Uncovering Multiple Intrinsic Chiral Phases in (PEA)2PbI4 Halide Perovskites

Two-dimensional (2D) halide perovskitesoffer a uniqueplatformfor investigating the ground state of materials possessing significantanharmonicity. In contrast to three-dimensional perovskites, their2D counterparts offer substantially fewer degrees of freedom, resultingin multiple well-defined crystal structures. In this work, we thoroughlyinvestigate the anharmonic ground state of the benchmark (PEA)(2)PbI4 compound, using complementary informationfrom low-temperature X-ray diffraction (XRD) and photoluminescencespectroscopy, supported by density functional theory calculations.We extrapolate four crystallographic configurations from low-temperatureXRD. These configurations imply that the ground state has an intrinsicdisorder stemming from two coexisting chiral sublattices, each witha bioriented organic spacer molecule. We further show evidence thatthese chiral structures form unevenly populated ground states, portrayinguneven anharmonicity, where the state population may be tuned by surfaceeffects. Our results uncover a disordered ground state that may induceintrinsic grain boundaries, which cannot be ignored in practical applications.