Charge- and anion-orderings in the quasi-one-dimensional organic conductor (TMTTF)$_2$NO$_3$

TL;DR

This study investigates charge and anion orderings in (TMTTF)$_2$NO$_3$, revealing that charge order is primarily influenced by TMTTF stacks, with anion ordering occurring at lower temperatures and affecting electronic and structural properties.

Contribution

It provides new insights into charge and anion ordering mechanisms in (TMTTF)$_2$NO$_3$, especially highlighting the minimal influence of anions on charge order compared to other compounds.

Findings

Charge imbalance 2δ correlates with the charge gap Δ_CO.

Anion ordering occurs at T_AO=50 K, affecting transport and vibrational features.

Charge order follows a universal relation independent of anion type.

Abstract

The quasi-one-dimensional organic conductors (TMTTF)$_2X$ with non-centrosymmetric anions commonly undergo charge- and anion-order transitions upon cooling. While for compounds with tetrahedral anions ($X$ = BF$_4^-$, ReO$_4^-$, and ClO$_4^-$) the charge-ordered phase is rather well understood, the situation is less clear in the case of planar triangular anions, such as (TMTTF)$_2$NO$_3$. Here we explore the electronic and structural transitions by transport experiments, optical and magnetic spectroscopy. This way we analyze the temperature dependence of the charge imbalance 2$\delta$ and an activated behavior of $\rho(T)$ with $\Delta_{\rm CO}\approx 530$~K below $T_{\rm CO} = 250$~K. Since (TMTTF)$_2$NO$_3$ follows the universal relation between charge imbalance 2$\delta$ and size of the gap $\Delta_{\rm CO}$, our findings suggest that charge order is determined by TMTTF stacks with little influence of the anions. Clear signatures of anion ordering are detected at $T_{\rm AO}=50$~K. The tetramerization affects the dc transport, the vibrational features of donors and acceptors, and leads to formation of spin singlets.