Kinetic retention of an anisotropic lattice response in pure FAPbI₃ by flash infrared annealing

Abstract

Far-from-equilibrium crystallization can access metastable states absent from equilibrium phase diagrams, but using the processing trajectory to select such states in functional semiconductors remains underexplored in halide perovskites. Here we examine pure formamidinium lead iodide (FAPbI₃), whose photoactive structure is usually retained with compositional or interfacial stabilization. A sub-second flash-infrared pulse produces films with higher photoluminescence and perovskite-diffraction retention than the antisolvent-processed references used in this study during accelerated thermal-light cycling. Laboratory X-ray diffraction, grazing-incidence diffraction, 4D-STEM and synchrotron nanoXRD reveal a room-temperature anisotropic lattice response that is not captured by a single cubic metric and is coupled to residual lattice distortion. Low-dose local electron diffraction contains additional reflections absent from the Pm-3m simulation and reproduced by a tilt-ordered tetragonal model, supporting local tetragonal ordering compatible with P4/mbm and I4/mcm. Scraping strongly reduces the metric anisotropy while simultaneously modifying texture and fragmentation, establishing mechanical sensitivity of the retained response. Together, the results support a non-epitaxial, alloying-free route for kinetically retaining an anisotropic metastable lattice response in pure FAPbI₃. Far-from-equilibrium crystallization accesses metastable states beyond equilibrium phase diagrams but remains underexplored in halide perovskites. Here, the authors show that sub-second flash-infrared processing enables the kinetic retention of an anisotropic metastable lattice state in pure FAPbI₃, yielding improved photoluminescence and stability.