperature and ultrahigh-pressure phase spaces (T < 300 K and 60 GPa < P < 180 GPa) remain largely unexplored, resulting in high-pressure phase boundaries in this region not yet being determined, and the Clapeyron slope remaining unknown. Here, we investigated the phase evolution of solid nitrogen at low temperatures (87–300 K) and high pressures (0–175 GPa) along two different thermodynamic pathways: the isothermal compression and the isobaric cooling. When we compress nitrogen above 130 GPa, below 150 K, we find an extended solid (named μ-N2) that exhibits Raman vibron fluctuations, which may be related to exciton–vibron coupling or electron–vibron coupling. Three missing low-temperature and high-pressure phase transition boundaries (ζ–κ, κ–μ, μ–η) have been identified by using a self-designed isothermal compression Raman spectroscopy. We also find that the dissociation of nitrogen significantly alters the Clapeyron slope in the phase diagram.