Organic Superconductivity Enhanced by Asymmetric-Anion Random

anion are reported, where MDT-TS is 5H-2-(1,3-diselenol-2-ylidene)-1,3,4,6-tetrathiapentalene. The X-ray oscillation photograph and the energy dispers...
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Chem. Mater. 2009, 21, 3521–3525 3521 DOI:10.1021/cm901456j

Organic Superconductivity Enhanced by Asymmetric-Anion Random Potential in (MDT-TS)I0.85Br0.41 [MDT-TS = 5H-2-(1,3-diselenole-2ylidene)-1,3,4,6-tetrathiapentalene] Yoshimasa Bando,*,† Tadashi Kawamoto,† Takehiko Mori,*,† Toru Kakiuchi,‡ Hiroshi Sawa,‡,§,# Kazuo Takimiya,^ and Tetsuo Otsubo^ †

Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-ku, Tokyo 152-8552, Japan, ‡Department of Materials Structure Science, The Graduate University for Advanced Studies, Ibaraki 305-0801, Japan, §Institute of Materials Structure Science, High Energy Accelerator Research Organization, Ibaraki 305-0801, Japan, ^ Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima 739-8527, Japan, and #Present address: Department of Applied Physics, Nagoya University Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received May 28, 2009. Revised Manuscript Received June 30, 2009

X-ray investigation and conducting and magnetic properties of a novel organic superconductor composed of an organic donor MDT-TS and an asymmetric I2Br- anion are reported, where MDT-TS is 5H-2-(1,3-diselenol-2-ylidene)-1,3,4,6-tetrathiapentalene. The X-ray oscillation photograph and the energy dispersion spectroscopy (EDS) measurements determine the composition to be (MDTTS)I0.85Br0.41, where the anion composition is approximately I2Br. The donor and the anion construct an incommensurate lattice similar to (MDT-TS)(AuI2)0.441. The magnetic susceptibility of the complex behaves anisotropically below 50 K, indicating an antiferromagnetic order, while a metal-insulator (M-I) transition is observed at a lower temperature, TMI = 30 K. Application of hydrostatic pressure suppresses the M-I transition, and a superconducting phase appears above 9.8 kbar. The midpoint Tc rises to 6.1 K under 13.8 kbar, which is the highest Tc among the MDT-TSF series salts. The asymmetric I-I-Br- anion reduces TMI by considerably influencing the incommensurate antiferromagnetic insulating state but does not destroy the superconductivity because the incommensurate lattice originally has a kind of random potential on the donors. Consequently, this is the first organic superconductor with an asymmetric linear anion. Introduction Many organic superconductors composed of linear halide anions have been reported.1 Examples are (BEDTTTF)2X, (DMET)2X and (MDT-TTF)2X (X, linear halide anion), where BEDT-TTF, DMET and MDT-TTF are bis(ethylenedithio)tetrathiafulvalene, dimethyl(ethylenedithio)diselenadithiafulvalene, and methylenedithiotetrathiafulvalene, respectively. Among these compounds, only the salts with centrosymmetric counteranions, X = I3-, AuI2-, ICl2-, and IBr2-, exhibit superconductivity, whereas the complexes involving an asymemetric anion *Corresponding author. Tel.: þ81-3-5734-2427. Fax: 81-3-5734-2876. E-mail: [email protected].

(1) (a) Ishiguro, T.; Yamaji, K.; Saito, G. Organic Superconductors, 2nd ed.: Springer: Berlin, 1998. (b) Mori, H. J. Phys. Soc. Jpn. 2006, 75, 051003. (c) Williams, J. M.; Ferraro, J. R.; Thorn, R. J.; Carlson, K. D.; Geiser, U.; Wang, H. H.; Kini, A.; Whangbo, M.-H. Organic Superconductors; Prentice-Hall: Upper Saddle River, NJ, 1992. (d) Shibaeva, R. P.; Yagubskii, E. B. Chem. Rev. 2004, 104, 5347. (2) (a) Kobayashi, H.; Kato, R.; Kobayashi, A.; Tokumoto, M.; Anzai, H.; Ishigur, T.; Saito, G. Chem. Lett. 1985, 14, 1293. (b) Emge, T. J.; Wang, H. H.; Beno, M. A.; Leung, P. C. W.; Firestone, M. A.; Jenkins, H. C.; Cook, J. D.; Carlson, K. D.; Williams, J. M.; Venturini, E. L.; Azevedo, L. J.; Schirber, J. E. Inorg. Chem. 1985, 24, 1736. (3) Kikuchi, K.; Honda, Y.; Ishikawa, Y.; Saito, K.; Ikemoto, I.; Murata, K.; Anzai, H.; Ishiguro, T. Solid State Commun. 1988, 66, 405. r 2009 American Chemical Society

