Electrical conductivity, thermoelectric power, and ESR of a new family of molecular conductors, dicyanoquinonediimine-metal [(DCNQI)2M] compounds

A new family of organic molecules, 2-$R_1$-5-$R_2$-DCNQI (with $R_1$,$R_2$=${\mathrm{CH}}_3$, ${\mathrm{CH}}_3$O, Cl, or Br; DCNQI=N,N’-dicyanoquinonediimine) works as a ligand as well as an electron acceptor to form highly conducting, charge-transfer and coordination compounds as (2-$R_1$-5-$R_2$-DCNQI${)}_2$M (with M=Cu, Ag, Li, Na, K, or ${\mathrm{NH}}_4$). These salts are investigated by the measurements of electrical conductivity, thermoelectric power, and electron spin resonance, which are appropriately understood by classifying them into three groups. Group-I DCNQI salts consisting of the salts with cations M other than Cu, undergo the Peierls transitions between 50 and 100 K. Their thermoelectric power is interpreted by the large-U limit of the Hubbard model. Group-II DCNQI salts, the Cu salts of the halogen-substituted DCNQI, also exhibit the Peierls transitions between 150 and 250 K, followed by the antiferromagnetic transitions around 10 K. With the one-dimensional tight-binding approximation, the bandwidth is estimated to be 0.4–0.5 eV. Group-III DCNQI salts, the Cu salts with $R_1$=$R_2$=${\mathrm{CH}}_3$ or ${\mathrm{CH}}_3$O retain metallic conductivity down to 1.5 K, whereas a magnetic transition takes place at 5.5 K. This may be the first organic conductor in which metallic conduction and a magnetic order coexist. The magnetic order is attributed to the comparatively localized ${\mathrm{Cu}}^{2+}$ spins present independently of the conduction electrons on DCNQI, where the average oxidation state of Cu has been estimated to be ${\mathrm{Cu}}^{1.3+}$. .AE