  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.
