From Ultrafast Photoinduced Small Polarons to Cooperative and Macroscopic Charge‐Transfer Phase Transition

We study by femtosecond infrared spectroscopy the ultrafast and persistent photoinduced phase transition of the Rb 0.94 Mn 0.94 Co 0.06 [Fe(CN) 6 ] 0.98 ⋅ 0.2H 2 O material, induced at room temperature by a single laser shot. This system exhibits a charge‐transfer based phase transition with a 75 K wide thermal hysteresis, centred at room temperature, from the low temperature Mn 3+ −N−C−Fe 2+ tetragonal phase to the high temperature Mn 2+ −N−C−Fe 3+ cubic phase. At room temperature, the photoinduced phase transition is persistent. However, the out‐of‐equilibrium dynamics leading to this phase is multi‐scale. Femtosecond infrared spectroscopy, particularly sensitive to local reorganizations through the evolution of the frequency of the N−C vibration modes with the different characteristic electronic states, reveals that at low laser fluence and on short time scale, the photoexcitation of the Mn 3+ −N−C−Fe 2+ phase creates small charge‐transfer polarons [Mn 2+ −N−C−Fe 3+ ]* within ≃250 fs. The local trapping of photoinduced intermetallic charge‐transfer is characterized by the appearance of a polaronic infrared band, due to the surrounding Mn 2+ −N−C−Fe 2+ species. Above a threshold fluence, when a critical fraction of small CT‐polarons is reached, the macroscopic phase transition to the persistent Mn 2+ −N−C−Fe 3+ cubic phase occurs within ≃ 100 ps. This non‐linear photo‐response results from elastic cooperativity, intrinsic to a switchable lattice and reminiscent of a feedback mechanism.