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)2X (X=PF(6) 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) approximately 0.6T(CO) indicative of a phase transition related to the charge ordering.