Charge Ordering in the TMTTF Family of Molecular Conductors

Abstract

Using one- and two-dimensional NMR spectroscopy applied to ${}^{13}\mathrm{C}$ spin-labeled $(\mathrm{TMTTF}{)}_2{\mathrm{AsF}}_6$ and $(\mathrm{TMTTF}{)}_2{\mathrm{PF}}_6$, we demonstrate the existence of an intermediate charge-ordered phase in the TMTTF family of charge-transfer salts. At ambient temperature, the spectra are characteristic of nuclei in equivalent molecules. Below a continuous charge-ordering transition temperature $T_{\mathrm{co}}$, there is evidence for two inequivalent molecules with unequal electron densities. The absence of an associated magnetic anomaly indicates only the charge degrees of freedom are involved and the lack of evidence for a structural anomaly suggests that charge-lattice coupling is too weak to drive the transition.