Wigner Crystal Type of Charge Ordering in an Organic Conductor with a Quarter-Filled Band:(DI−DCNQI)2Ag

${}^{13}\mathrm{C}-\mathrm{NMR}$ measurements have been performed to investigate the nature of the insulating state of a quasi-one-dimensional conductor, $(\mathrm{DI}-\mathrm{DCNQI}{)}_2\mathrm{Ag}$, with a quarter-filled band. The ${4k}_F$ charge-density wave has been evidenced below about 220 K by the line separation of the ${}^{13}\mathrm{C}-\mathrm{NMR}$ spectra, which shows a clear charge disproportionation instead of the lattice dimerization encountered in most cases. The present case demonstrates the Wigner crystallization of electrons in the charge-transfer salt and a crucial role of the long-range Coulomb interaction.