Evidence for a High-Pressure Isostructural Transition in Nitrogen

We observed an isostructural phase transition in the solid nitrogen \lambda-N_2 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 P2_1/c). Our ab initio calculations further confirm the phenomena, and suggest an optimized structure for the \lambda-N_2 phase. In addition, a new high-pressure amorphous phase of \eta '-N_2 was also discovered by a detailed investigation of the pressure-temperature phase diagram of nitrogen with the aim of probing the phase stability of \lambda-N_2. Our result may provide helpful information about the crystallographic nature of dissociation transitions in diatomic molecular crystals (H_2, O_2, N_2, etc).