Dynamic Distortions of Quasi-2D Ruddlesden-Popper Perovskites at Elevated Temperatures: Influence on Thermal and Electronic Properties

Ruddlesden-Popper hybrid halide perovskite are quasi-two-dimensional materials with a layered structure and structural dynamics that are determined by the interplay between the organic and inorganic layers. While their optical properties are governed by confinement effects, the atomistic origin of thermal and electronic properties of these materials is yet to be fully established. Here we combine computational and experimental techniques to study A$_2$PbI$_4$ (A=butylammonium (BA), phenethylammonium (PEA)) Ruddlesden-Popper perovskites and compare them with the quintessential perovskite CH$_3$NH$_3$PbI$_3$. We use first-principles density functional theory, molecular dynamics simulations based on machine-learned interatomic potentials, thermal measurements, temperature-dependent Raman spectroscopy, and ultraviolet photoelectron spectroscopy, to probe the thermal and electronic properties of these materials at elevated temperatures. Our molecular dynamics simulations demonstrate that dynamic fluctuations in the organic sublattice determine the bulk-average distortions of these materials at room-temperature, explaining significant differences in their electronic density of states close to the Fermi level. Furthermore, by analysing the organic layer dynamics in BA$_2$PbI$_4$ we provide a mechanistic explanation for the phase transition of this material at 274 K and observations from Raman measurements. Our results highlight the role of the organic interlayer for the electronic and thermal transport properties of Ruddlesden-Popper perovskites, paving the way for the design of new hybrid materials for tailored applications.