1. TEMPERATURE-INDUCED VALENCE AND STRUCTURAL INSTABILITY OF DMTTF-CA Paolo Ranzieri, Alberto Girlando Matteo Masino Università degli studi di Parma INSTM Parma University

2. Charge-Transfer (CT) crystals eeee D-----A 0<  <1

3. Neutral-Ionic Transition eeee eeee eeee D  D  D ..... D  A  D  A  D  A ..... COLLECTIVE CHARGE-TRANSFER D  D  D  D .... A  D  A  D  A  D  A  D ..... + - Stack dimerization and ferroelectricity

4. TTF-CA derivatives AcceptorDonor X=Cl, Br DMTTF-CA crystal

5. Vibrational Spectroscopy Ionicity variation

6. Infrared activation of the totally-symmetric molecular vibrations Spectroscopic effects of the stack distorsion

7. 65 K X-ray data E. Collet at al. Phys. Rev. B 63, 054105 (2001) Dimerization and cell doubling

8. DMTTF - CA Temperature-induced transition

9. Pretransitional phenomena  IR =  Raman ±  lattice  lattice =  Peierls M.Masino, A. Girlando, Z. G. Soos Chem. Phys. Lett. 369, 428 (2003)

10. Low-temperature phase Raman-active lattice modes

11. Many coupled modes contribute to the effective Peierls mode

12. Competition between the energy required for the formation of D + A - pair and the Madelung energy Conclusions Overlap between D-A modulated by Peierls phonons DMTTF-CA: transition driven by electron-phonon coupling followed by an electronic rearrangement Complex interaction among electronic, spin and vibrational degrees of freedom TTF-CA : transition driven by electronic interactions