such as I-I-Br-, do not show any superconducting transition.2,3 This has been attributed to the random distribution of the asymmetric anion. At low temperatures, the residual resistance of (DMET)2I2Br is higher than (DMET)2I3, indicating that electron scattering is enhanced by the random potential.3 In BEDT-TTF salts, anion disorder effects have been systematically studied by mixing anions, I3-, I2Br-, and IBr2-, where strong suppression of the superconductivity has been observed.4 MDT-TSF and MDT-TS are partially selenium substituted molecules of MDT-TTF (Scheme 1).5 These donors provide isostructural superconductors with noninteger donor and anion ratio represented as (MDTTSF)(AuI2)0.436 and (MDT-TS)(AuI2)0.441. The MDTTSF salt shows superconductivity at ambient pressure (Tc =4.2 K). Although preparation of an organic superconductor using I2Br- has been attempted by using MDT-TSF, the obtained salt has contained a large amount of iodine as represented as (MDT-TSF)I1.19Br0.08 (4) Tokumoto, M.; Anzai, H.; Murata, K.; Kajimura, K.; Ishiguro, T. Jpn. J. Apply. Phys. 1987, 26(Suppl. 26-3), 1977. (5) (a) Takimiya, K.; Kataoka, Y.; Aso, Y.; Otubo, T.; Fukuoka, H.; Yamanaka, S. Angew. Chem., Int. Ed. 2001, 40, 1122. (b) Takimiya, K.; Kodani, M.; Niihara, N.; Aso, Y.; Otsubo, T.; Bando, Y.; Kawamoto, T.; Mori, T. Chem. Mater. 2004, 16, 5120.

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Chem. Mater., Vol. 21, No. 15, 2009 Scheme 1. MDT-TSF Series Molecular Structuresa

a MDT-TTF, methylenedithiotetrathiafulvalene; MDT-TSF, methylenedithiotetraselenafulvalene; and MDT-TS, 5H-2-(1,3-diselenol-2ylidene)-1,3,4,6-tetrathiapentalene.

