+ Affiliations1Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, Meguro, Tokyo 152-8552, Japan2Department of Applied Chemistry, Ehime University, Matsuyama 790-8577, Japan3ACT-C JST, Honcho, Kawaguchi, Saitama 332-0012, Japan
Received December 3, 2015; Accepted June 17, 2016; Published July 29, 2016
The structural, transport, and magnetic properties of the new organic conductors (DMEDO-TTF)2X (X = ClO4 and BF4), where DMEDO-TTF is dimethyl(ethylenedioxy)tetrathiafulvalene, have been investigated. These compounds have a complete uniform stack structure, indicating that a quasi-one-dimensional 3/4-filled band without a dimerization gap is realized. The ClO4 and BF4 salts show a first-order metal–insulator (MI) transition at approximately 190 and 210 K, respectively, in the cooling process. The ground state is a nonmagnetic insulator on the basis of magnetic susceptibility measurements. Low-temperature X-ray diffraction measurements show that the MI transition originates in the anion ordering transition with a superstructure wave vector of \(\boldsymbol{{q}} = (0,1/2,0)\) corresponding to the stacking direction; the uniform donor stacking structure changes to the tetramerized structure with a large shift of the donors. The shift of the anion toward the central two donors in a tetramer indicates that the insulating phase is a charge-density-wave state.
©2016 The Physical Society of Japan
1. IntroductionOrganic conductors have a rich phase diagram, including metal, insulator, charge-density-wave, charge ordering, spin-Peierls, spin-density-wave, and superconducting states.1) Among these, quasi-one-dimensional (Q1D) 3/4-filled band systems have been extensively studied experimentally and theoretically. TMTCF (\(C =\text{S}\): tetramethyltetrathiafulvalene, \(C =\text{Se}\): tetramethyltetraselenafulvalene)-based organic conductors possess a variety of physical properties with various anions and show various ground states depending on the anions and external pressure.1)
The recently developed Q1D organic conductors (DMEDO-TTF)2MF6 (\(M =\text{P}\), As, and Sb), where DMEDO-TTF is dimethylethylenedioxy-tetrathiafulvalene [Fig. 1(a)], show a characteristic metal–metal (MM) structural phase transition.2) The MM transition temperatures are 130, 230, and 370 K for \(M =\text{P}\), As, and Sb, respectively. Organic conductors containing an ethylenedioxy group undergo a marked structural change at low temperatures, but the low-temperature phases of the salts of bis(ethylenedioxy)tetrathiafulvalene (BO) such as (BO)3Cu2(NCS)3 and (BO)2ReO4(H2O) are unclear owing to the considerable disorder at low temperatures.3–8) In contrast, the low-temperature structures of the DMEDO-TTF salts have been investigated extensively despite the significant donor rearrangement. The high-temperature metallic phase has an ordinary quasi-triangular network in which a donor molecule is surrounded by six donors. Below the MM transition temperature, the low-temperature metallic phase has a quasi-square network in which a donor is surrounded by eight donors.2) The donor–anion intermolecular interaction and the hydrogen-fluorine interaction play a crucial role in this MM transition. At low temperatures, (DMEDO-TTF)2MF6 shows a metal–insulator (MI) transition at 50 K; the ground state is a nonmagnetic insulator, although the origin of the MI transition is unclear. The MM transition temperature increases with increasing anion volume, whereas the MI transition temperature (\(T_{\text{MI}}\)) is independent of the anions.
Figure 1. (Color online) (a) DMEDO-TTF molecule. (b) Crystal structure projected onto the \(ac\)-plane, (c) donor layer projected along the molecular long axis, and (d) donor and anion columns projected along the molecular short axis of (DMEDO-TTF)2ClO4 at 293 K. In (c), the transfer integrals are \(t_{b} = 262.4\), \(t_{p1} = -43.5\), \(t_{p2} = -16.1\), and \(t_{p3} = -0.3\) meV. These values are 265.7, −45.8, −15.7, and 3.8 meV for the BF4 salt. (e) Energy band structure and Fermi surface of the ClO4 salt.
