J. Phys. IV France
Volume 114, April 2004
Page(s) 81 - 85

J. Phys. IV France
114 (2004) 81
DOI: 10.1051/jp4:2004114015

Structural instability and electronic localization in the 2:1 salts: The case of the Fabre and the (DMtTTF) 2CIO 4salts

S. Ravy1, 1, P. Foury-Leylekian1, D. Le Bolloc'h1, J.-P. Pouget1, J.M. Fabre2, R.J. Prado3 and P. Lagarde3

1  Laboratoire de Physique des Solides (CNRS UMR 8502), Bât. 510, Université Paris-Sud, 91405 Orsay Cedex, France
2  Laboratoire de Chimie Structurale Organique, Université de Montpellier, 34095 Montpellier Cedex 5, France
3  LURE (CNRS UMR 130), Bât. 209D, Université Paris-XI, BP. 34, 91898 Orsay Cedex, France

The charge ordering observed in the (TMTTF) 2X family has been studied by X-ray absorption spectroscopy. XANES measurements at the Sulfur K-edge show no evidence of charge disproportionation larger than 0.5 e, and EXAFS at the Phosphorus K-edge indicate no displacements of the PF 6 anion larger than 0.05 Å. The difficulty to observe a structural signature of this charge ordering is due to the triclinic symmetry of these salts. By contrast, in the monoclinic charge transfer salt (DMtTTF) 2ClO 4, a screw axis symmetry constrains the molecular stacks to be uniform. In this real $\scriptstyle 1/4$-filled system, charge localization is observed at about 150 K. We show that around this temperature an incommensurate modulation of reduced wave vector (0.58,0,-0.275) is stabilized. The transition is strongly hysteretic, but no long range order is established. We suggest that this modulation, which has the 4k F periodicity in the chain direction, stabilizes a local antiferroelectric state similar to the one previously observed in (TMTTF) 2SCN. Key words. Charge ordering, Structural phase transition.

© EDP Sciences 2004