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). zH(2)O (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). 5H(2)O and K0.4Co1.3Fe(CN)(6). 5H(2)O and to evaluate the composition ratios of Co-II and Co-III species quantitatively. The Co-II:Co-III 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)(6)Co-6 (the subscripts denote the coordination numbers) while those around Co are Co(NC)(6-1/y)(H2O)(1/y)Fe6-1/y. The first-nearest-neighbor Fe-C distances do not significantly change between Fe-II and Fe-III species, while the first-nearest-neighbor Go-NO distances are noticeably different between Co-II and Co-III ones. The difference of as much as 0.16-0.20 Angstrom between Co-II and Co-III, which corresponds well to that for typical Co tautomeric complexes, causes noticeable change of the lattice constant.