Transient X-ray Diffraction Reveals Nonequilibrium Phase Transition in Thin Films of CH₃NH₃PbI₃ Perovskite

Advantageous optoelectronic properties of methylammonium lead triiodide likely arise from coupling between photogenerated charge-carriers and the soft, deformable lattice. We investigate structural dynamics of MAPbI3 films using time-resolved X-ray diffraction versus pump–probe time delay and pump intensity. During the first nanosecond, the lattice anisotropically distorts from tetragonal to cubic at excitation intensities that are insufficient to thermally induce the first-order thermodynamic phase transition at 330 K. The high-symmetry structure then relaxes back to the starting phase with 11 and 236 ns time constants via a different transition pathway than observed either in the first nanosecond or in previous reports for MAPbI3. Early time dynamics are consistent with polaron formation and lattice strain stabilization while the slower recovery dynamics outlive radiative recombination and relate metastability. Fluence-independence of these lattice deformations in the low-power regime conveys relevance to optoelectronics including photovoltaics and highlights sustained involvement of nonequilibrium, photoinduced lattice reorganization in MAPbI3 under functional conditions.