EXAFS and thermodynamics of Fe( II) spin transition polymeric compounds

We have studied the temperature variations of the EXAFS spectra of three Fe ( II)/triazole-based spin transition polymeric compounds compared to a mono mer belonging to the same family. These compounds have various temperatures of transition and hysteresis loop widths. In the three polymers, the Fe-Fe -Fe alignment, detected by a multiple scattering signal at the double Fe-Fe distance, is preserved through the spin transition. For the four compounds , we have studied the variations versus temperature of the Debye-Waller fac tors for both FeN6 (first shell) and Fe-Fe-Fe (multiple scattering path, on ly for the polymers). We report a strong increase of sigma (2) (FeN6) throu gh the spin transition for the monomer and two of the polymeric species. Fo r the polymers an increase of sigma (2) (Fe-Fe-Fe) is also observed. These observations confirm the results available in the literature, and we confir m the interpretation as an increase of the vibrational part of the DW. For one particular polymer, we observe an unexpected behaviour: no significant increase of the Debye-Waller factor from low spin to high spin states. The EXAFS study of these non mono-crystalline species is the only way to discus s the local structure-thermodynamics properties relationships, and particul arly the so-called cooperative effect in the spin transition process.