POLYMERIC FE(II) SPIN CROSS-OVER COMPOUNDS - XAS STRUCTURAL RESULTS

Fe(II)triazole derivatives are spin cross-over compounds with various Te (-30-->+150 degrees C), large thermal hysteresis (Delta T-c approxi mate to 50 K) and an important thermochromic effect. These promising p roperties, which are already used in experimental imaging and memory d evices, are probably due to their polymeric structures. We have studie d these structures by EXAFS and X-Ray diffraction (in spite of the imp ortant long distance disorder observed). The key result of this study is me alignment of a long chain of Fe(II) ions in the Low Spin CLS) st ate of the polymeric species, proved by the presence of a characterist ic double Fe-Fe distance multiple scattering EXAFS signal (analysed by FEFF code). New results have been obtained recently on new members of the Fe(II) triazole family and nickel glyoxime model compounds presen ting similar metal-metal alignment, proving definitely the assumed ali gnment in the LS state. Systematic study of the temperature dependence of the multiple-scattering signal amplitude were performed in order t o explain the apparent vanishing of the alignment in me High Spin (HS) state.