Cation Driven Charge Transfer in (Co, Fe) Prussian Blues

The charge transfer between Cobalt and Iron ions in a series of (Co, Fe) Prussian blues has been investigated by synchrotron X‐ray absorption spectroscopy and powder diffraction. (Co, Fe) Prussian blues, A x Co y [Fe(CN) 6 ]‐nH 2 O where A = Li, Na, K, Rb and Cs, were prepared by dropwise mixing method. X‐ray absorption near edge spectra showed that cobalt was in a higher oxidation state when the interstitial alkali cation changed from Li, Na, to K, Rb, Cs. At the same time, the valence of Fe shifted from 3+ to 2+. The cation driven charge transfer ability follows the order of Li ≈ Na < K < Rb ≈ Cs. A similar trend was also found when the KCl concentration was increased during the synthesis of K‐containing Prussian blue. The higher the concentration of KCl, the more Fe 2+ ‐CN‐Co 3+ fragment in the (Co, Fe) Prussian blue, and hence the higher efficiency for photo‐induced magnetization. The Co‐N distance in Prussian blue became shorter, about 0.15 Å in Co 2+ salt compared to Co 3+ salt, and the unit cell dimension was reduced about 0.3 Å as well.