Pressure-induced transitions in solid nitrogen: Role of dispersive interactions

We have investigated the transition from molecular to fully covalent phases of solid nitrogen with an advanced ab initio quantum-mechanical method for crystals that rigorously describes weak dispersive interactions. It is demonstrated that the thermodynamic domain of stability of the cubic gauche phase (a promising high-energy- density material) extends down to pressures of about 60 GPa and that the high experimental transition pressure of 110 GPa is largely due to kinetic barriers which prevent the dissociation of triply bonded N₂ molecules. The role of temperature is complex: it helps the kinetics of the transition to fully covalent phases while it stabilizes the molecular phases through the entropic term.