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

The addition polymerization of charged monomers like C C2- 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 similar 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 similar to 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.