Evidence for a High-Pressure Isostructural Transition in Nitrogen

We observed an isostructural phase transition in the solid nitrogen λ-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 λ-N_(2) phase.In addition,a new high-pressure amorphous phase of η′-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 λ-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).