K₃Fe(CN)₆ under External Pressure: Dimerization of CN^- Coupled with Electron Transfer to Fe(III)

The addition polymerization of charged monomers like C≡;C^2- and C≡;N^- is scarcely seen at ambient conditions but can progress under external pressure with their conductivity significantly enhanced, which expands the research field of polymer science to inorganic salts. The reaction pressures of transition metal cyanides like Prussian blue and K₃Fe(CN)₆ are much lower than that of alkali cyanides. To figure out the effect of the transition metal on the reaction, the crystal structure and electronic structure of K₃Fe(CN)₆ under external pressure are investigated by in situ neutron diffraction, in situ X-ray absorption fine structure (XAFS), and neutron pair distribution functions (PDF) up to ∼15 GPa. The cyanide anions react following a sequence of approaching-bonding-stabilizing. The Fe(III) brings the cyanides closer which makes the bonding progress at a low pressure (2-4 GPa). At ∼8 GPa, an electron transfers from the CN to Fe(III), reduces the charge density on cyanide ions, and stabilizes the reaction product of cyanide. From this study we can conclude that bringing the monomers closer and reducing their charge density are two effective routes to decrease the reaction pressure, which is important for designing novel pressure induced conductor and excellent electrode materials.