New Phase Transitions Driven by Soft Phonon Modes for CsPbBr₃: Density Functional Theory Study

Abstract

Using first‐principles lattice dynamics calculations, new phase transitions driven by soft phonon modes in CsPbBr3, from the high‐symmetry cubic structure Pm3m to hypothetical structures, are predicted. The structural, electronic, and dielectric properties of the new phases are determined. A ferroelectric phase with Pmc21 symmetry is found to be more stable than the experimentally observed phases. It is suggested that it is a good candidate as a ground state. It is demonstrated that the out‐of‐phase rotations of the PbBr6 octahedra determine a more stable crystal lattice than the in‐phase rotations. All predicted structures show semiconductor character, with bandgap values between 1.44 and 1.87 eV. The bandgap increases with increasing PbBr bond length and the angle of rotation of the PbBr6 octahedron. The dielectric tensor, Born effective charges, and spontaneous polarization are also calculated. The optical dielectric constant shows low sensitivity to unit cell volume and symmetry of the structures. The Born effective charges are sensitive to lattice symmetry, as they are related to the directions of charge transfer between ions, whereas the cell volume has no significant effect on their values. The infrared and Raman vibration modes of the three structures are also determined.

Biaxial strain improving carrier mobility for inorganic perovskite: ab initio Boltzmann transport equation

“…The phonon dispersion curves obtained from density functional theory and Wannier interpolation were shown in figure 4, and some soft modes could be observed on cubic halide perovskites, which indicated that they were thermodynamically unstable. Cubic perovskite was stable at high temperature and unstable at low temperature, which had been reported in previous investigation [74][75][76]. Since the mobility was mainly determined by optical phonons with highfrequency, these soft modes would not greatly affect the mobility results.…”

Physics-based optimization of Landau parameters for ferroelectrics: application to BZT–50BCT

“…Here, we employ macroscopic measured properties to optimize the Landau parameters in the FE-MPF model, and we implicitly assume that the experimentally observed phase is the thermodynamically stable phase at the temperature of interest. Ab initio phonon calculations and stability analyses would validate these assumptions or reveal other more stable states [75]. This approach was out-of-scope of the current study, but future ab initio-derived data should be utilised in parameter optimization.…”