Observation of Tetragonal-to-Orthorhombic Phase Transition in Perovskite MAPbI₃ Single Crystal via Ultrafast Photoexcitation

Methylammonium lead iodide (MAPbI(3)) perovskitematerialshave attracted widespread attention owing to their charming performance.However, the lack of in-depth exploration of the temporal resolutionof the phase transition process has affected their further application.Here, we explored transient optical properties in tetragonal-orthorhombicphase transition behavior at the ultrafast time scale based on ourdelicately grown MAPbI(3) high-quality bulk single crystal.The distinct relaxation characteristics in the structural phase transitionwere found through the pump laser energy-dependent transient reflectionmeasurements while keeping the lattice temperature constant, suggestingthat the relaxation process dominated by electron-phonon interactionis related to the electronic structure of the crystal phase. The transientmeasurement of the lattice temperature slowly changing reveals thedynamic evolution of the whole phase transition process and also stronglyconfirms that the phase transition temperature range is only 1 K.Such an ultrafast dynamics study of crystals at the process of constantand variable temperatures provides a unique insight into the photophysicalprocess of perovskites and opens a new door for studying the ultrafastphase transition of crystals.