Accordingly, the DMEDO-TTF salts are an excellent playground to investigate the structural transition induced by the ethylenedioxy groups. The structural and physical properties of the MF6 salts depend on the anion volume. Therefore, we expect that these properties of DMEDO-TTF conductors are sensitive to the anion shape as well. In order to clarify the anion shape effect on the structural and physical properties of DMEDO-TTF conductors, new Q1D conductors (DMEDO-TTF)2X (\(X =\text{ClO$_{4}$}\) and BF4) are synthesized. The volume of ClO4− is larger than that of BF4−.9) Fabre et al. have reported the synthesis of DMEDO-TTF salts with these anions; the ClO4 salt has been obtained as a powder and the BF4 salt has been obtained as low-quality needles.10) The electrical conductivity of the BF4 salt has been reported as \(1\times 10^{-3}\) S cm−1 in Ref. 10, but the crystal structure and physical properties have not been clarified. In the present paper, we report the structural, transport, and magnetic properties of these salts. In these salts, the donors form a completely uniform stack, and the anions are disordered at room temperature. A first-order metal–nonmagnetic insulator transition is observed at 190 K (210 K) for the ClO4 (BF4) salt in the cooling process. Low-temperature X-ray diffraction measurements show that the MI transition is due to the anion ordering transition, in which the tetramerization of the donors with a large shift also occurs.
2. ExperimentSingle crystals of (DMEDO-TTF)2X (\(X =\text{ClO$_{4}$}\) and BF4) were grown by electrocrystallization under argon using DMEDO-TTF (0.014 mmol) and the corresponding tetra-n-butylammonium salt (0.020 mmol) in 15 mL of a mixture solution of methanol (MeOH), ethanol (EtOH), and chlorobenzene (PhCl), where the volume ratio of the solution was \(\text{MeOH}:\text{EtOH}:\text{PhCl}= 2 : 2 : 1\). The crystal structures were determined by X-ray single-crystal structure analyses. X-ray oscillation photographs were taken using a RIGAKU R-AXIS RAPID II imaging plate with Cu-\(K\alpha\) radiation from a rotating anode source with a confocal multilayer X-ray mirror (Rigaku VM-Spider, \(\lambda = 1.54187\) Å). For the low-temperature X-ray measurements, the samples were cooled to 105 K by a nitrogen gas-stream cooling method. The structures were solved by the direct method (SIR2008) and refined by the full-matrix least-squares procedure (SHELXL).11,12) Anisotropic thermal parameters were adopted for all non-hydrogen atoms. The energy band structures were calculated on the basis of the extended Hückel method and tight-binding approximation.13) The transfer integrals \(t_{i}\) were estimated from the intermolecular overlap integrals \(S_{i}\) as \(t_{i} = E\times S_{i}\), in which the energy level of the highest occupied molecular orbital (HOMO) E is taken to be −10 eV.
The electrical resistivities were measured by the four-probe method along the b-axis with low-frequency ac current (1.0–5.0 µA). Lock-in amplifiers were used for high-sensitivity detection. High-pressure resistivity measurements were performed using a clamped piston-cylinder cell with Daphne 7373 oil as the pressure-transmitting medium. The room-temperature pressure was determined by measuring the resistance of a Manganin wire.14) Because a pressure of about 1.5 kbar is released between 300 and 50 K, this value is subtracted from room-temperature values.15) The pressure values above 200 K are determined using Fig. 7 in Ref. 15. Electron spin resonance (ESR) spectra were measured using a conventional X-band spectrometer (JEOL JES-TE100). The sweep width of the magnetic field and the g-values were calibrated using the spectra of Mn2+/MgO with a hyperfine structure constant of 86.77 Oe and \(g_{0}\) of 2.00094. The measurements were carried out for a single crystal. The static magnetic susceptibility of the ClO4 salt was measured using a SQUID magnetometer (Quantum Design MPMS-5) under a magnetic field of \(H = 10\) kOe. The measurements were carried out for aligned crystals (2.14 mg) with \(H\parallel b\). The spin susceptibility \(\chi_{\text{spin}}\) was obtained after the subtraction of Pascal's law, \(\chi_{\text{dia}} = -3.33\times 10^{-4}\) emu/mol, where we defined 1 mol as (DMEDO-TTF)2ClO4.
