Evidence for a High-Pressure Isostructural Transition in Nitrogen.

We observed an isostructural phase transition in the solid nitrogen λ -N₂ at approximately 50 GPa accompanied by anomalies in lattice parameters, atomic volume and Raman vibron modes. The anomalies are ascribed to a slight reorientation of the nitrogen molecules, which does not seem to affect the monoclinic symmetry (space group P 2₁/ c). Our ab initio calculations further confirm the phenomena, and suggest an optimized structure for the λ -N₂ phase. In addition, a new high-pressure amorphous phase of η ′-N₂ was also discovered by a detailed investigation of the pressure-temperature phase diagram of nitrogen with the aim of probing the phase stability of λ -N₂. Our result may provide helpful information about the crystallographic nature of dissociation transitions in diatomic molecular crystals (H₂, O₂, N₂, etc). [ABSTRACT FROM AUTHOR]