Crystal structure refinement of the electron-transfer-active potassium manganese hexacyanoferrates and isomorphous potassium manganese hexacyanocobaltates

Abstract

We report on the crystal structure refinements in the novel electron-transfer-active Prussian Blue analogs (PBA) KMn4II[Co1−xIIIFexIII(CN)6]3·nH2O (n≃12). The series of novel PBA with the end members of KMn4[Co(CN)6]3·11.8H2O and KMn4[Fe(CN)6]3·10.5H2O have been synthesized for the first time, all showing a number of extra-reflections incompatible with ordinary face-centered cell of the Fm-3m symmetry group. We have analyzed the Rietveld patterns for x=0,0.53,1 and found that the extra-reflections could be well fitted using several primitive (P) cell symmetries. The best fitting quality was obtained using the noncentrosymmetric space group (S.G.) P4¯3m (Z=1) with the origin of coordinate system shifted into a zeolitic site. In this structure model, the Co–CN–Mn entities are bent owing to the charge introduced by the K+ insertion that induces also the electron transfer between Mn and Fe. Using Mössbauer spectroscopy the electron transfer activity is identified with the appearance of unsplit resonance at the isomer shift of typically −0.15mm/s evidencing the low-spin state for Fe3+ and Fe2+ species. In the same P4¯3m phases doped with Fe2+57 into the Mn site, a sequence of discrete values of quadrupole splitting (0mm/s, 0.9mm/s, 1.8mm/s) is observed and attributed to different conformations of the polyhedra, in which the ground states are orbital triplet, doublet and singlet, respectively.