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