(Scheme 1).6 We have, however, succeeded in preparing an organic charge-transfer salt with a composition close to I2Br, using a different donor MDT-TS (Scheme 1); the obtained compund is (MDT-TS)I0.85Br0.41. On account of the geometry of the seleniom substitution, this donor does not make any interchain Se-Se contacts in the crystal. Consequently, its salt has a relatively narrow band, and undergoes an M-I transition. Previousely we have investigated the AuI2 salt, (MDT-TS)(AuI2)0.441, and revealed that the insulating state below 50 K is an incommensurate antiferromagnetic insulating state.7 In this compound, a kind of the Mott insulating state is realized by the effectively half-filled band induced by the incommensurate anion potential.8 This salt shows a superconducting transition under hydrostatic pressure (Tc = 4.9 K at 12.7 kbar).5b,7 Here we report (MDTTS)I0.85Br0.41:9 the preparation and the crystal structure, the donor/anion ratio determination from X-ray photograph and EDS measurements, and the transport and magnetic properties. Under pressure, this salt shows a superconducting transition; to the best of our knowledge, this is the first organic superconductor containing an asymmetrical linear anion.1b Experimental Section Preparation of (MDT-TS)I0.85Br0.41. MDT-TS was prepared according to the previous report.5b Single crystals of the MDTTS salt were prepared by electrochemical oxidation of MDT-TS on a platinum electrode in a H-type cell fitted with a glass filter. Tetrabuthylammonium diiodobromide (n-Bu4N)I2Br, recrystallized from ethanol (Tokyo Kasei), was used as the supporting electrolyte. The reaction was performed at room temperature in a mixed solution of chlorobenzene (20 mL) and ethanol (2 mL) containing about 2 mg of MDT-TS and 15 mg of (n-Bu4N)I2Br, and a constant current of 0.5 μA was applied under nitrogen atmosphere for 1 week. Chlorobenzene was distilled after stirred with sulfuric acid overnight and washed with water. Ethanol was distilled from magnesium. X-ray Structural Analysis. Crystal structure was determined by single-crystal X-ray diffraction. The reflection data were measured by ω-2θ scan technique on a Rigaku automated four-circle diffractometer AFC-7R with graphite monochromatized MoKR radiation (λ=0.71069 A˚). All measurements were carried out (6) Takimiya, K.; Kodani, M.; Kataoka, Y.; Aso, Y.; Otsubo, T.; Kawamoto, T.; Mori, T. Chem. Mater. 2003, 15, 3250. (7) (a) Kawamoto, T.; Bando, Y.; Mori, T.; Takimiya, K.; Otsubo, Y. Phys. Rev. B. 2005, 71, 052501. (b) Kawamoto, T.; Bando, Y.; Mori, T.; Konoike, T.; Yamaguchi, T.; Terashima, T.; Uji, S.; Takimiya, K.; Otsubo, T. Phys. Rev. B. 2008, 77, 224506. (8) (a) Yoshioka, H.; Seo, H.; Fukuyama, H. J. Phys. Soc. Jpn. 2005, 74, 1922. (b) Seo, H.; Yoshioka, H.; Otsuka, Y. J. Phys. Conf. Ser 2008, 132, 012018. (9) Mori, T.; Bando, Y.; Kawamoto, T.; Terasaki, I.; Takimiya, K.; Otsubo, T. Phys. Rev. Lett. 2008, 100, 037001.

Bando et al. at room temperature. The crystal structure was solved by the direct method, SHELX-86.10 The structure was refined by the full-matrix least-squares refinement by applying anisotropic temperature factors for all non-hydrogen atoms. The hydrogen atoms were determined from the calculation. All calculations were performed on the crystallographic software package teXsan. Determination of Composition. The X-ray oscillation photograph was taken by using the synchrotron radiation X-ray at BL-1B of the Photon Factory, KEK, Tsukuba. The X-ray wavelength (λ=0.688401(5) A˚) was decided by using the lattice constants of CeO2.11 EDS was recorded on an EDAX PV-9900 at The Center for Advanced Materials Analysis in Tokyo Institute of Technology with an accelerated voltage 16 keV, where the Br KR line, which potentially overlaps to the Se KR line, does not appear. The relative ratio of selenium and iodine was estimated using KR line of selenium (11.22 keV) and LR line of iodine (3.94 keV). The chemical composition was estimated on the basis of the information of X-ray photograph and EDS. Transport Properties. The electrical resistivity of the MDT-TS salt was measured by the four-probe method. By applying an alternative current of 100 μA along the needle direction corresponding to the crystallographic a axis, the resistivity was estimated from the voltage drop measured by a lock-in amplifier. The high-pressure resistivity measurements were carried out by the conventional Be-Cu clamped cell technique, in which we used Daphne 7373 oil as pressure medium. The applied pressure was estimated from the change of the resistance of a manganin wire inserted in the clamped cell.12 Because the pressure was released by ca. 1.5 kbar at liquid helium temperatures, this value was subtracted from the pressure values estimated at room temperature.13 For all measurements, gold wires of φ15 μm were attached to the single crystal with carbon paste. Thermoelectric power was measured by the two-terminal method along the a axis down to 2.0 K. Magnetic Properties. The magnetic susceptibility measurements were performed on a SQUID magnetometer under the magnetic fields (H = 10 kOe) of H//a and H//b, and the amount of the sample was 1.41(3) mg for H//a and 1.06(4) mg for H//b.

