Spin-crossover phase transition of a chain Fe(II) complex studied by x-ray-absorption fine-structure spectroscopy

Temperature dependence of Fe K-edge x-ray-absorption fine-structure spectra of a chain Fe(II) complex ${[Fe(4{-}amino{-}1,2,4{-}triazole)}₃{](CH}₃{SO}₃{)}₂{{{·}}2H}₂O$ was investigated to obtain local structural information on the spin-crossover phase transition at {~}280 K. The analysis of the x-ray-absorption near-edge structure spectra successfully yielded the low-spin/high-spin composition ratios as a function of temperature. The analysis of the extended x-ray-absorption fine structure was performed not only for the nearest-neighbor Fe-N shell but for higher-neighbor shells such as the Fe-Fe coordination. For the temperature-dependent analysis of the nearest-neighbor Fe-N shells, the elongation of the distance and the softening of the stretching vibration were revealed, implying significant weakening of the bond on the transition from the low- to the high-spin states. This indicates that the Fe-N stretching vibration contributes to the entropy difference significantly. The abrupt change of the interatomic distance and the divergent behavior of the Debye-Waller factor for the nearest-neighbor Fe-N shell were clearly observed upon the phase transition. These findings in the spin-crossover phase transition are discussed by comparing the order-disorder transition from a local-structure point of view.