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.薄膜铁电材料一直以来被应用于电容型及非易失性存储器件中。这类材料依赖于面外取向的极化特性，以适配CMOS（互补金属氧化物半导体）架构的集成与寻址需求。然而，由表面电荷诱导产生的内部退极化场，会抑制超薄铁电薄膜的面外极化——而这类超薄薄膜本可具备更低的矫顽场，实现低功耗运行。本研究揭示了n=2型Ruddlesden–Popper（鲁登斯登-波珀）家族中极性纵向光学（LO）模式的稳定机制，该模式可诱导出面外铁电性，且在开路边界条件下仍能稳定存在，同时有别于超铁电性。我们的第一性原理计算表明，LO模式的稳定机制在硫族化合物与卤化物中普遍存在，其本质为非谐三线性模式耦合。进一步研究发现，面外铁电性可通过晶体学容忍因子进行预测，且本研究将以此展开…… # 准二维材料中的面外铁电性与强磁电性 [https://doi.org/10.5061/dryad.59zw3r2dr](https://doi.org/10.5061/dryad.59zw3r2dr) ## 数据与文件结构说明 *I*4₁*cd* 相结构的SrTb₂Fe₂S₇、*I*4₁*cd* 相结构的BaLa₂In₂S₇，以及*Pc*相结构的KAg₂Mn₂Cl₇。