Spatially Resolved Observation of Ferroelectric-to-Paraelectric Phase Transition in a Two-Dimensional Halide Perovskite.

2D halide perovskite ferroelectrics have garnered significant attention due to their potential applications and intriguing fundamental properties. However, their temperature-dependent ferroelectric behaviors, particularly at the nanoscale, remain poorly understood. In this study, the nanoscale ferroelectric domain evolution with temperature and ferroelectric-to-paraelectric phase transition in (BA)(2)(MA)Pb(2)Br(7) films are investigated using piezoresponse force microscopy (PFM). Angle-resolved lateral PFM (LPFM) reveals a complex in-plane ferroelectric domain structure. Temperature-dependent LPFM measurements clearly show that the Curie temperature (T(C)) is ≈353 K, as confirmed by other macroscopic measurements. Notably, it is observed that the ferroelectric-to-paraelectric phase transition initiates locally even below T(C). As the temperature increases, large ferroelectric domains fragment into smaller ones and the regions with the novel LPFM phase signal emerge, indicating a local phase transition. Furthermore, temperature-dependent LPFM spectroscopy demonstrates a progressive weakening of the ferroelectricity. The analysis based on Landau-Ginzburg-Devonshire theory identifies a second-order phase transition, consistent with the gradual evolution of nanoscale ferroelectric domains observed in LPFM images. This spatially resolved observation of phase transition provides critical insights into the temperature-dependent ferroelectric properties of 2D halide perovskite ferroelectrics and establishes a foundational framework for their future device applications.