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).