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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]