Structural softening of solid nitrogen under pressure by first-principles calculations

Solid nitrogen has received widespread attention. In this paper, the calculated structural, electronic, phonon and mechanical properties are used to investigate the phase transition of solid nitrogen (space group P4₂/mnm). Acoustic phonons can be divided into two parts. One acoustic phonon shows that tetragonal solid nitrogen is dynamically stable at c-axis, while the remaining two acoustic phonons have virtual frequencies at a,b-axis. The virtual frequency of phonon indicates that tetragonal solid nitrogen has a structural softening process. We point out that the softening process of acoustic phonon is considered to be related to the phase transition. Based on the results in our calculation, the phase transition of tetragonal solid nitrogen starts at a,b-axis along [001‾] direction (from k-point Z to Γ). The negative group velocity of phonon is considered as the speed of phase transition. The phase transition velocity is increasing under pressure, and the low pressure corresponds to low velocity.