First-Principles Study of the Structural Phase Transition and Physical Properties in NaI under High Pressure

It has been shown that NaI undergoes a pressure-induced phase transition which is different from other alkali metal halides such as NaCl. Previous X-ray diffraction experiments show that NaI will experience a structure phase transition from B1 to B33 phase under high pressure. However, there may exist pressure difference of the structure phase transition in experiments due to the lack of pressure transmit medium and the high water absorption of NaI. In this study, we predict the structural phase transition from B1−B33 phase at 20 GPa based on density functional theory of the first principles calculation method. The results of the theoretical calculations confirm the previous reported data, but the calculated pressure of the phase transition is slightly lower than the experimental value. In addition, detailed evolution of physical properties in NaI under high pressure was investigated. The band gap of NaI shows a gradual closure with increasing pressure, while its brittleness and ultraviolet light reflectance exhibit an enhancement. This work establishes a theoretical foundation for exploring the potential applications of alkali metal halides under extreme conditions.