Results and Discussion Crystal Structure and Composition of (MDT-TS)I0.85Br0.41. Single-crystal structure analysis reveals that the present MDT-TS salt is isostructural to (MDT-TS)(AuI2)0.441 (Figures 1a and 1b).14 In the crystal, the donor molecules stack uniformly in a head-to-head manner along the a axis (Figure 1a), and the donor columns are arranged in 2-fold periodicity in a zigzag manner along the b axis (Figure 1b). The linear anions are sandwiched between the methylenedithio units and form an infinite chain in the same direction as the donor stacking. The X-ray oscillation photograph along the a axis shows several satellite reflections which are not indexed on the basis of the donor lattice, a=3.9943(4) A˚ (represented by h, Figure 1c), because the anion lattice is (10) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112. (11) National Astronomical Observatiory. Rika Nenpyo (Chronological Scientific Tables 2006); Maruzen Co., Ltd.: Tokyo, 2005. (12) Fujiwara, H.; Kadomatsu, H.; Tohma, K. Rev. Sci. Instrum. 1980, 51, 1345. (13) Murata, K.; Yoshini, H.; Yadav, H. O.; Honda, Y.; Shirakawa, N. Rev. Sci. Instrum. 1997, 68, 2490. (14) Crystallographic data; orthorhombic, Pnma, a = 3.986(3) A˚, b = 12.488(1) A˚, c = 25.251(2) A˚, V = 1256(1) A˚3 and Z = 4.

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Figure 1. Crystal structure of (MDT-TS)I0.85Br0.41, (a) view along the molecular long axis, and (b) view along the a axis. (c) X-ray oscillation photograph. (d) The energy band structure and the Fermi surface.

incommensurate with the donor subcell. Using the observed anion layer lines, the anion lattice is determined to be a0 =3.1671(4) A˚. In (MDT-TS)(AuI2)0.441, the anion layer line coming from the AuI2 length (9.045(6) A˚), which corresponds to 3a0 , has been observed.7b In the present salt, reflections originating from the I2Br- length are not detected. This is quite similar to other MDT-TSF complexes.6 The ratio of the donor and anion lattices (a/a0 =1.2612(2)) determines the composition of the present salt to be (MDT-TS)A1.261 (A=iodine or bromine). Similarly to other mixed halide salts of MDT-TSF, the ratio of the donor and the iodine is estimated by the EDS measurement to be 1:0.85.6 Combined with the above donor/anion ratio, the chemical composition is (MDTTS)I0.85Br0.41. Therefore, the anion is very close to I2Br, similarly to the starting material. It is reasonable to assume the monovalent trihalide anion in the polyhalide chain. Under this assumption, the degree of charge transfer is 0.4204(2). The anion lattice 3a0 =3  3.1671 A˚ of the present salt is larger than 9.045 A˚ of the AuI2 salt, in agreement with the fact that I2Br is longer than AuI2.1c In these incommensurate salts, the difference of the anion length does not change the lattice volume, but changes the donor: anion ratio by keeping the donor spacing approximately constant. Accordingly, it is reasonable that the large I2Br anion leads to smaller degree of charge transfer (0.4204) than the AuI2 salt (0.441). Electronic Band Structure Calculation. To investigate the electronic structure of (MDT-TS)I0.85Br0.41, we have calculated the intermolecular overlap integrals Si between the highest occupied molecular orbitals :: (HOMOs) on the basis of the extended Huckel molecular (15) Mori, T.; Kobayashi, A.; Sasaki, Y.; Kobayashi, H.; Saito, G.; Inokuchi, H. Bull. Chem. Soc. Jpn. 1984, 57, 627.

