First-Principles Study of the High-Pressure Phase Transition and Physical Properties of Rubidium Nitrate

The high-pressure structure and physical properties of RbNO₃ at zero temperature was systematically explored using first-principles calculations based on density generalized theory combined with CALYPSO crystal structure predictions. The accuracy of four different functionals was compared based on experimental data of the RbNO₃-Ⅳ phase, and the revised PBE for solids (PBEsol) functional was found to be the most reliable. The zero-temperature phase transition sequence of RbNO₃ predicted is R3m→Pnma→Pmmn (experimental phaseⅤ) based on the PBEsol functional, and the two phase transition pressures are 1.7 and 8.2 GPa. The two phase transitions are first-order phase transitions, and the volume collapse rates reach 3.73% and 2.54%, respectively. This suggests that RbNO₃ at low pressure may have new low-temperature phases those are different from P3₁ structure given by experiment in the room temperature and ambient pressure. In the energy-stabilized pressure interval, all three phases satisfy the “Born-Huangkun” elastic stability criteria, and there is no phonon virtual frequency phenomenon in the whole Brillouin zone, which indicates that they are dynamically stable structures. The electronic property analysis showed that the three phases are semiconductors, and the band gap changes caused by the phase transition are generally small, but the pressure generally inhibits the charge transfer from alkali metal ions to nitrate ions. The high-pressure phase transition sequence predicted in this paper and the elasticity, lattice dynamics, and electronic structure properties of the individual phases can provide a reference for subsequent experimental and theoretical studies.