3. ResultsThe crystallographic data of both (DMEDO-TTF)2ClO4 and (DMEDO-TTF)2BF4 at room temperature are shown in Table I. The lattice constants show that the ClO4 and BF4 salts are isostructural. The unit cell volume of the ClO4 salt is larger than that of the BF4 salt. This corresponds to the difference between the anion volumes; \(V_{\text{ClO$_{4}^{-}$}} - V_{\text{BF$_{4}^{-}$}}\sim 8\) Å3 is obtained from the present compounds.9) Figures 1(b)–1(d) show the crystal structure of the ClO4 salt at room temperature. A unit cell contains eight donors and four anions, affording a donor-to-anion ratio of \(2:1\). The donors and anions are located on the twofold screw and twofold axes, respectively, and the anions are disordered on the twofold axis [Figs. 1(b) and 1(d)]. If we choose the noncentrosymmetric space group \(\mathit{Cc}\) for the present salt, the maximum shift/error for the parameter refinement does not converge to zero. The donors form a completely uniform stack along the b-axis in a head-to-tail manner. This is in contrast to the MF6 salts, which form dimerized columns.2) In the donor columns, the slip distance along the molecular long axis and the interplanar distance are 1.03 and 3.55 Å for the ClO4 salt and 1.04 and 3.54 Å for the BF4 salt, respectively. The intrastack transfer integrals are 262.4 and 265.7 meV for the ClO4 and BF4 salts, respectively, and the values are approximately five times as large as those for the interstack direction. The calculated band structure and Fermi surface of the ClO4 salt are shown in Fig. 1(e). The Q1D Fermi surface is shown in an extended zone scheme because the energy bands degenerate at the Z–U–M zone boundary owing to the c glide plane.16)
 »View table | Table I. Crystallographic data of the (DMEDO-TTF)2X salts. |
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The electrical conductivities at room temperature are \(6.6\times 10^{2}\) and \(2.6\times 10^{2}\) S cm−1 for the ClO4 and BF4 salts, respectively. The present conductivity of the BF4 salt is approximately \(10^{5}\) times larger than that reported in Ref. 10. Figure 2 shows the temperature dependence of the electrical resistivity of the ClO4 and BF4 salts at ambient pressure. Both salts exhibit metallic behavior below room temperature and show an MI transition. The resistivity shows clear hysteresis, indicating a first-order phase transition; the MI transition temperature (\(T_{\text{MI}}\)) of the ClO4 (BF4) salt is 190 K (210 K) in the cooling process and 220 K (240 K) in the heating process. The transition temperature of the BF4 salt is 20 K higher than that of the ClO4 salt. We will later show that this MI transition is associated with a marked structural rearrangement. The charge activation energy \(\Delta_{\text{charge}}\) is estimated as 56 and 66 meV for the ClO4 and BF4 salts, respectively, where we assume the resistivity behavior as \(\rho(T)\propto \exp(\Delta_{\text{charge}}/T)\) below \(T_{\text{MI}}\), as shown in the inset of Fig. 2.
Figure 2. (Color online) Temperature dependence of resistivity at ambient pressure. The open and solid symbols are for the cooling and heating processes, respectively. The arrows indicate the thermal processes. The inset shows the Arrhenius plots for the cooling process. Solid lines in the inset are fits to the data.
The temperature dependence of the relative resistance under various pressures in the cooling process is shown in Fig. 3; the several small jumps below 150 K for both salts are due to microcracks. For both salts, the MI transition temperature increases as the pressure increases. Furthermore, both salts show continuous semiconducting behavior below room temperature under high pressures, and the resistance jump associated with the structural phase transition disappears. When the pressure is increased at room temperature (insets in Fig. 3), the resistance increases abruptly at \(P_{\text{c}}\approx 2\) kbar (4 kbar) in the ClO4 (BF4) salts. When the pressure is decreased, the resistance finally returns to the original value with a large hysteresis. The semiconducting behavior below room temperature appears under pressures above \(P_{\text{c}}\); the semiconducting state is induced by the pressure. It appears that the structural phase transition is pushed up to room temperature by the pressure \(P_{\text{c}}\), but it is necessary to determine the high-pressure structure to confirm this hypothesis. The semiconducting behavior is not completely suppressed even under a pressure of ∼11 kbar.
Figure 3. (Color online) Temperature dependence of the relative resistance of (DMEDO-TTF)2ClO4 (a) and (DMEDO-TTF)2BF4 (b) under various pressures. The insets show the pressure dependence of the relative resistance at room temperature. The arrows indicate the pressure cycles. Although a value of 1.5 kbar is subtracted from the room-temperature values in the main panel, the pressure values at \(T_{\text{MI}}\) above 200 K are determined using Fig. 7 in Ref. 15. The horizontal axes of the insets show the room-temperature pressure values.
