3.2.4. (TMTSF)8(I3)5

A simple picture of the oxidation of the TMTSF molecules in the 8:5 salt is obtained by assuming that R is almost neutral as it is a monomer. Then the molecules 4P + 2Q will have the charge of +5 in total since the composition of the 8:5 salt is P:Q:R:I− <sup>3</sup> = 4:2:2:5. In other words, the average valence of P and Q is (5/6)+. This is, however, not reasonable since the Δ*E*<sup>0</sup> − Δ*E*1<sup>+</sup> of P and Q is smaller than that of the TMTSF(1/2)+ of the 2:1 NO3, NbF6, and Ni(tds)2 salts as in Figure 12d.

The monomer R has a smaller negative value than that of the monomer C in the 5:2 salt. This is not strange since the molecular plane of R is flat such as P and Q (Figure 1a), while C has a curved structure (Figure 5a). The small difference in the Δ*E*<sup>0</sup> − Δ*E*1<sup>+</sup> among P, Q, and R probably suggests that all the TMTSF molecules in the 8:5 salt accompany to form energy bands and their average valence is considered to be (5/8)+ as expected simply from the composition. On the other hand, P and R are probably the most and least oxidized, respectively, on the basis of the Δ*E*<sup>0</sup> − Δ*E*1<sup>+</sup> in Figure 12d.

Even though the knowledge of the valence of TMTSF is available, the highly symmetric but the rather complicated donor arrangement of the 8:5 salt makes the discussion of its band structure difficult without any band calculations. The sixteen HOMO's (highest occupied molecular orbitals) in one TMTSF layer will give the sixteen energy bands separated by narrow band gaps. When ten out of thirty two electrons in the HOMO's are removed (5/16-filled by holes in total), the Fermi level

will be inside such a small band gap. This picture is consistent with the weak metallic behavior of the *ρ* for *I* longest edge below room temperature, and also with the semiconducting behavior for *I* ⊥ plane, respectively.

Or, in the real space picture, the possible difference in the effective valence of P, Q, and R probably implies that the electronic system is in a charge-disproportionated (or ordered) state in the 8:5 salt.
