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

Abstract

Mn 2+ and Cd 2+ 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 M 3 [Fe(CN) 6 ] 2 ⋅xH 2 O and MCs 2 [Fe(CN) 6 ] (where M=Mn, Cd) were refined in the space group Fm3m. The mixed salts, MK 2 [Fe(CN) 6 ]⋅2H 2 O, were found to be orthorhombic (space group Pmn2 1 ). 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, M 2 [Fe(CN) 6 ]⋅8H 2 O (monoclinic P2 1 /n) also transform, in this case liberating M 2+ 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 137 Cs + from radioactive waste solutions. The XRD data were complemented with structural information from infrared (IR), Mössbauer and water vapor adsorption techniques. © 2004 International Centre for Diffraction Data.