Structural, Electronic, and Vibrational Properties of N₂O₄ under Pressure from First-Principles Study

This article uses first principles to study a single molecular phase of N₂O₄. The structural, electronic, and vibrational properties of N₂O₄ under high pressure are calculated. The a-axis and c-axis compressibilities are basically the same. The bandgap of N₂O₄ decreases as the pressure increases; this is because under the action of pressure, the delocalized electrons in the system increase, so the bandgap decreases. There are two main types of N₂O₄ vibration: telescopic vibration and bending vibration. Herein, the calculated Raman spectrum and infrared (IR) spectrum are compared with the experimental values, and the agreement is better under normal pressure. With the increase of pressure, both the Raman and IR peaks of N₂O₄ appear blueshifted. This is because the applied pressure reduces the volume of the unit cell and reduces the distance between atoms, causing the movement between the atoms to intensify, so the vibration intensity increases. At the same time, under the action of pressure, N₂O₄ increases the IR activity in the high-frequency region and reduces the IR activity in the low-frequency region.