Phonon Driven Ferroelectricity and Raman Active Modes in Hybrid Organic-Inorganic Perovskites.

Hybrid organic-inorganic perovskites (HOIPs) have emerged as promising ferroelectric semiconductors, yet the phonon signatures governing their ferroelectricity remain poorly understood. Here, by analyzing the temperature-dependent Raman peak profiles of highly ordered ferroelectric domains in HOIPs, a framework to systematically investigate the dimensionality (n)-dependent phonons that are critical to ferroelectric behaviour is established. By tracking phonon evolution across the ferroelectric-to-paraelectric phase transition in HOIPs with different n, characteristic modes associated with the ferroelectric symmetry-breaking process are identified. Notably, in the ferroelectric phase of (BA)(MA)PbBr (n = 3), these modes exhibit a redshift compared to those in (BA)(MA)PbBr (n = 2), reflecting a reduced energy barrier for ferroelectric switching. Density functional theory (DFT) calculations further correlate these modes with their spectral signatures in Raman spectroscopy, particularly highlighting zone-boundary modes that diminish upon transitioning to the paraelectric phase. Polarized Raman mapping further reveals adjacent ferroelectric domains with orthogonal polarization orientations, directly linking phonon activity to domain configuration. This work elucidates the role of phonons in HOIP ferroelectricity, offering insights for tailoring domain-related properties in ferroelectric devices.