Extended d-electron state of Fe(CN)₆ unit in Prussian blue analogue

The Prussian blue analogue, A_xM[Fe(CN)₆]_y zH₂O (A and M are an alkali metal and a transition metal, respectively), is one of the candidates for a positive electrode material of a lithium-ion secondary battery. For further development of the electrode material, deeper understanding of the redox states of the constituents, i.e., M and the Fe(CN)₆ unit, is indispensable. Here, we investigated the electronic state of the redox constituents of K_0.34Co[Fe(CN) ₆]_0.753.6H₂O by means of the electrostatic potential method combined with the maximum entropy method combined with the Rietveld analysis (MEM/Rietveld). The electrostatic potential U(r) revealed that both the reduced and oxidixed states of Fe extend to the surrounding CN groups, which may cause the robustness of the cyano-bridged transition metal network.