Evidence for Lattice Effects at the Charge-Ordering Transition in (TMTTF)$_2$X

TL;DR

This study reveals lattice effects at the charge-ordering transition in (TMTTF)$_2$X, showing how charge modulation and anion shifts induce ferroelectric order and identifying an additional phase transition near the charge-ordering temperature.

Contribution

It provides new insights into the lattice effects and the role of anion displacements in charge ordering and ferroelectricity in (TMTTF)$_2$X compounds.

Findings

Distinct lattice effects at $T_{CO}$ in uniaxial expansivity

Charge modulation coupled with anion shifts induces ferroelectric order

Evidence for an additional phase transition at $T_{int} \,\simeq\ 0.6 \cdot T_{CO}$

Abstract

High-resolution thermal expansion measurements have been performed for exploring the mysterious "structureless transition" in (TMTTF)$_{2}$X (X = PF$_{6}$ and AsF$_{6}$), 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 $\textbf{\textit{c*}}$-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 $T_{int}$ $\simeq$ 0.6 $\cdot$ $T_{CO}$ indicative of a phase transition related to the charge ordering.