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

Abstract

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 $\ensuremath{\beta}$ structure to the ordered $\ensuremath{\alpha}$ and $\ensuremath{\gamma}$ 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 $\ensuremath{\delta}$ parent structure. Space-group symmetries, consistent with the assumed structural mechanisms, are proposed for the ${\ensuremath{\delta}}_{\text{loc}}$, $\ensuremath{\zeta}$, $\ensuremath{\kappa}$, $\ensuremath{\iota}$, and $\ensuremath{\theta}$ molecular phases. Above 140 GPa, in the molecular dissociation region, the local structure of the $\ensuremath{\eta}$ phase is assumed to realize a link between the molecular $\ensuremath{\zeta}$ and $\ensuremath{\kappa}$ phases and the high-temperature cubic-gauche (cg)-N phase. An analogy is pointed out between the amorphous $\ensuremath{\eta}$ to superhard cg-N transition and the mixed vortex state to superconducting transition of type II superconductors.