Crystal structures of some manganese(ll) and cadmium hexacyanoferrates (II,III) and structural transformations related to the sorption of Cesium

Mn²⁺ and Cd²⁺ form a family of isostructural hexacyanoferrates(II,III). Their crystal structures, including those of mixed compositions containing K⁺ and Cs⁺ as charge balance cations, were resolved and refined from XRD powder patterns. The crystal structures of M₃[Fe(CN)₆]₂· xH ₂O and MCs₂[Fe(CN)₆] (where M=Mn, Cd) were refined in the space group Fm3m. The mixed salts, MK₂[Fe(CN) ₆]· 2H₂O, were found to be orthorhombic (space group Pmn2₁). The orthorhombic structure results from a local distortion due to monohydrated potassium ions located in interstitial sites. On ionic exchange in an aqueous solution containing Cs⁺, the orthorhombic distortion disappears and the cubic cell is obtained. Cs ⁺ is a large ion, which practically fills the available interstitial voids stabilizing the cubic structure. In solutions of K⁺ and Cs ⁺ the single salts, M₂[Fe(CN)₆]· 8H ₂O (monoclinic P2₁ /n) also transform, in this case liberating M²⁺ ions and forming the corresponding mixed salts. An analogous but slow structural transformation was also observed in the anhydrous forms of these single salts. These structural transformations could be relevant to the use of these compounds as ion exchangers and particularly for the sorption of ¹³⁷Cs⁺ from radioactive waste solutions. The XRD data were complemented with structural information from infrared (IR), Mössbauer and water vapor adsorption techniques.