orbital calculation by using the atomic coordinates and the determined degree of charge transfer.15 The sulfur 3d and selenium 4d atomic orbitals are not included in the calculation similarly to the previous reports of the MDT-TSF series salts.16 The calculated overlap integrals are Sa = -8.15  10-3, Sp1 = -0.70  10-3, and Sp2 = 2.5310-3 (Figure 1a), which are almost the same as those of (MDT-TS)(AuI2)0.441.5b,7 The tight-binding band structure is estimated from the transfer integrals ti estimated from ti = E  Si, where the energy level of HOMO is E = -10 eV. The band structure and Fermi surface of the present complex depicted in Figure 1d are almost the same as the other MDT-TSF series salts (Figure 1d). This compound has a closed Fermi surface, and the electronic structure is two-dimensional. Transport Properties at Ambient Pressure. Figure 2a shows the temperature dependence of the resistivity of (MDT-TS)I0.85Br0.41. The complex shows an M-I transition, taking a resistivity minimum at TF =74 K. A drastic increase of the resistivity is obvious from the peak of the logarithmic derivatives observed at 30 K (inset in Figure 2a), and this temperature is defined as the M-I transition temperature TMI. This temperature is lower than TMI =50 K defined in the same way for the AuI2 salt. Even by comparing the raw temperature dependence of the resistivity, the resistivity increase is obviously shifted to lower temperatures than the AuI2 salt (Figure 2a). Chemical pressure is not likely because lattice volumes are almost the same.5b,7b Using this salt, we have previously reported giant nonlinear conductivity with negative (16) (a) Kawamoto, T.; Mori, T.; Terakura, C.; Terashima, T.; Uji, S.; Takimiya, K.; Aso, Y.; Otsubo, T. Eur. Phys. J. B 2003, 36, 161. (b) Kawamoto, T.; Mori, T.; Terakura, C.; Terashima, T.; Uji, S.; Tajima, H.; Takimiya, K.; Aso, Y.; Otsubo, T. Phys. Rev. B 2003, 67, 20508(R). (c) Kawamoto, T.; Mori, T.; Enomoto, K.; Konoike, T.; Terashima, T.; Uji, S.; Takamori, A.; Takimiya, K.; Otsubo, T. Phys. Rev. B 2006, 73, 024503.

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Figure 2. (a) Temperature dependence of the resistivity of (MDT-TS)I0.85Br0.41, compared with (MDT-TS)(AuI2)0.441.7b Inset is the Arrhenius plot and the logarithmic derivatives. (b) Temperature dependence of the thermoelectric power.

Figure 3. Temperature dependence of the static magnetic susceptibility (circles for H//a and squares for H//b). The solid curve is the onedimensional Heisenberg model (J = -74 K) fitted to the observed data of H//b. The inset shows the low-temperature data.

differential resistivity and spontaneous current oscillation (organic thyristor).9 The temperature dependence of the thermoelectric power is measured along the crystallographic a axis (Figure 2b). The thermoelectric power decreases with decreasing the temperature and show a drastic increase below around 70 K, corresponding to TF. Although the thermoelectric power of the superconductor, (MDTTSF)(AuI2)0.436, decreases linearly toward zero, (MDTTS)I0.85Br0.41 has a finite intercept at T f 0, indicating strong correlation. Below 20 K, the thermoelectric power decreases again. Similar unusual behavior has been observed in (MDT-TS)(AuI2)0.441,7b (DMIT)2I3 (DMIT = 4,5-(ethylenedithio)-4’,50 -dimethyl-1,10 ,3,30 -tetrathiafulvalene)17 and ChTM-TTP salts (ChTM-TTP = 2-[4,5-(1,2cyclohexylenedithio)-1,3-dithio-2-ylidene]-5-[4,5-bis(methylthio)-1,3-dithiol-2-ylidene]-1,3,4,6-tetrathiapentalene).18 Magnetic Properties. The static magnetic susceptibility measured by a SQUID magnetometer (Figure 3) gradually increases with decreasing temperature, though the resistivity decreases in the same temperature region. The increase has been observed in the AuI2 salt and indicates strong electronic correlation.7b Figure 3 shows a curve fitted to the one-dimensional Heisenberg model, where the exchange interaction J = -74 K is slightly (17) Saito, K.; Sato, A.; Kikuchi, K.; Nishikawa, H.; Ikemoto, I.; Sorai, M. J. Phys. Soc. Jpn. 2000, 69, 3602. (18) Kawamoto, T.; Ashizawa, M.; Mori, T.; Yamaura, J.; Kato, R.; Misaki, Y.; Tanaka, K. Bull. Chem. Soc. Jpn. 2002, 75, 435. (19) Estes, W. E.; Gavel, D. P.; Hatfield, W. E.; Hodgson, D. J. Inorg. Chem. 1978, 17, 1415.

