Class: Metal-Cyanide Framework
Related Structure Types: Perovskite, Rhenium Oxide
Prussian blue and an iron cyanide pigment that has been known and widely used since its first synthesis in 1706 by german paint maker Johann Diesbach. Historically two primary forms of the material are known soluble and insoluble. While both are in fact quite insoluble in water the two forms are chemically distinguishable by the charge balancing cations sitting within the pores of the host structure. The soluble Prussian blue is charge balanced with potassium ions as K[Fe2(CN)6]•xH2O and is most common. The insoluble for and an all iron phase with the formula Fe4[Fe(CN)6]3. Unless otherwise noted, here Prussian blue will refer to the ‘soluble’ phase. It should be noted that defects in the form of cyanide vacancies are quite common in these materials and material properties are often reported for a specific chemical formula that accounts for the nonstoichiometric composition.
Given the structural disorder in the framework, the lattice can be approximated to fit the space group Fm3-m and localization of interstitial cations and water are responsible for a reduction in symmetry therein. This crude approximation of symmetry can be made since the carbon and nitrogen atoms in the lattices are highly disordered between the two sites and have been reported fluxional. Typically, cation localization reduces the symmetry to a rhombic or primitive cubic lattice. It is also noteworthy that even very pure sample of Prussian blue is riddled with cyanide vacancies, substituted with coordinated waters.
The characteristic blue color results from an intervalence charge transfer absorption band that engulfs most the visible spectrum. Indeed, Prussian blue is the quintessential Robin-Day class II mixed valence material. While the bulk electronic conductivity in Prussian blue is low 10–7 S cm–1, it remains redox active and both the all ferric (Prussian Yellow) and all ferrous state (Prussian White, Everitt’s Salt) are chemically and electrochemically accessible. However, the endmember Prussian yellow is quite reactive and to some extent mythological. Usually, Berliner Green is accepted as the all ferric state, which is easily isolable. This electrochemical reactivity has been leveraged for use as electrodes materials in batteries and electrochromic devices.