Lattice dynamics of quasi-2D halide perovskites from first-principles

Quasi-2D halide perovskites have emerged as a promising platform for a diverse set of applications from light-emitting diodes to ferroelectric memory devices. However, because they contain vastly different atom types, masses, and interatomic interactions, it is challenging to study, from first-principles, how their lattice vibrations modulate optoelectronic properties of interest. Here, we present ab initio phonon frequencies and dispersion for (HA)₂CsPb₂I₇ and (HA)₂(MA)Pb₂I₇. We first benchmark the impact of DFT exchange-correlation functional choice, showing that dispersion corrections are needed to reproduce the experimental equilibrium structure. We then compute phonon dispersion, finding that the low energy phonon branches are mainly dispersive in the in-plane direction (within the perovskite layer), with minimal interlayer coupling. Further, while many phonon modes are inherited from the parent systems (bulk perovskites and ligand molecular crystals), there exist unique coupled vibrations not consistent with vibrations of the pure constituent layers. Our work provides a crucial foundation for future theoretical work on exciton-phonon coupling and other thermal and mechanical properties in quasi-2D halide perovskites.