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X-ray-absorption fine-structure spectra were measured and analyzed for several FeCo cyanides MₗCoₘFe (CN) ₆H₂O (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) ₆5H₂O and K₀. ₄Co₁. ₃Fe (CN) ₆5H₂O 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) ₆Co₆ (the subscripts denote the coordination numbers) while those around Co are Co (NC) ₆-₁/ₘ (H₂O) ₁/ₘFe₆-₁/ₘ. The first-nearest-neighbor Fe-C distances do not significantly change between Fe^II and Fe^III species, while the first-nearest-neighbor Co-NO distances are noticeably different between Co^II and Co^III ones. The difference of as much as 0. 16--0. 20 between Co^II and Co^III, which corresponds well to that for typical Co tautomeric complexes, causes noticeable change of the lattice constant.
Yokoyama et al. (Thu,) studied this question.