Nitrogen Phase Diagram at High P-T Conditions by the T-USPEX Method

It is known that under normal conditions nitrogen exists in a gaseous state and makes up most of the Earth's atmosphere. However, under high P-T conditions, nitrogen can transform into a condensed state, being a nonmolecular polymeric structure. At 110-180 GPa, polymeric nitrogen structures are known in three crystalline phases: cubic gauche (cg-N), layered polymer (LP-N), and black phosphorus (BP-N). Recently, a hexagonal polymer phase (HLP-N) has also been found in an experiment at pressures above 244 GPa. One of the most attractive areas for the application of such nonmolecular nitrogen is the creation of high energy density materials (HEDMs). Therefore, it is important to determine the conditions of the phase transition between these structures and to construct a phase diagram of nitrogen at high pressures and temperatures. In this paper, a phase diagram of nitrogen was calculated at pressures from 120 to 250 GPa and temperatures from 1500 to 3500 K using the T-USPEX algorithm.