**1. Introduction**

The organic donor TMTSF (= tetramethyltetraselenafulvalene.) is known to give the first organic superconductor (TMTSF)2PF6 (*T*<sup>c</sup> = 0.9 K, *p*<sup>c</sup> ∼ 1.2 GPa) [1] and first ambient-pressure organic superconductor (TMTSF)2ClO4 (*T*<sup>c</sup> = 1.2 K) [2]. These superconductors are most studied members of Bechgaard salts, which are the 2:1 radical salts of TMTSF and monovalent anions (PF6 −, AsF6 −, SbF6 −, TaF6 <sup>−</sup>, BF4 <sup>−</sup>, ClO4 −, NO3 −) [3,4]. In addition to the starting seven members, yet eight isostructural salts have been synthesized with NbF6 <sup>−</sup> [5], ReO4 <sup>−</sup> [6,7], BrO4 − [8], FSO3 − [8], PF2O2 − [9], CF3SO3 − [10,11], H2F3 − [12], and SiF5 − [13], respectively.

Their crystal structures are characterized by the weakly dimerized stacks of the planar TMTSF molecules along the most-conducting *a*-axis. There are weak interaction between the neighboring stacks along the second-conducting *b*-axis forming the TMTSF layers. Finally, each TMTSF layer is separated by an anion layer resulting in a sandwich structure.

The physical properties of the PF6 and ClO4 salts are so interesting that the studies of the TMTSF salts seem to have been mainly focused on the starting members of Bechgaard salts including the two. Many experimental and theoretical efforts have been devoted to reveal the nature of, for example, possible exotic superconductivity [14–18], spin-density-wave (SDW) transitions [19–27], field-induced SDW transitions [28–35], and a variety of angular-dependent magnetoresistance oscillations [36–44], all of which is closely related to the low dimensionality of electronic systems of Bechgaard salts.

On the other hand, much less attention has been paid to "exotic" TMTSF salts other than Bechgaard ones. To the best of the authors' knowledge, there are reports on the six charge-transfer complexes with acceptor molecules [45–50] as well as twenty five salts with inorganic or complex anions, where TMTSF molecules have the average valence of (1/2)+ [51–55], (2/3)+ [56–61], (3/4)+ [62,63], (4/5)+ [64], and 1+ [65–71], respectively. Actually, the number of the exotic TMTSF salts is larger than that of the fifteen Bechgaard ones mentioned above. Although the superconductivity has never been reported, new phenomena to be explored will be probably provided by the variety of their exotic crystal structures giving different oxidation states and/or packing patterns of TMTSF molecules from that of Bechgaard salts.

In this paper, we present three novel exotic TMTSF salts with the same counter anion I − <sup>3</sup> , namely (TMTSF)8(I3)5 (8:5 salt), (TMTSF)5(I3)2 (5:2 salt), and (TMTSF)4(I3)4·THF (4:4 salt, THF = tetrahydrofuran), respectively. Their crystal structures are of different types from those of any TMTSF salts ever reported. We carried out the X-ray crystal structure analyses; and the electrical resistivity measurements at ambient pressure as well as at pressures up to 1.7 GPa.

The information on the valence of donors and/or acceptors is helpful to understanding the electronic states of charge-transfer complexes with partial charge transfer. In some literature of the exotic TMTSF salts, they attempted to estimate the TMTSF valence on the basis of bond lengths within the TMTSF molecule [46,60,63,71]. It is, however, shown below that such an method does not give plausible estimates for the present I3 salts.

Instead, we estimated the valence from the difference between calculated energies of the neutral and cationic TMTSF molecules using the conformations observed for the crystals. By comparing these energies with that obtained for the quantum mechanically optimized conformations, it is shown that one can obtain reasonably quantitative valence of TMTSF molecules in the crystals. This method is probably applicable to the other exotic TMTSF salts as well as to salts of other organic donors and/or acceptors.