Figure 4 shows the temperature dependences of the ESR peak-to-peak linewidth and the normalized spin susceptibility under a magnetic field parallel to the b-axis. A single Lorentzian lineshape is observed. The spin susceptibility shows a sharp drop at approximately 150 K without a divergent increase in the linewidth, leading to a spin-singlet state. The difference between the transition temperatures observed in the ESR and transport measurements also indicates that the MI transition is a first-order phase transition. The scattered ESR data below 100 K are due to the contribution of the impurity spins.
Figure 4. Temperature dependences of the ESR linewidth (\(H\parallel b\)) (upper panels) and the normalized spin susceptibility (lower panels) of (DMEDO-TTF)2ClO4 [(a) and (b)] and (DMEDO-TTF)2BF4 [(c) and (d)]. The open and solid symbols are for the cooling and heating processes, respectively. The arrows indicate the thermal processes.
Figure 5 shows the temperature dependence of the static magnetic susceptibility (\(\chi_{\text{spin}}\)) of the ClO4 salt. The magnitude of \(\chi_{\text{spin}}\) is approximately \(3.6\times 10^{-4}\) emu/mol at 300 K, which is the same as that of (DMEDO-TTF)2PF6.2) \(\chi_{\text{spin}}\) slightly decreases with decreasing temperature to 190 K and shows a sharp drop to a spin-singlet state. The increase in \(\chi_{\text{spin}}\) below 50 K is due to the impurity. The spin activation energy \(\Delta_{\text{spin}}\) is estimated from the simple singlet-triplet model given by \(\chi_{\text{spin}}(T)\propto (1/T)\exp(-\Delta_{\text{spin}}/T)\). The obtained spin activation energy of the ClO4 salt is \(\Delta_{\text{spin}} = 40\) meV as shown in the inset of Fig. 5; \(\Delta_{\text{spin}}\) is close to the charge activation energy \(\Delta_{\text{charge}} = 56\) meV. In the heating process, \(\chi_{\text{spin}}\) shows clear hysteresis, indicating the first-order phase transition.
Figure 5. (Color online) Temperature dependence of the static magnetic susceptibility \(\chi_{\text{spin}}\) (\(H\parallel b\)) of (DMEDO-TTF)2ClO4. The open and solid symbols are for the cooling and heating processes, respectively. The arrows indicate the thermal processes. The inset shows a semilog plot of \(\chi_{\text{spin}} T\) versus \(1000/T\). The solid line in the inset is the fit to the data.
In order to clarify the origin of the MI transition, low-temperature X-ray diffraction measurements were performed. Figure 6 shows the X-ray oscillation photographs. Below \(T_{\text{MI}}\), superlattice reflections with the wave vector \(\boldsymbol{{q}}= (0,1/2,0)\) corresponding to \(2k_{\text{F}}\) are observed for both salts, where \(k_{\text{F}}\) is the Fermi wave number; the b-axis is doubled at the MI transition. This means that the anion occupies one of the two sites alternately along the b-axis. The C-centered unit cell changes to a primitive cell with the space group \(P\bar{1}\) as shown in Table I.
Figure 6. X-ray oscillation photographs of (DMEDO-TTF)2ClO4 at 200 K (a) and 185 K (b) and of (DMEDO-TTF)2BF4 at 240 K (c) and 200 K (d).
Figure 7 shows the crystal structure of the ClO4 salt at 105 K. The stacking b-axis is doubled but the interlayer a-axis is halved. Thus, the unit cell volume containing eight donor molecules is approximately the same. However, the direction of the interstack c-axis changes markedly (Fig. 8), so the lattice angles are completely different (Table I). The crystallographically independent molecules are four donors and two anions [Figs. 7(b) and 7(c)], and the donors and anions are located on general positions. The anions are ordered in the insulating phase and alternately oriented along the donor stacking direction [Fig. 7(c)]. In comparison with the high-temperature average position, the Cl atom is alternately shifted to approach the donor column. Donors tetramerize along the stacking direction (b-axis); there are four overlap modes of \(b1\), \(b2\), \(b3\), and \(b4\). The intrastack transfer integrals are \(t_{b1} = 317.8\), \(t_{b2} = 287.8\), \(t_{b3} = 285.1\), and \(t_{b4} = 224.2\) meV; a tetramer is formed by the stacking modes of \(b1\), \(b2\), and \(b3\) [Figs. 7(b) and 7(c)]. Since a tetramer contains two carriers (holes), a spin-singlet state is realized. This indicates that the anion ordering transition of the ClO4 salt is the driving force of the MI transition.
