Nature of the Long-Range Structural Changes Induced by the Molecular Photoexcitation and by the Relaxation in the Prussian Blue Analogue Rb1.8Co₄[Fe(CN)₆]3.3·13H₂O. A Synchrotron X-ray Diffraction Study

The photomagnetic properties of CoFe Prussian blue analogues arise from a photoinduced Co III (LS)−Fe II (LS) → Co II (HS)−Fe III (LS) [LS = low spin, HS = high spin] electron transfer accompanied by a spin state change of the cobalt and a significant expansion of the cobalt coordination sphere. The local bond-lengthening around the cobalt induces long-range structural changes that we studied by X-ray powder diffraction using synchrotron radiation. Beyond the structural characterization of an excited metastable state, the study shows how a photoinduced molecular bond-lengthening is made possible in a face-centered cubic Prussian blue analogue by the presence of [Fe(CN) 6 ] vacancies: in their vicinity, the lengthening of the cobalt to ligands bonds is easier. The excitation process starts at the cobalt neighbors of the vacancies through a continuous or second-order transformation (in a single phase), which triggers a cooperative propagation through a first-order discontinuous transition (between two phases).