Characterization of magnetic CoFe cyanides by x-ray-absorption fine-structure spectroscopy

X-ray-absorption fine-structure spectra were measured and analyzed for several FeCo cyanides MₓCoyFe(CN)₆·zH₂O (M=Na, K, Rb) which are known to exhibit interesting magnetic properties depending on temperature. X-ray-absorption near-edge structure spectra allow us to confirm very clearly the spin transitions in Na0.4Co1.3Fe(CN)₆·5H₂O and K0.4Co1.3Fe(CN)₆·5H₂O and to evaluate the composition ratios of CoII and CoIII species quantitatively. The CoII:CoIII ratios are essentially important physical quantities with which to understand magnetic properties. The local structures around Fe and Co atoms were determined by the Fe and Co K-edge extended x-ray-absorption fine-structure spectra. It is confirmed that the local structures around Fe are Fe(CN)₆Co₆ (the subscripts denote the coordination numbers) while those around Co are Co(NC)_6-1/y(H₂O)1/yFe_6-1/y. The first-nearest-neighbor Fe-C distances do not significantly change between FeII and FeIII species, while the first-nearest-neighbor Co-NO distances are noticeably different between CoII and CoIII ones. The difference of as much as 0.16--0.20 {} between CoII and CoIII, which corresponds well to that for typical Co tautomeric complexes, causes noticeable change of the lattice constant.