Figure 7. (Color online) (a) Crystal structure projected onto the \(ac\)-plane, (b) donor layer projected along the molecular long axis, and (c) donor and anion columns projected along the molecular short axis of (DMEDO-TTF)2ClO4 at 105 K. In (b), the transfer integrals are \(t_{b1} = 317.8\), \(t_{b2} = 287.8\), \(t_{b3} = 285.1\), \(t_{b4} = 224.2\), \(t_{c1} = -47.5\), \(t_{c2} = -46.5\), \(t_{c3} = -40.5\), \(t_{c4} = -40.4\), \(t_{p1} = 69.6\), \(t_{p2} = 71.1\), \(t_{p3} = 27.2\), \(t_{p4} = 28.8\), \(t_{p5} = 17.7\), and \(t_{p6} = 17.3\) in meV. Rectangles denote tetramers. In (c), arrows close to anions denote the anion shift direction.
Figure 8. (Color online) Donor layer projected along the molecular long axis of (DMEDO-TTF)2ClO4 at 293 K (a) and 105 K (b).
The molecular arrangement markedly changes at the MI transition. Although the long axis of the donor is tilted by 55° from the c-axis in the metallic phase [Fig. 1(b)], this angle is enlarged to 65° in the insulating phase [Fig. 7(a)], and the donors rotate by approximately 10° with respect to the anion layer. In the metallic phase, the ethylenedioxy (EDO) sides of the donor make a zigzag line along the interstack direction [horizontal direction in Fig. 8(a)]. In the insulating phase, this zigzag line changes to a straight line along the c-axis [Fig. 8(b)], indicating the large shift of the donor molecules along the stacking direction. Such a structural transformation is similar to the MM transition in (DMEDO-TTF)2PF6.2)
4. DiscussionThe present compounds have a complete uniform stacking structure because the donor molecules are on the twofold screw axis. This is the same as the case of (o-DMTTF)2Cl, where o-DMTTF is ortho-dimethyltetrathiafulvalene.17) This compound shows charge ordering at 80 K and the spin-Peierls transition is observed at 50 K.18) The mechanism of the metal–insulator transition differs from that of the present compounds because the anions of the Cl salt are ordered even at room temperature.
Anion ordering transitions have been observed in many TMTCF conductors with noncentrosymmetric anions.19,20) The molecular stacking axis is the a-axis for the TMTCF conductors. The anion ordering usually induces an MI transition with the superlattice wave vector \(\boldsymbol{{q}}= (1/2,1/2,1/2)\). For example, (TMTSF)2BF4 shows an MI transition at the anion ordering temperature \(T_{\text{AO}} = 36\) K.21) However, (TMTSF)2ClO4 does not show an MI transition at \(T_{\text{AO}} = 24\) K because the superlattice wave vector \(\boldsymbol{{q}}= (0,1/2,0)\) does not include the molecular stacking direction.22–24) The superlattice wave vector of the present DMEDO-TTF compounds, \(\boldsymbol{{q}}= (0,1/2,0)\), is along the molecular stacking direction, and the anion ordering induces the tetramerization.
The anion ordering transition disappears under external pressure in (TMTSF)2ClO4.24) For (TMTSF)2ReO4, the superlattice wave vector changes from \((1/2,1/2,1/2)\) to \((0,1/2,1/2)\) under high pressure.25) For the present DMEDO-TTF compounds, it is not clarified whether the anion-ordered insulating phase below \(T_{\text{MI}}\) at ambient pressure is the same as the semiconducting state induced by the pressure at room temperature. High-pressure X-ray diffraction may clarify this problem.
The structural phase transition of (DMEDO-TTF)2ClO4 is not simply an ordering of the O atoms in the ClO4 anion because the Cl atoms are shifted. For the Cl(A) atom, the shift along the b-axis, \(\Delta r_{\|}\), is 0.30 Å, and the shift perpendicular to the b-axis, \(\Delta r_{\bot}\), is 0.23 Å. For the Cl(B) atom, these are \(\Delta r_{\|} = 0.31\) and \(\Delta r_{\bot} = 0.24\) Å. For both Cl(A) and Cl(B) atoms, the total shift is approximately 0.38 Å. Although the anion shift of the present ClO4 salt is similar to that of (TMTSF)2ReO4, the total shift is larger than that of the ReO4 salt (∼0.12 Å).26,27) The shift directions of the ClO4 anions indicate that the anions approach the ethylenedioxy side of the donor molecules [Fig. 7(c)]. In the present compound, the donors are also shifted along the molecular stacking direction; the interstack donor arrangement changes from a zigzag to a straight line (Fig. 8). This indicates that the hydrogen-oxygen interaction is related to the structural phase transition.
