Out-of-plane ferroelectricity and robust magnetoelectricity in quasi two-dimensional materials

Thin film ferroelectrics have been pursued for capacitive and nonvolatile memory devices. They rely on polarizations that are oriented in an out-of-plane direction to facilitate integration and addressability with CMOS architectures. The internal depolarization field, however, formed by surface charges can suppress the out-of-plane polarization in ultrathin ferroelectric films that could otherwise exhibit lower coercive fields and operate with lower power. Here we unveil stabilization of a polar longitudinal optical (LO) mode in the n=2 Ruddlesden–Popper family that produces out-of-plane ferroelectricity, persists under open-circuit boundary conditions, and is distinct from hyperferroelectricity. Our first-principles calculations show the stabilization of the LO mode is ubiquitous in chalcogenides and halides and relies on anharmonic trilinear mode coupling. We further show that the out-of-plane ferroelectricity can be predicted with a crystallographic tolerance factor, and we use these..., , , # Out-of-plane-ferroelectricity-and-robust-magnetoelectricity-in-quasi-two-dimensional-materials

[https://doi.org/10.5061/dryad.59zw3r2dr](https://doi.org/10.5061/dryad.59zw3r2dr)

## Description of the data and file structure

*I*41*cd* structure of SrTb2Fe2S7, *I*41*cd* structure of BaLa2In2S7 and *Pc* structure of KAg2Mn2Cl7.