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 monomer 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 factors for both FeN₆ (first shell) and Fe-Fe-Fe (multiple scattering path, only for the polymers). We report a strong increase of σ²(FeN₆) through the spin transition for the monomer and two of the polymeric species. For the polymers an increase of σ²(Fe-Fe-Fe) is also observed. These observations confirm the results available in the literature, and we confirm 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 discuss the local structure-thermodynamics properties relationships, and particularly the so-called cooperative effect in the spin transition process.