In the anion-ordered phase of (DMEDO-TTF)2ClO4, the differences between the interplanar distances in a donor column are less than 0.1 Å (\(d_{z,b1} = 3.49\), \(d_{z,b2} = 3.42\), \(d_{z,b3} = 3.42\), and \(d_{z,b4} = 3.47\) Å). However, the transfer integrals show clear tetramerization (\(t_{b1}:t_{b2}:t_{b3}:t_{b4}\approx 1:0.9:0.9:0.7\)). This is due to the slip along the molecular long axis. The slip distance of the \(b4\) mode (\(d_{x,b4} = 1.44\) Å) is the largest among all the modes in the tetramer (\(d_{x,b1} = 0.95\), \(d_{x,b2} = 1.31\), and \(d_{x,b3} = 1.31\) in Å). The absolute value of the transfer integral decreases as the slip distance increases in this slip region (\(d_{x}\leq 1.6\) Å).28) The tetramerization is consistent with the spin-singlet ground state.
The ClO4 anions shift toward the central donor molecules (A and B) in a tetramer and away from the donors C and D [Fig. 7(c)]. This anion displacement induces the charge disproportionation in donors; the hole density of the central donors in a tetramer increases and that of the terminal donors in a tetramer decreases. This indicates that the insulating phase is a \(2k_{\text{F}}\) charge-density-wave state. In order to determine the value of the charge disproportionation in the tetramer, the parameter \(\delta = (b + c) - (a + d)\), which is empirically known to be proportional to the charge on TTF derivatives, is estimated, where a and d (b and c) are the C=C double (C–S single) bond lengths of DMEDO-TTF shown in Fig. 1(a).29) Although the estimated δ is the same for all molecules within the error [\(\delta_{\text{A}} = 0.71(4)\), \(\delta_{\text{B}} = 0.76(4)\), \(\delta_{\text{C}} = 0.74(4)\), and \(\delta_{\text{D}} = 0.77(4)\)], the error is considerably large. This indicates that the difference in charge between the donor molecules is quite small. Optical study including the Raman spectrum is a potential method to elucidate this point.
Figure 9 shows the temperature–pressure phase diagram of (DMEDO-TTF)2X (\(X =\text{ClO$_{4}$}\) and BF4) in the cooling process. Although the MI transition temperature of the BF4 salt is higher than that of the ClO4 salt at ambient pressure, the transition temperature of the ClO4 salt is higher than that of the BF4 salt under external pressure above ∼1 kbar. The anion volume of BF4− is smaller than that of ClO4−. This indicates that the BF4 salt is located at a higher pressure than the ClO4 salt, i.e., so-called the chemical pressure. However, the transition temperature of the ClO4 salt is more sensitive to the external pressure (physical pressure) than that of the BF4 salt. This indicates that the relationship between physical pressure and chemical pressure is not simple for the present compounds, the reason for which is unclear.
Figure 9. (Color online) Phase diagram of (DMEDO-TTF)2X (\(X =\text{ClO$_{4}$}\) and BF4). The closed circles indicate the transition temperatures in the cooling process. The open triangles show the transition pressures at room temperature. The dashed lines are guides to the eye.
5. ConclusionsThe isostructural salts (DMEDO-TTF)2X (\(X =\text{ClO$_{4}$}\) and BF4) have been synthesized, where the donors form a completely uniform stack and the anions are disordered at room temperature. The first-order metal–nonmagnetic insulator transition occurs at 190 and 210 K for the ClO4 and BF4 salts, respectively, in the cooling process. The MI transition temperature increases with increasing pressure. The semiconducting state is induced under pressurization at room temperature. The X-ray oscillation photographs show the superlattice reflections corresponding to \(2k_{\text{F}}\) below \(T_{\text{MI}}\) indicating the anion ordering. In the insulating phase, the donors are tetramerized, and the anion is ordered with a shift toward the central two donors in a tetramer; this indicates that the ground state is a charge-density-wave state.
Acknowledgment
This work was partially supported by JSPS KAKENHI Grant Numbers 24540364 and 16K05436.
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