#### **2. Results**

#### *2.1. (TMTSF)*8*(I*3*)*<sup>5</sup>

#### 2.1.1. Crystal Structure

The crystal structure of the 8:5 salt is shown in Figure 1; and its crystallographic and refinement data are summarized in Table 1. Crystallographic data files are available as Supplemental Materials. The appearance of single crystals are black thick cuboids reflecting the orthorhombic crystal system with the space group *Cmcm*.

There are four TMTSF stacks along the *c*-axis in the unit cell with the period of six TMTSF molecules. There are two crystallographically independent TMTSF molecules (P and Q) in the stack, which is made of repetition of trimers (PQP). The normal to the molecular plane defined with the four Se atoms is tilted from the stacking direction (||*a*) by 2.23◦ for P, while that of Q is not tilted as its carbon and selenium atoms are on a mirror plane (||*bc*).

The stacks are separated by another independent TMTSF molecules (R) along the *b*-axis; and by I − <sup>3</sup> chains along the *a*-axis. Although there exist shorter Se··· Se contacts than the sum of van der Waals radii of two Se atoms (<4.0 Å) between Q and R as in Figure 2, such contacts do not form

networks along the *b*-axis. Thus, the electronic system is probably quasi-one-dimensional along the *c*-axis. Here we adopted 2.0 Å as the van der Waals radius of Se by Pauling [72], while 1.9 Å by Bondi [73] is used in some literature [13,56,58,60]. The length and number of such short contacts between neighboring molecules are the primitive but important measure to find strong intermolecular interactions; namely the stronger interactions along a direction can give the wider band dispersion than that along the other directions.

**Figure 1.** The crystal structure of (TMTSF)8(I3)5 at room temperature viewed along (**a**) the *c*-axis and (**b**) the *b*-axis. The crystallographically independent TMTSF molecules are labeled P, Q, and R, respectively.

The distance between the molecular planes, which is defined by the four Se atoms in each molecule, is almost the same for that between P and Q (*d*<sup>1</sup> = 3.56(1) Å) and between P and P (*d*<sup>2</sup> = 3.559(8) Å), respectively (Figure 2). On the other hand, the average short Se··· Se contacts between P and Q and between P and P are 3.95 Å and 3.82 Å suggesting that the interaction between P and P is slightly stronger than that between P and Q.

While dimer units of donors are widely observed as building blocks of organic conductors such as Bechgaard salts, TMTSF trimers are also not so rare in the exotic TMTSF salts. For example, there are reports on (TMTSF)3[Ti2F8(C2O4)] [56], (TMTSF)3*M*(CN)4 (*M* = Pt, Ni) [57], (TMTSF)3W6O19(DMF)2 (DMF = *N*,*N*-dimethylformamide) [58], (TMTSF)3[Cr(NCS)4(phen)]2·CH2Cl2 (phen = 1,10-phenanthroline) [59], and (TMTSF)3(TFPB)2 (TFPB = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) [60], (TMTSF)3Ta2F10O [61], respectively. In most of these salts, the average valence of a TMTSF molecule is (2/3)+ suggesting the stability of the [(TMTSF)3] <sup>2</sup><sup>+</sup> unit.


**Table 1.** X-ray crystallographic and refinement data for (TMTSF)8(I3)5, (TMTSF)5(I3)2, and (TMTSF)4(I3)4·THF, respectively.

† *Z* = the number of chemical formula units per unit cell.

**Figure 2.** Donor arrangement in a half unit cell of (TMTSF)8(I3)5. Hydrogen atoms are not shown for clarity. The crystallographically independent TMTSF molecules are labeled P, Q, and R, respectively. The broken lines show the Se··· Se contacts shorter than 4.0 Å, namely the sum of van der Waals radii of two Se atoms (see text). The interplanar distance between P and Q is *d*<sup>1</sup> = 3.56(1) Å; and that between P and P is *d*<sup>2</sup> = 3.559(8) Å, respectively.

