Solid state electrochemistry, X-ray powder diffraction, magnetic susceptibility, electron spin resonance, Mossbauer and diffuse reflectance spectroscopy of mixed iron(III)-cadmium(II) hexacyanoferrates

Coprecipitates of Cd-II, K-I and Fe-III with hexacyanoferrate ions [Fe(CN)( 6)](4-) have been studied by solid-state electrochemistry (voltammetry of i mmobilized microparticles), magnetic susceptibility measurements, X-ray pow der diffraction, electron spin resonance, Mossbauer and diffuse reflectance spectroscopy. Most suprisingly? all experimental results point to the form ation of a continuous series of complex mixed phases without the formation of phase mixtures. Although Cd-II and Fe-III ions differ too much in their ionic radii to allow the formation of simple substitution mixed hexacyanofe rrates, they are capable of forming different kinds of complex insertion an d substitution mixed crystals because of the zeolitic structure of both the iron and the cadmium hexacyanoferrate. Low cadmium concentrations can be f ound in the zeolitic cavities of iron hexacyanoferrate (Prussian blue), and they start to widen the lattice and facilitate, at higher concentrations, the direct substitution of high-spin iron(III) ions by cadmium ions. In cas es of an excess of cadmium, the formation of cadmium hexacyanoferrate with iron(III) ions in the interstitials of the zeolitic structure is observed. These mixed phases show strong charge transfer bands in the visible range a nd have the appearance of "diluted" Prussian blue. For the first time, this indicates that the charge transfer between the carbon-coordinated low-spin iron(II) ions and the high-spin iron(III) ions can also occur when the lat ter are situated in the cavities of a host hexacyanoferrate. In Prussian bl ue the interstitial iron(III) ions are responsible for a very strong charge transfer interaction between the low-spin iron(II) ions and the nitrogen-c oordinated high-spin iron(III) ions. Upon substitution of the very small am ount of interstitial iron(III) ions in Prussian blue by potassium and cadmi um ions the Kubellca-Munk function diminishes by more than 30%, indicating a tremendous decrease in iron(III)-iron(II) interaction.