Direct Observation of Short-Range Structural Coherence During a Charge Transfer Induced Spin Transition in a CoFe Prussian Blue Analogue by Transmission Electron Microscopy

The local structure within the Co–Fe atomic array of the photoswitchable coordination polymer magnet, K_0.3Co­[Fe­(CN)₆]_0.77·<i>n</i>H₂O, is directly observed during charge transfer induced spin transition (CTIST), a solid–solid phase change, using high-resolution transmission electron microscopy (HRTEM). Along with the low-spin (LS) or thermally quenched high-spin (HS) states normally observed in CTIST solids at low temperature, slow cooling of K_0.3Co­[Fe­(CN)₆]_0.77·<i>n</i>H₂O results in an intermediate phase containing both HS and LS domains with short coherence length. By mapping individual metal–metal distances, the nanometer-scale HS domains are directly visualized within the LS array. Temperature-dependent analyses allow monitoring of HS domain coarsening along the warming branch of the CTIST, providing direct visualization of the elastic process and insight into the mechanism of phase propagation. Normally sensitive to electron beam damage, the low-temperature TEM measurements of the porous coordination polymer are enabled by using appropriate ionic liquids instead of usual conductive thin-film coatings, an approach that should find general utility in related classes of materials