The fluid-like nature of solid cubic halide perovskites

X-ray diffraction ‘see’ in the high temperature phase of ABX₃ halide perovskites a macroscopically averaged cubic structure with a single formula unit (FU) per cell. Yet DFT calculations on this structure reveal a number of anomalies: i) It has dynamically unstable phonons, ii) its band gap is lower than both experiment and molecular dynamics (MD) predictions, iii) the trends with A cation show inconsistency with experiment e.g., cubic FASnI₃ has larger gap than the orthorhombic CsSnI₃, and iv) it has higher total energy than the relaxed supercell with many repeated FUs. We find via DFT that the real microscopic structure of cubic halide perovskites is polymorphous, i.e., it contains a dynamic fluid-like distribution of different local motifs, each having differently tilted/rotated/B-atom displaced BX₆ octahedra and differently oriented A molecules, the average of which is the fictitious monomorphous cubic structure. The polymorphous configurations have stable phonons and much larger band gaps, forming the correct description of trends in gaps and structures in the ABX₃ group of materials. We will present the main effects that govern band gaps in perovskites, via carefully constructed static supercell approximants to the dynamic MD structure.