Electron–Phonon-Driven Temperature-Adaptive Optoelectronics in the van der Waals Ferroelectric SnP2S6

Abstract Thermochromic ferroelectrics that combine reversible optical tunability with intrinsic polarization are promising platforms for adaptive optoelectronics, neuromorphic devices, and temperature-sensitive photonics. Here, we uncover the microscopic origin of thermochromism in the layered van der Waals ferroelectric SnP2S6 through a comprehensive combination of temperature-dependent Raman spectroscopy, photoelectron spectroscopy, optical spectroscopy, and density functional theory. Despite the absence of crystallographic symmetry changes between 100 and 400 K, SnP2S6 exhibits pronounced and reversible color evolution driven by substantial bandgap renormalization. Quantitative analysis reveals strong electron–phonon coupling with an effective phonon energy of ≈38 meV, while Raman spectroscopy identifies a dominant out-of-plane Sn–S vibrational mode that governs the bandgap modulation. Across the ferroelectric-to-paraelectric transition (≈345 K), local orbital rehybridization at the conduction band edge is directly observed via temperature-dependent Sn M4,5 X-ray absorption spectroscopy, evidencing enhanced Sn 5s–5p mixing without global electronic reconstruction. These findings establish electron–phonon interaction and local symmetry dynamics as the key design parameters controlling the optical response in SnP2S6, providing a microscopic framework for engineering temperature-adaptive van der Waals ferroelectric devices.