Study of oxidation states of the transition metals in a series of Prussian blue analogs using x-ray absorption near edge structure (XANES) spectroscopy

There have been renewed interests in metal-organic framework classes of materials such as Prussian blue analogues (PBAs) due to their potential usage in energy storage applications. In particular, due to their high surface areas, controllable structures and excellent electrochemical properties, PBAs such as hexacyanometalates M^II₃[A^III(CN)₆]_2*nH₂O (M = Mn, Fe, Co, Ni, Cu, Zn; A = Co, Fe, Cr; n = no. of water molecules present), M^II₂[Fe^II(CN)₆]_2*nH₂O (M = Mn, Co, Ni, Cu, Zn) and mixed hexacyanometalates(III) (Fe_1-xCo_x)₃[B^III(CN)₆]₂·nH₂O (x = 0.25, 0.5, 0.75; B = Co, Fe) could have possible usage as a new class of cathode and even anode materials for rechargeable batteries. Detailed knowledge of the oxidation states of the transition metals in PBAs is required to improve efficiency and durability of such devices. Furthermore, a link between the thermal expansion observed in these materials and the oxidation state of the transition metal is of interest to synthesize materials with a desired thermal expansion behavior, Here we demonstrate the use of Synchrotron based X-ray absorption near-edge structure (XANES) spectra to identify transition metal oxidation states. Our analysis reveals the presence of divalent, trivalent and/or mixed valence transition metals in the materials as well as high-spin and low-spin complexes.