Abstract
We study properties of nitrogen at high pressure and temperature (100-120GPa, 2000-3000K) where molecular and polymeric phases compete both in solid and liquid phase. We employ abinitio MD simulations with the SCAN functional and study the pressure-induced polymerization in liquid nitrogen for system sizes up to 288 atoms in order to reduce finite-size effects. The transition is studied upon both compression and decompression, and at 3000K, it is found to take place between 110 and 115GPa, coming close to experimental data. We also simulate the molecular crystalline phase close to the melting line and analyze its structure. We show that the molecular crystal in this regime is highly disordered, in particular, due to pronounced orientational and also translational disorder of the molecules. Its short-range order and vibrational density of states are very close to those of the molecular liquid revealing that the system likely represents a plastic crystal with high entropy.
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