Ultrafast Structural Dynamics of a Photoexcited Mn−Fe Charge-Transfer Material in the Polaronic and Phase Transition Regimes

The Rb_0.94Mn_0.94Co_0.06[Fe(CN)₆]_0.98 material was designed to exhibit a 75 K wide thermal hysteresis around room temperature, associated with a bistability between the low temperature Mn^IIIFe^II tetragonal phase and the high temperature Mn^IIFe^III cubic phase. We have recently shown that a single laser pulse can be used to drive, at room temperature, the photoinduced phase transition from the Mn^IIIFe^II tetragonal phase to the Mn^IIFe^III cubic phase. By using a specific powder sample streaming technique, we could monitor by time-resolved X-ray diffraction how the ultrafast out-of-equilibrium dynamics, imposed by the structural relaxation around the local photoinduced charge transfer, changes with the fluence of the laser pulse. In this paper we present a detailed structural analysis of the crystalline reorganizations associated with the charge-transfer in different regimes of photoexcitation. In the low excitation polaronic regime, the photoinduced charge transfer is local and accompanied by an anisotropic distortion of the Mn coordination sphere and of the tetragonal lattice. In the high excitation regime, a macroscopic and cooperative charge-transfer phase transition occurs towards the cubic lattice, where the coordination around the Mn reaches octahedral symmetry.