On the other hand, the existence of the TMTSF monomer separating TMTSF stacks like R in the present 8:5 salt was reported for (TMTSF)3[Cr(NCS)4(phen)]2·CH2Cl2 [59] and (TMTSF)4(TMTSF)Nb6Cl18·(CH2Cl2)0.5 [62,63]. This is also the case with the 5:2 salt shown below. When a rather strong interaction is expected between such a monomer and a donor stack, it is difficult to estimate the valence of TMTSF molecules because the crystallographically independent molecules naturally have different charges from one another and a TMTSF molecule easily possesses an irrational charge when it participates in forming energy bands. The estimation of the valence of TMTSF molecules in the present salts is discussed in Section 3.

There exist three types of disorders in the 8:5 salt at room temperature as shown in Figure 3. The first is a conformational disorder of three methyl groups of the TMTSF molecule Q. Each of the methyl groups at the carbon C5, C9, and C10 is disordered between two energetically favorable states.

**Figure 3.** Some molecules in the unit cell of (TMTSF)8(I3)5 selected to show the three types of disorders. (1) The conformational disorder of three methyl groups at the carbons C5, C9, and C10. (2) The positional disorder of the I− <sup>3</sup> anions of I4–I5–I6 and I7–I8–I9. (3) The orientational disorder of the I− <sup>3</sup> anions of I11–I10–I12 and I11B–I10–I12B.

The second is the positional disorder of the I− <sup>3</sup> anions on a mirror plane and they are labeled I4–I5–I6 and I7–I8–I9, respectively. The crystal structure analysis was carried out by assuming the same probability of 1/2 for each of the red and blue sites. The slight bending of I–I–I is probably spurious and caused by the overlap of the electron density of the disordered anions.

The third is the orientational disorder of the I− <sup>3</sup> anion whose center is labeled I10. The crystal structure was solved by assuming I11–I10–I12 (pink) has the probability of 1/2 and each of the two orientations of I11B–I10–I12B (lime green and orange) has 1/4.

In addition to the orthorhombic crystal system, these disorders and the large unit cell containing the thirty two TMTSF molecules and twenty I− <sup>3</sup> anions are characteristic to the 8:5 salt.

#### 2.1.2. Electrical Resistivity

The electrical resistivity *ρ* of (TMTSF)8(I3)5 was measured at ambient pressure and under hydrostatic pressures up to 1.73 GPa.

The single crystals are rather thick and plate-like. The dimensions are 0.5 × 0.2 × 0.07 mm<sup>3</sup> on average for the seven samples. Although the correspondence between the crystal edges and the lattice vectors has not been determined, the anisotropy of *ρ*—those along the most and least grown directions are ∼60 and ∼2 S·cm<sup>−</sup>1—suggests that the longest crystal edge is parallel to the stacking direction of TMTSF molecules (*c*-axis).

The *ρ* along the most-grown edge (*I* longest edge) is weakly metallic below room temperature, but resistance jumps make its intrinsic behavior very unclear and hysteretic. On the other hand, the reproducibility was good when the electrical current was applied along the least-grown edge, in other words perpendicular to the widest surface (*I* ⊥ plane). Such behavior is very similar to that observed for the Bechgaard salts. Thus, we measured the pressure dependence of the *ρ* with *I* ⊥ plane.

The result obtained for the sample #1908 is shown in Figure 4a. At ambient pressure, the *ρ* weakly increases below 300 K on cooling and becomes almost temperature independent below about 190 K. The *ρ* starts to increase very rapidly below 88 K suggesting a kind of phase transition or a change in the electronic state. After the increase in the *ρ* becomes gradual approaching 60 K, another rapid increase starts at 53 K.

The behavior of the *ρ* at higher pressures is similar to that observed at ambient pressure. The *ρ* becomes smaller and, at 1.7 GPa at room temperature, it reaches about 3.3% of that at ambient pressure. The temperatures of the steps also become lower at higher pressures.

