Charge and anion ordering in the quasi-one-dimensional organic conductor ${\left(\mathrm{TMTTF}\right)}_2{\mathrm{NO}}_3$

The quasi-one-dimensional organic conductors (${\mathrm{TMTTF})}_{2}X$ with noncentrosymmetric anions commonly undergo charge- and anion-order transitions upon cooling. While for compounds with tetrahedral anions ($X={\mathrm{BF}}_4^{-}, {\mathrm{ReO}}_4^{-}$, and ${\mathrm{ClO}}_4^{-}$) 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 $2\delta$ and an activated behavior of $\rho \left(T\right)$ with ${\mathrm{\Delta }}_{\mathrm{CO}}\approx 530\mathrm{K}$ below $T_{\mathrm{CO}}=250\mathrm{K}$. Since (${\mathrm{TMTTF})}_2{\mathrm{NO}}_3$ follows the universal relation between charge imbalance $2\delta$ and size of the gap ${\mathrm{\Delta }}_{\mathrm{CO}}$, 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 $T_{\mathrm{AO}}=50\mathrm{K}$. The tetramerization affects the dc transport, the vibrational features of donors and acceptors, and leads to formation of spin singlets.

Figure 1

(a) Crystal structure of ${\left(\mathrm{TMTTF}\right)}_2{\mathrm{NO}}_3$ where TMTTF stands for tetramethyltetrathiafulvalene at the normal state with finite dimerization and disordered anions, $d_1$ and $d_2$ are intra- and interdimmers distances respectively. (b) Suggested low-temperature state with alternating orientation of the anions that leads to a doubling of the unit cell and tetramerization along the TMTTF molecular stack. (c) From the left to the right shape of the planar ${\mathrm{NO}}_3^{-}$ anion, the difference between two anion orientations, and all possible orientations are shown.

Figure 2

Transport properties of ${\left(\mathrm{TMTTF}\right)}_2{\mathrm{NO}}_3$ along the chain direction $a$. (a) The black solid curve corresponds to the temperature dependence of the dc resistivity; note the double logarithmic scale on the left and bottom axes. The blue dotted curve gives the derivative d $\ln \rho /\mathrm{d}(1/T)$ corresponding to the linear right axis. (b) The Arrhenius plot of the temperature-dependent resistivity allows the extraction of an activation energy $\mathrm{\Delta }=530\mathrm{K}$, indicated by the dashed red line.

Figure 3

ESR parameters of ${\left(\mathrm{TMTTF}\right)}_2{\mathrm{NO}}_3$ obtained from $X$-band measurements as a function of temperature. (a) The intensity $I\left(T\right)$ for the $a$ and $b$ orientation normalized to the room temperature value. The ordering of the anions at $T_{\mathrm{AO}}=50\mathrm{K}$ causes a drop of the spin susceptibility. (b) The temperature-dependent linewidth $\mathrm{\Delta }H$ measured along different directions: $a$ axis (black squares), $b$ direction (green triangles), and diagonal direction (blue stars). Strong evidence for the presence of charge order in this compound comes from the fact that at $T_{\mathrm{CO}}=250\mathrm{K}$, the slope in the diagonal direction changes, as clearly shown in the inset, where the derivative of the smoothed data is plotted. This leads to a crossing (at around 125 K) of the temperature dependence for the linewidth obtained in the diagonal direction and the linewidth for the $b$ orientation.

Figure 4

(a) Temperature evolution of the charge-sensitive infrared-active molecular vibration ${\nu }_{28}$ observed in the mid-infrared spectra of ${\left(\mathrm{TMTTF}\right)}_2{\mathrm{NO}}_3$ for $E\parallel c$. For clarity reasons, the curves are shifted with respect to each other. (b) Resonance frequency of the ${\nu }_{28}\left({\mathrm{B}}_{1u}\right)$ mode as a function of temperature obtained from fitting the spectra with two Lorentzians below ${\mathrm{T}}_{\mathrm{CO}}$ (blue and red triangles). The error bars represent the uncertainty of the fit procedure; more important, however, the peaks gradually diminish on raising the temperature and cannot be reliably distinguished above ${\mathrm{T}}_{\mathrm{CO}}=250\mathrm{K}$ as demonstrated in Fig. 5. The gray area indicates that we present the fit results only below $250\mathrm{K}$. The overall behavior resembles a gradual phase transition more than a rapid jump common to first-order transition. Note the slight upward shift for $T<T_{\mathrm{AO}}$. (c) By using Eq. (1) the charge imbalance $2\delta$ can be directly calculated from the peak separation $\mathrm{\Delta }\nu$.

Figure 5

Vibrational spectra of ${\left(\mathrm{TMTTF}\right)}_2{\mathrm{NO}}_3$ in the range of the ${\nu }_{28}$ mode. (a) The 15 K spectrum can be readily fitted with two Lorentzians, indicated by the red and blue curves; the overall fit is represented by the green line. (b) At $T=290\mathrm{K}$ the curve is much broader (note, the frequency scale differs by a factor of 2) and the mode is less pronounced. We present a fit of the data with one contribution (cyan line) and two contributions (green line) and conclude that a two-component fit is not sensible.

Figure 6

Amount of charge disproportionation $2\delta$ plotted versus the charge-order gap ${\mathrm{\Delta }}_{\mathrm{CO}}$ obtained from temperature-dependent transport measurements [9, 12]. The red line indicates the linear dependence between $2\delta$ and ${\mathrm{\Delta }}_{\mathrm{CO}}$.

Figure 7

Temperature evolution of the optical conductivity in the region of the fundamental ${\nu }_2$ mode of ${\mathrm{NO}}_3^{-}$ anion measured for $E\parallel c$ axis. The curves are displaced for clarity. The dashed black line tracks the resonance frequency of the ${\nu }_2$ molecular vibration upon cooling and visualizes the splitting below $T_{\mathrm{AO}}=50\mathrm{K}$.

Figure 8

Sketch of the ${\left(\mathrm{TMTTF}\right)}_2{\mathrm{NO}}_3$ tetramer in the CO and AO state at low temperatures. Here the dimers A and ${\mathrm{A}}^{\prime }$, as well as B and ${\mathrm{B}}^{\prime }$, are linked via inversion symmetry. The dashed lines indicate the shortest -${\mathrm{CH}}_3\cdots \mathrm{O}$ distances; ($\delta +$) corresponds to the charge-rich sites and $(\delta -)$ to the charge-poor ones, forming a 0110 pattern along the $a$ direction.

Figure 9

Temperature dependence of the mid-infrared spectra of ${\left(\mathrm{TMTTF}\right)}_2{\mathrm{NO}}_3$ probed with $E\parallel a$. Around 1410 to 1420 ${\mathrm{cm}}^{-1}$ the ${\nu }_{4,gu}\left({\mathrm{A}}_g\right)$ mode becomes activated by the tetramerization in the AO state.