Structural Instabilities Related to Highly Anharmonic Phonons in Halide Perovskites

Hybrid perovskites have emerged over the past five years as absorber layers for novel high-efficiency low-cost solar cells combining the advantages of organic and inorganic semiconductors. Unfortunately, electrical transport in these materials is still poorly understood. Employing the linear response approach of density functional theory, we reveal strong anharmonic effects and a double-well phonon instability at the center of the Brillouin zone for both cubic and orthorhombic phases of inorganic CsPbI₃. Previously reported soft phonon modes are stabilized at the actual lower-symmetry equilibrium structure, which occurs in a very flat energy landscape, highlighting the strong competition between the different phases of CsPbI₃. Factoring these low-energy phonons into electron-phonon interactions and band gap calculations could help better understand the electrical transport properties in these materials. Furthermore, the perovskite oscillations through the corresponding energy barrier could explain the underlying ferroelectricity and the dynamical Rashba effect predicted in halide perovskites for photovoltaics.