One can see that the weak metallic behavior appears in the intermediate temperature region above 0.88 GPa. Please note that the hydrostatic pressure was applied using a clamped type pressure cell with Daphne 7373 oil as pressure medium as described in Section 4. Then the inside pressure decreases on cooling [74,75] resulting in the increase in the *ρ*, which sometimes cancels out the weak metallic behavior. Thus, the observation of the metallic behavior for the 8:5 salt suggests the existence of the small number of free charge carriers and a Fermi pocket, or a narrow band gap at Fermi level.

In the temperature dependence at ambient pressure, we see subtle hysteresis above 190 K as shown by the arrows (Figure 4a). Although the overall behavior under pressure is very similar to that at ambient pressure, no hysteresis was recognized at and above 0.30 GPa. It suggests that the apparent hysteresis has the same origin as the resistance jumps for *I* longest edge and is caused by an extrinsic effect like micro-cracks.

The steps below 88 K and 53 K are clearly seen also in the Arrhenius plot of *ρ* shown in Figure 4b. In addition to these steps, the Arrhenius plot reveals the existence of the third step above 200 K at each pressure. Figure 4c shows the numerical derivative of the Arrhenius plot. The calculations were carried out after smoothing the data. The rapid increases in the *ρ* in Figure 4a,b are recognized as the peaks in Figure 4c. Here we assume each peak corresponds to a phase transition or a change in the electronic state.

It should be noted that the pressure medium Daphne 7373 gradually solidifies around 200 K at ambient pressure; and the solidification temperature increases up to 300 K at 2.2 GPa [74,75]. The solidification results in rather rapid decrease in the inside pressure (∼0.1 GPa) on cooling and can be detected as a subtle extrinsic anomaly in the *ρ* as indicated by the arrows in Figure 4c. This is, however, not the case with the result at ambient pressure as the sample was in vacuum. Therefore, the the third anomaly above 200 K is not that caused by the pressure change.

We can define the possible transition temperatures *T*c1 and *T*c2 as the peak tops in Figure 4c as well as that of the cross point in Figure 4a,b. The *T*c1 and *T*c2 defined by the cross point is 2–3 K higher than that at the peak top. On the other hand, it is difficult to define the *T*c3, since the steps in Figure 4a,b, and the peaks in Figure 4c are incomplete. This shows that the step-like change in the high temperature region in the Arrhenius plot probably starts above 310 K.

Figure 4d shows the *T*–*p* phase diagram made by plotting the *T*c1 and *T*c2 defined as the cross points. The pressures at low temperatures were corrected by using the clamped pressure at room temperature and the temperature dependence of the pressure inside the clamped-type cell with Daphne 7373 as the pressure medium [74,75]. The nature of the phase transitions is unclear at present, but it is obvious that we need much higher pressure to suppress the semiconducting/insulating states (high-*R* 1 and high-*R* 2) than 1.7 GPa.

**Figure 4.** (**a**) Temperature dependence of the electrical resistivity *ρ* of (TMTSF)8(I3)5 measured perpendicular to the most grown crystal surface. (**b**) Arrhenius plot of the same data in (**a**). Please note that the values in the horizontal axis is *T* but the scale is linear in *T*<sup>−</sup>1. (**c**) Numerical derivative of the Arrhenius plot in (**b**) calculated after smoothing the data. A series of data at each pressure was shifted from one another for clarity. The arrows at high temperatures indicate the subtle anomalies caused by the solidification of the pressure medium (see text). (**d**) Temperature–pressure phase diagram of (TMTSF)8(I3)5 below 150 K.

#### *2.2. (TMTSF)*5*(I*3*)*<sup>2</sup>

#### 2.2.1. Crystal Structure

The crystal structure of (TMTSF)5(I3)2 at room temperature is shown in Figure 5; and its crystallographic and refinement data are summarized in Table 1. The crystal system is monoclinic with the space group *P*21/*n*, but *β* = 90.355◦ is close to 90◦.

