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 $M_x{\mathrm{Co}}_y{\mathrm{Fe}\left(\mathrm{CN}\right)\mathrm{}}_6\cdot z{\mathrm{H}}_2\mathrm{O}$ $(M=\mathrm{Na},$ 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 ${\mathrm{Na}}_{0.4}{\mathrm{Co}}_{1.3}{\mathrm{Fe}\left(\mathrm{CN}\right)\mathrm{}}_6{\mathrm{\cdot }5\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{K}}_{0.4}{\mathrm{Co}}_{1.3}{\mathrm{Fe}\left(\mathrm{CN}\right)\mathrm{}}_6{\mathrm{\cdot }5\mathrm{H}}_2\mathrm{O}$ and to evaluate the composition ratios of ${\mathrm{Co}}^{\mathrm{II}}$ and ${\mathrm{Co}}^{\mathrm{III}}$ species quantitatively. The ${\mathrm{Co}}^{\mathrm{II}}{:\mathrm{C}\mathrm{o}}^{\mathrm{III}}$ 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 ${\mathrm{Fe}\left(\mathrm{CN}\right)\mathrm{}}_6{\mathrm{Co}}_6$ (the subscripts denote the coordination numbers) while those around Co are ${\mathrm{Co}\left(\mathrm{NC}\right)\mathrm{}}_{6-1/y}({\mathrm{H}}_2\mathrm{O}{)}_{1/y}{\mathrm{Fe}}_{6-1/y}.$ The first-nearest-neighbor Fe-C distances do not significantly change between ${\mathrm{Fe}}^{\mathrm{II}}$ and ${\mathrm{Fe}}^{\mathrm{III}}$ species, while the first-nearest-neighbor Co-NO distances are noticeably different between ${\mathrm{Co}}^{\mathrm{II}}$ and ${\mathrm{Co}}^{\mathrm{III}}$ ones. The difference of as much as 0.16–0.20 Å between ${\mathrm{Co}}^{\mathrm{II}}$ and ${\mathrm{Co}}^{\mathrm{III}},$ which corresponds well to that for typical Co tautomeric complexes, causes noticeable change of the lattice constant.