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, Na_0.4Co_1.3Fe(CN)₆·5H₂O, 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 Fe^II(S=0)-CN-Co^III(S=0) configuration to Fe^III(S=1/2)-CN-Co^II(S=3/2). 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.