Photoinduced phase nucleation driven by multiscale charge transfer and symmetry change dynamics

Understanding photoinduced phase transitions is key for developing optoelectronic devices based on the ultrafast control of multifunctional materials. Still, how macroscopic transformations emerge from local excitations remains poorly understood, as it is experimentally challenging to isolate the multiscale electronic and structural dynamics. Here, we use femtosecond X-ray techniques to track the non-equilibrium dynamics leading to macroscopic transformation in a Prussian Blue analogue. The experimental data evidence a sequence of phenomena with different equilibration time scales. The initial electronic excitation leads to reverse Jahn-Teller distortion within 50 femtoseconds, which generates intermetallic charge-transfer polarons within 200 femtoseconds. Our study reveals how these photoinduced polarons generate significant lattice strain and trigger the phase nucleation within 60 picoseconds, where the lattice expansion leads to a self-amplifying macroscopic phase transition.