Local structure of a trapped photoexcited state of a Fe-Co cyanide studied by x-ray-absorption fine-structure spectroscopy

The local structure of a trapped photoexcited state of a Fe-Co cyanide magnetic material, ${\mathrm{Na}}_{0.4}{\mathrm{Co}}_{1.3}\mathrm{Fe}(\mathrm{CN}{)}_6\cdot 5{\mathrm{H}}_2\mathrm{O},$ was investigated by Fe and Co K-edge x-ray-absorption fine-structure spectroscopy. The excited state was prepared by visible-light irradiation at 36 K. In order to obtain detailed structural information, temperature dependence of the local structure was also studied without light irradiation. The Fe-C distance shows almost no change by light irradiation (1.92 Å before and after irradiation), while the average Co-N/O distance is noticeably elongated (1.94 and 2.08 Å before and after irradiation, respectively). This phase transformation is caused by the charge transfer from the ${\mathrm{Fe}}^{\mathrm{II}}\mathrm{}(S=0\mathrm{})-\mathrm{CN}-{\mathrm{Co}}^{\mathrm{III}}\mathrm{}(S=0\mathrm{})\mathrm{}$ configuration to ${\mathrm{Fe}}^{\mathrm{III}}\mathrm{}(S=1/2\mathrm{})-\mathrm{CN}-{\mathrm{Co}}^{\mathrm{II}}\mathrm{}(S=3/2\mathrm{}).$ The local structure of the trapped photoexcited state was found to be identical with that of the high-temperature phase, although the high-temperature phase was created much more effectively by light irradiation than thermal activation.