There are two TMTSF stacks along the *a*-axis in the unit cell. The stack at the center of the unit cell is crystallographically identical to that at the corners, which are related by the two-fold screw axis along the *b*-axis to each other.

Each stack is made of the repetition of trimers, where the donors are labeled as B, A, and B, respectively. Please note that the center of the molecule A is located at an inversion center. The tilt angle of the normal to the molecular plane from the *a*-axis is 24.26◦ and 23.79◦ for A and B, respectively. In this sense, the trimer stack of the 5:2 salt is rather different from that of the 8:5 salt, where the molecular plane is almost perpendicular to the stacking direction.

The TMTSF stacks are separated by the TMTSF monomers C and the I− <sup>3</sup> anions as in the 8:5 salt. The molecular plane of C is, however, not flat as the monomer R in the 8:5 salt. This suggests that C is in a different oxidation state from that of R.

Figure 6 shows the arrangement of the TMTSF molecules of the 5:2 salt. There are six Se··· Se short contacts between the neighboring TMTSF molecules in each stack. The interplanar distance between A and B is *d*<sup>1</sup> = 3.45(3) Å; and that between B and B is *d*<sup>2</sup> = 3.39(4) Å, respectively. Thus the much stronger interactions than that in the 8:5 salt are expected for the 5:2 salt. In addition, each monomer C shares four short contacts with one of neighboring stacks. Thus, we cannot simply conclude that C is neutral. The valence of each TMTSF molecule is estimated together with that in the other I3 salts in Section 3.

**Figure 5.** The crystal structure of (TMTSF)5(I3)2 at room temperature viewed along (**a**) the *a*-axis and (**b**) the (*b* + *c*)-direction, respectively. The crystallographically independent TMTSF molecules are labeled A, B, and C.

**Figure 6.** Donor arrangement in (TMTSF)5(I3)2. Hydrogen atoms are not shown for clarity. The crystallographically independent TMTSF molecules are labeled A, B, and C, respectively. The broken lines show the Se··· Se contacts shorter than the sum of van der Waals radii of two Se atoms (4.0 Å). The interplanar distance between A and B is *d*<sup>1</sup> = 3.45(3) Å; and that between B and B is *d*<sup>2</sup> = 3.39(4) Å, respectively.

#### 2.2.2. Electrical Resistivity

The *ρ* was measured along the most grown crystal axis. Although the correspondence between the crystal edges and the lattice vectors was not determined, it is natural to assume the crystal grows the best along the TMTSF stacks. The room temperature electrical conductivity is typically 50 S·cm−<sup>1</sup> but scattered between 1 and 200 S·cm−<sup>1</sup> among the seven single crystals measured.

The temperature dependence of the *ρ* is shown for the sample #1910 in Figure 7a. The *ρ* increases with decreasing temperature suggesting the existence of a band gap at Fermi level. Anomalous change in slope was observed around 190 K. It is clearly seen in the Arrhenius plot in Figure 7b. The slope becomes small below 190 K. Assuming the activation type temperature dependence, the tentative activation energy *<sup>E</sup>*<sup>a</sup> above and below 190 K is estimated as 1.3 × <sup>10</sup><sup>2</sup> and 34 meV, respectively.

The application of hydrostatic pressure up to 1.73 GPa does not change the behavior very much. The room temperature resistivity was decreased to about 25% of that at ambient pressure. The tentative *E*a was also reduced to 90 and 22 meV above and blow 185 K, respectively.

**Figure 7.** (**a**) Temperature dependence of the electrical resistivity *ρ* of (TMTSF)5(I3)2 measured along the longest crystal edges. (**b**) Arrhenius plot of the same data in (**a**). Please note that the values in the horizontal axis is *T* but the scale is linear in *T*<sup>−</sup>1.
