Probing the Ionic Dielectric Constant Contribution in the Ferroelectric Phase of the Fabre-Salts

• Published in: arXiv.org 2018-01
• Main Authors: de Souza, Mariano, Squillante, Lucas, Sônego, Cesar, Menegasso, Paulo, Foury-Leylekian, Pascale, Jean-Paul Pouget
• Format: Article
• Language: English
• Subjects: Anions; Critical temperature; Deformation mechanisms; Dielectric strength; Donors (electronic); Ferroelectric materials; Ferroelectricity; Freezing; Melting points; Order disorder; Organic materials; Permittivity; Relaxors; Thermal expansion
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1801.00626

In strongly correlated organic materials it has been pointed out that charge-ordering could also achieve electronic ferroelectricity at the same critical temperature \(T_{co}\). 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 \(T_{co}\) requires the break of inversion symmetry, which should be accompanied by a lattice deformation. In this work we investigate the role of the monovalent counter-anion \(X\) in such mechanism. For this purpose, we measured the quasi-static 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 intra-stack electronic response. The \(c^{*}\) dielectric constant (\(\epsilon'_{c^*}\)) 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 \(\epsilon'_{c^*}\) measurements %conjugated with earlier investigations of the \(c^*\) lattice thermal expansion, show that the dielectric response is thermally broaden below \(T_{co}\) 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 PF\(_6\)-H\(_{12}\) salt, where \(T_{co}\) occurs at the freezing point, a relaxor-type ferroelectricity is observed. Also, because of the slow kinetics of the anion sub-lattice, 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.