First-Principles Investigation of the High-Pressure Phase Transition in Representative Alkali Metal Halides

Utilizing first-principles calculations based on density functional theory, this study investigates the geometric, electronic, and mechanical properties of NaCl, KCl, and KBr crystals in phase Ⅰ and phase Ⅱ structures under varying pressures. The relationships between these properties and the phase transition points are explored. Additionally, the Gibbs free energy method was employed to judge the phase transition points of NaCl, KCl, and KBr crystals. The results show that in the phase Ⅰ structure of NaCl, the band gap value increases with pressure from 0 to 30 GPa. However, in the range of 30−50 GPa, the band gap value decreases, indicating that 30 GPa is the phase transition point for NaCl phase Ⅰ. This suggests that pressure-induced changes in electronic structure can be indicative of metal halide phase transition points to some extent. However, pressure-induced alterations in crystal structure, phonon spectrum, and mechanical stability cannot reliably indicate alkali metal halide phase transition points. Furthermore, the phase transition points for NaCl, KCl and KBr calculated by Gibbs free energy method are 22.26, 3.47 and 3.11 GPa, respectively.