Local structure and electronic state of a photomagnetic material of CoW cyanide studied by x-ray-absorption fine structure spectroscopy

The local structure and electronic state of a photomagnetic material ${\mathrm{Cs}}_{0.8}{\mathrm{Co}}_{1.1}(3\mathrm{}-\mathrm{cyanopyridine}{)}_{1.9}\mathrm{W}(\mathrm{CN}{)}_8\cdot 2.1{\mathrm{H}}_2\mathrm{O}$ have been investigated by means of x-ray-absorption fine structure (XAFS) spectroscopy. The Co $K-$ and W $L-$edge x-ray-absorption near-edge structure has clarified that upon the transition from the high-temperature (HT) phase to the low-temperature (LT) one, a Co $3d$ electron is transferred to the W $5d$ level; the electronic state of Co changes from the high-spin divalent state ${(d}^7,$ spin momentum $S=3/2)$ to the low-spin trivalent state ${(d}^6,S=0),$ while the W state correspondingly varies from the pentavalent state ${(d}^1,S=1/2)$ to the tetravalent one ${(d}^2,S=0).\mathrm{}$ Extended XAFS analysis has revealed that the coordination around W and Co is $\mathrm{W}(\mathrm{CN}{)}_8{\mathrm{Co}}_4$ and $\mathrm{Co}\left(\mathrm{NC}{)}_4\right(3\mathrm{}-\mathrm{cyanopyridine}{)}_2{\mathrm{W}}_4,$ respectively, and that the Co-N distance in the LT phase is decreased by $\sim 0.17\hspace{0.5em}\AA$ compared to that in the HT one, while the structure around W shows little change. We have also found x-ray-induced phase transition of the present material at 30 K. The x-ray-induced phase was found to be structurally and electronically identical to the HT phase.