Methylammonium lead iodide (MAPbI3) perovskite nanocrystals (NCs) offer desirable optoelectronic properties with prospective utility in photovoltaics, lasers, and light-emitting diodes (LEDs). Structural rearrangements of MAPbI3 in response to photoexcitation, such as lattice distortions and phase transitions, are of particular interest, as these engender long carrier lifetime and bolster carrier diffusion. Here, we use variable temperature X-ray diffraction (XRD) and synchrotron-based transient X-ray diffraction (TRXRD) to investigate lattice response following ultrafast optical excitation. MAPbI3 NCs are found to slowly undergo a phase transition from the tetragonal to a pseudocubic phase over the course of 1 ns under 0.02-4.18 mJ/cm2 fluence photoexcitation, with apparent nonthermal lattice distortions attributed to polaron formation. Lattice recovery exceeds time scales expected for both carrier recombination and thermal dissipation, indicating meta-stability likely due to the proximal phase transition, with symmetry-breaking along equatorial and axial directions. These findings are relevant for fundamental understanding and applications of structure-function properties.
Keywords: lattice dynamics; metastable state; methylammonium lead iodide; perovskite; photoinduced phase transition; polaron; transient X-ray diffraction.