Theoretical description of pressure- and temperature-induced structural phase transition mechanisms of nitrogen

A theoretical description is proposed for the pressure- and temperature-induced structural transitions of nitrogen. Three regions of the phase diagram are distinguished corresponding to different types of transition mechanisms and parent structures. Combined ordering and displacive reconstructive transitions are found in the lowest-pressure region, from the parent disordered beta structure to the ordered alpha and gamma structures. In a second region, which extends from about 2 to 140 GPa at room temperature, group-subgroup related structures occur, which realize ferroelastic or ferroelectric distortions of the delta parent structure. Space-group symmetries, consistent with the assumed structural mechanisms, are proposed for the delta(loc), zeta, kappa, iota, and theta molecular phases. Above 140 GPa, in the molecular dissociation region, the local structure of the eta phase is assumed to realize a link between the molecular zeta and kappa phases and the high-temperature cubic-gauche (cg)-N phase. An analogy is pointed out between the amorphous eta to superhard cg-N transition and the mixed vortex state to superconducting transition of type II superconductors.