Charge-ordering transition in (TMTTF)₂X explored via dilatometry

Abstract

Charge-ordering phenomena have been highly topical over the last few years. A phase transition towards a charge ordered state has been observed experimentally in several classes of materials. Among them, many studies have been devoted to the family of quasi-one dimensional organic chargetransfer salts (TMTTF)2X, where (TMTTF) stands for tetramethyltetrathiafulvalene and X for a monovalent anion (X = PF6, AsF6 and SbF6). However, the relationship between the electron localization phenomena and the role of the lattice distortion in stabilizing the charge-ordering pattern is poorly documented in the literature. Here we present a brief overview of selected literature results with emphasis placed on recent thermal expansion experiments probing the charge-ordering transition of these salts.

Donor–anion interactions in quarter-filled low-dimensional organic conductors

“…16). [89][90][91] The decrease rate is strongest for measurements along the c* direction where donor layers and anion cavities alternate. The lattice contraction should be caused by a decrease of the cavities effective volume upon heating due to the thermal enhancement of the orientational disorder of methyl groups and the rotation and reorientation disorders of the anion.…”

“…[151] Furthermore, lattice expansion coefficients, exhibit anomalies at T CO related to the specific heat anomaly expected for a second-order phase transition. [91] The CO transition of the SbF 6 salt exhibits a giant lambda type critical anomaly ( Fig. 22a) [92] while the CO transition of the AsF 6 and PF 6 salts exhibit a mean-field jump.…”

“…Thermal dependence of the uniaxial lattice expansion along the c*-axis in (a) (TMTTF) 2 SbF 6 (from the data of ref [91]. (Copyright 2013 by IOP Publishing), and (b) (TMTTF) 2 AsF 6 and (TMTTF) 2 PF 6 (from the data of ref [152].…”

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

“…Thus, SbF 6 -H 12 is not expected to exhibit important disorder effects, modifying the Curie-Weiss dependence of the dielectric constant. This is not the case for the AsF 6 -H 12 (T co = 105 K) because the ferroelectric transition occurs when the anions just begin to freeze with the saturation of the c * negative lattice thermal expansion regime around 125 K 3,16 . Compared to the SbF 6 -H 12 salt, a longer tail of the dielectric response develops below T co in the AsF 6 -H 12 salt.…”

Enhancement of thermal expansion of organic charge-transfer salts by strong electronic correlations

“…Materials that have been studied included heavy fermion compounds [2], organic chargetransfer salts [3][4][5][6], iron pnictide superconductors [7,8], and LiV 2 O 4 [9]. The Grüneisen parameter , which is proportional to the ratio of the thermal expansion to the specific heat, can be two orders of magnitude larger than the values of order unity found for elemental solids [2], and may diverge at a quantum critical point [10].…”