J. Phys. IV France 114 (2004) 81-85  EDP Sciences, Les Ulis DOI: 10.1051/jp4:2004114015 81 Structural instability and electronic localization in the 2:1 salts: The case of the Fabre and the (DMtTTF) 2 CIO 4 salts S. Ravy 1, * , P. Foury-Leylekian 1 , D. Le Bolloc’h 1 , J.-P. Pouget 1 , J.M. Fabre 2 , R. J. Prado 3 and P. Lagarde 3 1 Laboratoire de Physique des Solides (CNRS UMR 8502), Bât. 510, Université Paris-Sud, 91405 Orsay Cedex, France 2 Laboratoire de Chimie Structurale Organique, Univesité de Montpellier, 34095 Montpellier Cedex 5, France 3 LURE (CNRS UMR 130), Bât. 209D, Université Paris-XI, BP 34, 91898 Orsay Cedex, France Abstract. The charge ordering observed in the (TMTTF) 2 X 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) 2 ClO 4 , a screw axis symmetry constrains the molecular stacks to be uniform. In this real ¼-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) 2 SCN. Key words. Charge ordering, Structural phase transition. 1. INTRODUCTION The Bechgaard ((TMTSF) 2 X) and Fabre ((TMTTF) 2 X) salts and more generally the 2:1 organic charge transfer salts, are among the richest materials ever discovered. Indeed, while Mott localization, spin-Peierls state, antiferromagnetism and superconductivity are known and studied for two decades [1], ferroelectricity (FE) and antiferroelectricity (AFE) have recently been recognized [2,3] as being part of the general phase diagram of the (TMTTF) 2 X compounds. Dielectric permittivity [2] and NMR measurements [4] in (TMTTF) 2 PF 6 , AsF 6 , SbF 6 , and (TMTTF) 2 ReO 4 , SCN compounds [5], lead to the conclusion that these FE (q=0) and AFE (q≠0) states are associated to a charge ordering (CO), and result from the lost of the symmetry center relating the molecules of the chain. On resistivity curves the q=0 CO gives rise to small though visible anomalies [6], while diffraction methods fail to evidence structural anomalies associated to the CO transition [7]. 2. (TMTTF) 2 X COMPOUNDS It is known [8] that sulfur K-edge X-ray absorption spectroscopy (XANES) measurements are very sensitive to the electronic state of sulfur involved in a molecule. XANES spectra on (TMTTF) 2 X have been measured on the SA32 beamline at LURE, for X= PF 6 , AsF 6 and SbF 6 , above and below the CO transition temperatures. Figure 1 displays typical spectra of (TMTTF) 2 PF 6 (T CO =70 K) at 100 K and 10 K and their derivative. The position of the S K-edge is found to be the same, with a precision of about 0.1 eV. The same results have been obtained for the two other salts. Assuming that a charge * Corresponding author: S. Ravy, e-mail: ravy@lps.u-psud.fr