Temperature dependence of Fe K-edge x-ray-absorption fine-structure spectra of a chain Fe(II) complex ${\left[\mathrm{Fe}\right(4-\mathrm{a}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{o}-1,2,4-\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{z}\mathrm{o}\mathrm{l}\mathrm{e})\mathrm{}}_3{\left]\right(\mathrm{C}\mathrm{H}}_3{\mathrm{SO}}_3{)}_2{\mathrm{\cdot }2\mathrm{H}}_2\mathrm{O}$ 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.

• Received 22 June 1998

DOI:https://doi.org/10.1103/PhysRevB.58.14238