First-principles study of the structural phase transition process of solid nitrogen under pressure.

Area of Science:

• Solid-state physics
• Materials science
• Computational chemistry

Background:

• Solid nitrogen exhibits diverse structures, making its phase transitions a key research area.
• Understanding these transitions is crucial for materials science and condensed matter physics.

Purpose of the Study:

• To investigate the structural softening of rhombohedral solid nitrogen under pressure.
• To propose a new criterion for predicting solid nitrogen's structural phase transitions.
• To analyze the mechanism and velocity of phase transition in solid nitrogen.

Main Methods:

• First-principles calculations were employed to study structural softening.
• Acoustic phonon analysis was used to determine the phase transition pathway.
• The newly proposed Egret criterion was applied to predict phase transitions.

Main Results:

• Structural softening in rhombohedral solid nitrogen under pressure was successfully studied.
• The Egret criterion was proposed to predict the entire phase transition process.
• The phase transition initiates along a specific direction ([−1, −1, 0]) with a slow velocity.
• The criterion accurately predicted the emergence and stability of ξ-N2 at 17 and 22 GPa.

Conclusions:

• The study provides insights into the phase transition mechanism of solid nitrogen.
• The Egret criterion offers a reliable tool for predicting solid nitrogen's phase behavior.
• Results align well with existing nitrogen phase diagrams, validating the findings.