Probing the ionic dielectric constant contribution in the ferroelectric phase of the Fabre salts

Abstract

In strongly correlated organic materials it has been pointed out that charge ordering could also achieve electronic ferroelectricity at the same critical temperature Tco. A prototype of such phenomenon are the quasi-one-dimensional (TMTTF)2X Fabre salts. However, the stabilization of a long-range ferroelectric ground state below Tco requires the break of inversion symmetry, which should be accompanied by a lattice deformation. In this paper we investigate the role of the monovalent counteranion X in such mechanism. For this purpose, we measured the quasistatic dielectric constant along the c∗-axis direction, where layers formed by donors and anions alternate. Our findings show that the ionic charge contribution is three orders of magnitude lower than the intrastack electronic response. The c∗ dielectric constant (ϵc∗′) probes directly the charge response of the monovalent anion X, since the anion mobility in the structure should help to stabilize the ferroelectric ground state. Furthermore, our ϵc∗′ measurements show that the dielectric response is thermally broaden below Tco if the ferroelectric transition occurs in the temperature range where the anion movement begin to freeze in their methyl groups cavity. In the extreme case of the PF6-H12 salt, where Tco occurs at the freezing point, a relaxor-type ferroelectricity is observed. Also, because of the slow kinetics of the anion sublattice, global hysteresis effects and reduction of the charge response upon successive cycling are observed. In this context, we propose that anions control the order-disorder or relaxation character of the ferroelectric transition of the Fabre salts. Yet, our results show that x-ray irradiation damages change the well-defined ferroelectric response of the AsF6 pristine salt into a relaxor.