Dynamics of Photoinduced Phase Transitions in a Prussian Blue Analog Studied by CN Vibrational Spectroscopy

Background: Recent progress in the study of photoinduced phase transition phenomena in cyano-bridged metal complexes called Prussian blue analog by means of CN vibrational spectroscopy is reviewed. The material discussed in this article is one of the most attractive and well-studied material, RbxMn[Fe(CN)6]y·nH2O, which shows a thermal phase transition accompanying a charge transfer and a change in magnetization around room temperature. Owing to a unique opportunity in this material to investigate the domains as well as the initial and final phases, by using the CN stretching vibration modes, domain growth dynamics and the nuclear formation processes are investigated in detail. Raman Spectroscopy: Raman spectroscopy is used to distinguish ion pairs corresponding to the low and high temperature phases and the boundary. The resonant Raman spectroscopy reveals the coupling of the CN ions with the local electronic states, and gives some information about the local lattice structure. Domain Growth: Time dependence of the phase fractions and the amount of boundary under irradiation of light is investigated. Continuous frequency shift suggests relaxation of the local strain during the growth of the domain. The domain growth process is found to be dependent on the excitation power density and the behavior is described by a kinetic model based on mean field approximation. Ultrafast Dynamics: A picosecond infrared spectroscopy is utilized to observe the ultrafast phenomena following the femtosecond pulse excitation. In addition to the decrease of the initial phases, creation of a large number of boundary component is found for photoinduced phase transition in both directions.