Evidence for Lattice Effects at the Charge-Ordering Transition in (TMTTF)2X

High-resolution thermal expansion measurements have been performed for exploring the mysterious structureless transition in (TMTTF)(2)X (X=PF6 and AsF6), where charge ordering at T-CO coincides with the onset of ferroelectric order. Particularly distinct lattice effects are found at T-CO in the uniaxial expansivity along the interstack c(*) direction. We propose a scheme involving a charge modulation along the TMTTF stacks and its coupling to displacements of the counteranions X-. These anion shifts, which lift the inversion symmetry enabling ferroelectric order to develop, determine the 3D charge pattern without ambiguity. Evidence is found for another anomaly for both materials at T-int similar or equal to 0.6T(CO) indicative of a phase transition related to the charge ordering.