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

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

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

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

Vibrational Spectroscopy Ionicity variation

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

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

DMTTF - CA Temperature-induced transition

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

Low-temperature phase Raman-active lattice modes

Many coupled modes contribute to the effective Peierls mode

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