A chemical model of intermediate states implied in the switching properties of CoFe Prussian blue analogues: how a cell parameter lengthening can cause a crystal field parameter increase

A series of CoFe Prussian blue analogues of chemical formula Rb2Co4−xZnx[Fe(CN)6]3.3·11H2O (x = 0, 1, 1.95 and 2.7) has been synthesized along which the MII/CoIII ions ratio at the Co site has been tuned. The long range order and the electronic structure of the Co ions have been investigated by combined powder X-ray diffraction and X-ray absorption spectroscopy (XAS) measurements. The cell parameter of the face-centered cubic structure lengthens as the MII/CoIII ions ratio increases without phase demixing. The study of the electronic structure of the Co ions by XAS shows that the coordination polyhedra of the CoII(HS) ions play an important role in the flexibility of the cubic structure. The variation of the cell parameter in the series of compounds is accompanied by the variation of the CoII–NC bond angle which allows the expansion or contraction of the cubic structure accompanying the electronic switch without phase demixing. Due to this structural re-arrangement, a lengthening of the cell parameter unusually produces an increase of the Co ion crystal field. Such a re-arrangement occurs in the course of the photo-induced electron transfer.