Accurately Probing [Fe(CN)6] Defects in Prussian Blue Analogues

Prussian blue analogues A x M[Fe(CN)6]1-y & centerdot;zH2O (A = Na, K, M = Fe, Mn) possess a rich crystal chemistry that allows their use as insertion materials for batteries, water treatment, and smart windows, among others, applications. Despite being simple in appearance, their synthesis is complex and often results in defective, nonstoichiometric materials. These defects can be of neutral charge, such as interstitial water molecules, or charged, such as A+ vacancies or [Fe(CN)6] n- vacancies, whose presence implies charge balance by the remaining transition metal centers. In this work, a compound close to the Berlin green Fe[Fe(CN)6] nominal composition was synthesized, and its crystal structure, electrochemical properties, and electronic structure were investigated. Although usual characterization methods such as X-ray diffraction, X-ray absorption spectroscopy, and 57Fe Mossbauer spectroscopy agree well with the formation of Fe[Fe(CN)6], the material can only be intercalated to Na1.4Fe[Fe(CN)6]. This lack of capacity was hypothesized to arise from "hidden" [Fe(CN)6] vacancies. The charge balance associated with these vacancies implies partial reduction of the low-spin Fe, as confirmed by low-temperature 57Fe Mossbauer spectroscopy and further high-angular-resolution diffraction experiments. This study suggests that numerous materials previously described as "Berlin green" in the literature might actually be substantially hexacyanoferrate defective.