Abstract

M. de Souza, P. Foury-Leylekian, A. Moradpour, J.-P. Pouget, and M. Lang

A charge-ordering transition (CO) can be defined as the self-arrangement of charge carriers in a well-defined superstructure. The latter is a direct consequence of strong electronic correlation effects: if the inter-site Coulomb repulsion V is sufficiently strong, the reduction of the total electrostatic energy is achieved by the formation of a non-uniformly distributed charge configuration. The phase transition towards a CO state was already observed experimentally in several materials. Found among them, is the family of quasi-one -dimensional organic charge-transfer salts (TMTTF)2X, where (TMTTF) stands for tetramethyltetrathiafulvalene and X for a monovalent anion. For the latter substances, the transition into the charge-ordered phase is accompanied by the onset of ferroelectricity. To date, no evidence of structural changes accompanying the charge ordering has been reported in the literature. Due to this, the CO transition in these substances has been known as a mysterious “structureless” phase transition. In this article, we report evidence for lattice effects at the CO transition, indicating that for the (TMTTF)2X salt with X = PF6 and AsF6, both charge and lattice degrees of freedom, namely subtle anion displacements, should be involved in the CO transition. In addition, we provide the first evidence of another phase transition at Tint = 0.6 · TCO, which is likely to be related to the CO process.

Phys. Rev. Lett. 101, 216403 (2008)