Multiscale phase nucleation driven by photoinduced polarons in a volume-changing material.

Ultrafast control of functional materials by light is key for developing optoelectronic devices. Still, the mechanisms by which microscopic photoinduced excitations evolve into a macroscopic phase transformation remain poorly understood, partly due to the experimental challenges of isolating multiscale electronic and structural dynamics. Here we use femtosecond X-ray techniques to track the non-equilibrium dynamics leading to macroscopic transformation in a ferroelastic charge-transfer Prussian blue analogue. The experimental data evidence a sequence of phenomena. The initial electronic excitation leads to reverse Jahn-Teller distortion within 50 fs, causing intermetallic charge-transfer polarons within 200 fs. Polarons generate considerable lattice strain and trigger phase nucleation within 60 ps. This elastically driven cooperativity, launched by the photoinduced polarons, offers an efficient route to the generation and stabilization of photoinduced phases in many volume-changing materials.