Figure 4. (a) Temperature dependence of the resistivity under various pressures. (b) Resistivities under various magnetic fields at 9.8 kbar.

smaller than that of the AuI2 salt (J = -85 K).19 The magnetic susceptibility decreases after making a peak around 75 K and behaves anisotropically below TN = 50 K (Figure 3 inset), meaning an antiferromagnetic transition at this temperature. The easy axis is the crystallographic b axis. The increase at low temperatures is due to the paramagnetic impurities. In the AuI2 salts, TN = 50 K is the same as TMI,5b whereas in the present I2Br salt, TMI = 30 K is lower than TN = 50 K. The difference between TMI and TN may be associated with the random potential coming from the anion disorder. The I-I-Br unit has no inversion symmetry and is randomly oriented in the linear chain, thereby generating random potential, which can potentially cause electron localization. The random potential is also reflected by the larger roomtemperature resistivity of the I2Br salt than that of the AuI2 salt (Figure 2a). The insulating state is an antiferromagnetic state achieved by the incommensurate anion potential, which produces an effectively half-filled state in the partially filled band, consequently leading to a Mott insulating state.8 Because such a Mott insulating state is obscured by the anion random potential in the I2Br salt,

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On the basis of the transport and magnetic measurements, the temperature-pressure phase diagram of (MDT-TS)I0.85Br0.41 is depicted in Figure 5 together with that of (MDT-TS)(AuI2)0.441. TMI disappears above 13.0 kbar, but TF remains, which is expected to disappear above 13.8 kbar. The present salt shows lower TMI, TF, and Pc and higher Tc than those of the AuI2 salt, as results of the anion disorder. Conclusion

Figure 5. Temperature-pressure phase diagram of (MDT-TS)I0.85Br0.41 (closed symbols and solid curves), compared with (MDT-TS)(AuI2)0.441 (open symbols and dashed curves).

TMI is reduced. Random potential usually enhances localization, but in the present case it reduces TMI. Temperature Dependence of the Resistivity under High Pressure. Figure 4a shows the temperature dependence of the resistivity under various hydrostatic pressures. Application of pressure reduces TF and TMI, and under the critical pressure of Pc =9.8 kbar, a considerable drop in the resistivity appears at Tc = 4.2 K (onset), though an increase of the resistivity is still observed below TF. Because the magnetic field suppresses the resistivity drop as shown in Figure 4b, the resistance drop indicates the presence of a superconducting state. This is the first organic superconductor with an asymmetric linear anion, because anion disorder generally destroys superconductivity.1 Because even the ordered incommensurate anions work as a kind of random potential, additional randomness due to the asymmetric anion does not seem to give serious influence on the superconductivity. The midpoint Tc rises to 6.1 K at 13.8 kbar, which is higher than 4.9 K (12.7 kbar) at the maximum of the AuI2 salt, and is even the highest superconducting transition temperature among the MDT-TSF series ambient-pressure organic superconductors.20 In the present pressure region, the increase of the resistivity is not completely suppressed. (20) Mori, T.; Kawamoto, T Annu. Rep. Prog. Chem. 2007, 103, 134.

We have investigated the organic superconductor, (MDT-TS)I0.85Br0.41, where the anion composition close to I2Br is likely to cause the anion disorder. The M-I transition at TMI = 30 K is lower than that of (MDTTS)(AuI2)0.441, and different from TN = 50 K. Under hydrostatic pressure, superconductivity is observed above 9.8 kbar, and Tc reaches 6.1 K at 13.8 kbar, which is the highest Tc among the MDT-TSF series organic superconductors. The asymmetric anion in the incommensurate lattice reduces TMI by obscuring the Mott insulating state, but does not influence Tc. Accordingly, the asymmetric anion assists the high-pressure effect without reducing the density of states, resulting in the relatively high Tc. Since MDT-TS salts are the most strongly correlated materials among the MDT-TSF series, the border to the Mott insulating state realizes the high Tc. Because the role of random potential as investigated in the present work has not been familiar in organic conductors, further experimental and theoretical works are necessary to fully understand the relation to the incommensurate antiferromagnetic insulating state and the superconducting state. Acknowledgment. We are grateful to the members of center for advanced materials analysis in Tokyo Tech. for EDS measurement. This work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal 2004G226). This work is partly supported by a Grant-in-Aid for Scientific Research (B) (17340114). Supporting Information Available: Crystallographic information file (CIF) for (MDT-TS)I0.85Br0.41. This material is available free of charge via the Internet at http://pubs.acs.org.