Structural frustration, metastability and cascade of polar phases in a layered lead bromide perovskite

Kinetically trapped perovskites offer a unique platform for controlling functional properties through non-equilibrium phase formation. Here, we report that a layered hybrid perovskite, CPA₂PbBr₄, exhibits an unusual kinetic trapping phenomenon where rapid cooling stabilizes a metastable polar phase (IV, Pna2₁) instead of the thermodynamic ground state. This compound displays a cascade of polar phases below 350 K: the room-temperature phase III (Cmc2₁), the metastable phase IV, and the low-temperature phase V (P2₁), all of which are noncentrosymmetric and polar. The polar order in each phase is characterized by distinct CPA⁺ cation orientations and PbBr₆ octahedral distortions, giving rise to switchable dielectric response and second-harmonic generation (SHG). SHG studies reveal that thermal cycling suppresses SHG intensity of phase III without structural changes, attributed to generation of strain that limits coherence length. The material exhibits efficient broadband photoluminescence (PL) with strong thermochromism, arising from self-trapped excitons in the distorted lattice. Pyroelectric measurements and P-E hysteresis loops confirm spontaneous polarization values of 1.2-2.2 µC cm^-2 across the polar phases. Our findings demonstrate that kinetic control of phase selection provides a powerful strategy for tuning polar order and multifunctional properties in hybrid perovskites, with implications for optoelectronic and photonic applications.