2025 AIChE Annual Meeting

(284g) Halide Perovskites As Electronic Plastic Crystals

Authors

Richard Remsing, Rutgers University
Halide perovskites, of the form ABX3, are an emerging class of materials with a wide range of applications, from solar cells, X-ray detectors, and LEDs. The origin of the low- frequency modes of perovskites has been widely analyzed and debated as they have a significant impact on the properties of the materials. Our recent research has classified perovskites as electronic plastic crystals, where the orientation of the local electron density is dynamically disordered and couples to lattice fluctuations in the system. One such mode is the out- of-phase octahedral tilting, which distorts the cage around the A-cation, breaking the local symmetry from Oh to D2d. This distortion leads to large fluctuations in the band gap, a crucial factor in the performance of perovskite-based devices. B-site off-centering modes and other octahedral distortions also play a significant role in the properties of these materials, with direct implications for the design and optimization of perovskite-based devices. All of these modes involve the inorganic lattice vibrations of the perovskite, so the B-site cation has been a focal point of numerous studies, with debates centered on what role lone-pair expression has on the instabilities of the soft lattice. We have undertaken a comprehensive analysis of how each atom’s local electron density can couple to these important modes and their impact on ion diffusion, thermoelectricity, and phase transitions. Using ab initio simulations, we have characterized the electron density fluctuations by focusing on the structure and dynamics of localized electron pairs, represented by the positions of Maximally Localized Wannier Function Centers (MLWFCs). Our examination of the symmetry fluctuations of the local electron density in perovskites provides electronic-scale insights into the dynamics of these materials.