¹³C NMR Analyses of Successive Charge Ordering in (TMTTF)₂ReO₄

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¹³C NMR measurements were performed for a one-dimensional organic conductor, (TMTTF)₂ReO₄. The existence of an intermediate charge ordering (CO) phase was clarified for a TMTTF salt with a T_d symmetry counter anion by the ¹³C NMR absorption line and spin–lattice relaxation rate, ¹³C T₁⁻¹. The ¹³C NMR spectra, which are characteristic of nuclei in equivalent molecules at room temperature, indicated two inequivalent molecules with unequal electron densities below 225 K. Moreover, the spin-singlet transition associated with ReO₄ anion ordering was confirmed at around 158 K by ¹³C NMR. The ¹³C NMR lines show a marked change at 158 K. The possible redistribution of the electronic charge at the anion ordering temperature as well as the origin of the charge ordering phenomena are discussed.