Structure, Insertion Electrochemistry, and Magnetic Properties of a New Type of Substitutional Solid Solutions of Copper, Nickel, and Iron Hexacyanoferrates/Hexacyanocobaltates

Substitutional solid solutions of metal hexacyanometalates in which low-spin iron(III) and cobalt(III) ions populate
the carbon-coordinated sites were synthesized and studied by powder diffraction including Rietveld refinement,
cyclic voltammetry of immobilized microparticles, diffuse reflection vis-spectrometry, and magnetization techniques.
The continuous solid solution series of potassium copper(II), potassium nickel(II), and iron(III) [(hexacyanoferrate(III))1-x
(hexacyanocobaltate(III))x] show that the substitution of low-spin iron(III) by cobalt(III) in the hexacyanometalate
units more strongly affects the formal potentials of the nitrogen-coordinated copper(II) and high-spin iron(III) ions
than those of the remaining low-spin iron(III) ions. In the case of copper(II) and iron(III) [(hexacyanoferrate(III))1-x(hexacyanocobaltate(III))x]
the peak currents decrease much more than can be explained by stoichiometry, indicating that the charge propagation
is slowed by the substitution of low-spin iron(III) by cobalt(III). The Rietveld refinement of all compounds confirmed
the structure initially proposed by Keggin for Prussian blue and contradicts the structure described later by Ludi.
The dependencies of lattice parameters on composition exhibit in all series of solid solutions studied similar, although
small, deviations from ideality, which correlate with the electrochemical behavior. Finally, a series of solid solutions
of the composition KNi0.5IICu0.5II[FeIII(CN)6]1-x[CoIII(CN)6]x, where both the nitrogen- and carbon-coordinated metal
ions are mixed populated and were synthesized and characterized. These are the first examples of solid solutions
of metal hexacyanometalates with four different metal ions, where both the nitrogen- and the carbon-coordinated
sites possess a mixed population.