Charge and anion ordering in the quasi-one-dimensional organic conductor (TMTTF)2NO3

The quasi-one-dimensional organic conductors (${\mathrm{TMTTF})}₂X$ with noncentrosymmetric anions commonly undergo charge- and anion-order transitions upon cooling. While for compounds with tetrahedral anions ($X={\mathrm{BF}}₄^{\ensuremath{-}}, {\mathrm{ReO}}₄^{\ensuremath{-}}$, and ${\mathrm{ClO}}₄^{\ensuremath{-}}$) the charge-ordered phase is rather well understood, the situation is less clear in the case of planar triangular anions, such as (${\mathrm{TMTTF})}₂{\mathrm{NO}}₃$. 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 $2\ensuremath{\delta}$ and an activated behavior of $\ensuremath{\rho}(T)$ with ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CO}}\ensuremath{\approx}530\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ below ${T}_{\mathrm{CO}}=250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Since (${\mathrm{TMTTF})}₂{\mathrm{NO}}₃$ follows the universal relation between charge imbalance $2\ensuremath{\delta}$ and size of the gap ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CO}}$, 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 ${T}_{\mathrm{AO}}=50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The tetramerization affects the dc transport, the vibrational features of donors and acceptors, and leads to formation of spin singlets.