Displacive In-Plane Ferroelectricity with Domain-Specific Curie Temperature in Van der Waals Semiconductors

Van der Waals bilayers assembled by mechanical stacking of 2D layers have been shown to realize symmetry-breaking mechanisms, such as sliding- or moiré ferroelectricity, which are without equivalent in 3D ferroelectrics. Here, we discuss emergent, highly unconventional behavior in thicker semiconducting few-layer SnS(e) van der Waals ferroelectrics crystallizing in a distorted polar structure, obtained by bottom-up synthesis. Electron diffraction at variable temperature demonstrates a gradual displacive transformation between the polar low-temperature and symmetric high-temperature phase, with significant variability in both the lattice constants and the Curie temperature (T_C) where the transition is completed. Strikingly, such variations are even found among stripe domains in individual crystals, where lattice constants and distortion angles change periodically between adjacent domains. The resulting spontaneously formed homo-materials superlattices imply that such ferroelectrics adopt domain-specific Curie temperatures, in sharp contrast with the long-held notion of T_C as a global materials parameter for a given ferroelectric.