Mapping spatially heterogeneous phase transitions in two-dimensional perovskite single crystals by low-frequency Raman microspectroscopy

Structural phase transitions in two-dimensional (2D) lead halide perovskites are intimately linked to their physicochemical and optoelectronic properties, yet the spatial heterogeneity of these transitions within individual crystals remains poorly understood. Here, we employed temperature-dependent low-frequency Raman (LFR) microspectroscopy to elucidate the structural phase transitions of 2D perovskites with different alkyl-chain lengths of the organic cations: BA2PbI4, HA2PbI4, and OA2PbI4, where BA, HA, and OA denote butylammonium, hexylammonium, and octylammonium, respectively. The temperature-dependent evolution of lattice phonon modes provides a direct and sensitive spectroscopic probe of structural transformations, enabling us to distinguish among the phase transition behaviors of these three perovskites. By leveraging LFR imaging, we revealed pronounced spatial heterogeneity in the order-to-disorder phase transition occurring at ∼36-37 °C in a single OA2PbI4 microcrystal. Mapping the intensity of a characteristic lattice phonon band at 22 cm-1 revealed substantial variations in the phase transition temperature across the crystal, suggesting the presence of distinct microdomains within a single crystal. This spatial heterogeneity may arise from local variations in the intrinsic defect density. Our results demonstrate that temperature-dependent LFR microspectroscopy offers a powerful means of visualizing spatially heterogeneous phase transitions and provides new insight into the local structural landscape of 2D perovskites.