Synthesis, structure and conducting properties of radical cation salt α-(ET)3(NO3)2

Synthetic Metals 123 (2001) 535±539

Synthesis, structure and conducting properties of radical cation salt a-(ET)3(NO3)2 N.D. Kushcha,b,*, S.V. Konovalikhina, G.V. Shilova, L.I. Buravova, P.P. Kushcha, T.G. Togonidzec, K.V. Vand a

Institute of Problems of Chemical Physics, RAS, 142432 Chernogolovka, Moscow Region, Russia b Walther-Meissner-Institute, D-85748 Garching, Germany c Institute of Solid State Physics, RAS, 142432 Chernogolovka, Moscow Region, Russia d Institute of Experimental Mineralogy, RAS, 142432 Chernogolovka, Moscow Region, Russia Received 1 February 2001; accepted 6 March 2001

Abstract High-quality crystals of a radical cation salt a-(ET)3(NO3)2 were synthesised. Their structure was examined and re®ned. Conducting properties of the salt were studied at different pressures. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Organic conductor based on radical cation salt; Electrocrystallisation; X-ray diffraction; Conductivity

1. Introduction Bis(ethylenedithio)tetrathiafulvalene (ET) is one of the most widely used donors to obtain organic metals and superconductors [1]. A large number (more than 50 [1± 4]) of organic superconductors based on ET were synthesised. Among them there are the superconductors with a record (for the radical cation salts) temperature of the superconducting transition [1±4]. The interest to this donor is still alive, and works on creating new organic conductors are being actively. The creation of new conductor takes place, as a rule, either through synthesis of new donors and salts on their base, or through searching of new counterions to already known donors. We suggest using quaternary onium salts including silicon in their composition, in particular (R4NO)xSi (OCH3)4 x, where R is alkyl or aryl, x ˆ 1 4, as the new counterion in the radical cation ET salts. Depending on x, the anion charge can vary from ( 1) to ( 4). This suggest a different degree of the conduction band ®lling in the ET salts obtained with these anions. The variation in the bond ®lling degree may result in appearance of new interesting features in the structure and properties of the radical cation salts. However, as our investigation shows, the electrochemical oxidation of ET does not lead to preparation of radical cation salts with silicon-containing *

Corresponding author. Fax: ‡7-96-515-3588. E-mail address: [email protected] (N.D. Kushch).

counterions. Instead of the expected silicon-containing complexes of ET we have obtained ones incorporated either NO3 or Cl ions, depending on solvent. In the chlorobenzene medium the crystals of a-(ET)3(NO3)2 grew. The NO3 ions were introduced into the reaction medium together with Ph4POSi(OCH3)3 electrolyte where they were present as an admixture. The substitution of 1,1,2-trichloroethane for chlorobenzene leads to a formation of different composition of ET chlorides. It should be noted that radical cation salt a-(ET)3(NO3)2 was described earlier on [5]. According to Weber et al. [5] a-salt was obtained simultaneously with the other (b and g) phases by electrocrystallisation of ET in tetrahydrofuran. Bu4N(NO3) was used as supporting electrolyte. In spite of the structure of the salt was studied, the question of charge distribution on the ET molecules in the stacks remained unclear. This paper reports on the synthesis high-quality crystals of a radical cation salt a-(ET)3(NO3)2, investigation of its crystal structure and conducting properties at different pressure. 2. Experimental Single crystals of the a-(ET)3(NO3)2 salt were prepared electrochemical oxidation of ET in chlorobenzene with an addition of absolute ethanol (10 vol.%) at 208C under

0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 3 5 6 - 3

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N.D. Kushch et al. / Synthetic Metals 123 (2001) 535±539

galvanostatic conditions (I ˆ 0:3 mA). The crystals grew as long (up to 2.5 mm) black needles in 2±4 weeks. The Ph4POSi(OCH3)3 salt containing about 1% NO3 anion was used as an electrolyte. Ph4POSi(OCH3)3 was obtained as described by Lortz and Schin [6]. Calculated, % C: 68.07, H: 6.09, (P ‡ Si): 12.39; found, % C: 67.85, H: 5.73, (P ‡ Si): 10.98, N: 0.3. An X-ray study was carried out for a single crystal with a length of 0.85 mm. The main crystal data: (C10H8S8)3(NO3)2, M ˆ 1278:0, a ˆ 5:877(6), Ê , b ˆ 102:91(7)8, V ˆ b ˆ 31:130(10), c ˆ 12:842(9) A 3 Ê 2290(1) A , monoclinic, space group P21/a, Z ˆ 2, d calc ˆ 1:85 g/cm3. Re®nement of the unit cell parameters and measurement of 618 re¯ections with intensi®es I  2s…I† was carried out on a four-circle automatic KM4 diffractometer (KUMA DIFFRACTION, Mo-Ka radiaÊ , graphite monochromator, o/2y scantion, l ˆ 0:7111 A ning, (2y) max ˆ 40:088). The crystal structure was determined by direct method by the SHELX-86 programme [7]. Hydrogen atoms were not localised. All non-hydrogen atoms of the structure were re®ned by the full-matrix least squares using an anisotropic approximation by the complex of SHELXL-93 programmes [8]. The ®nal R value is 0.065. The atomic co-ordinates are listed in Table 1, bond lengths and angles are given in Table 2. Table 1 Atom co-ordinates (104) and equivalent isotropic thermal parameters in the crystal a-(ET)3(NO3)2 Atom S(1) S(2) S(3) S(4) C(1) C(2) C(3) C(4) C(5) S(5) S(6) S(7) S(8) S(9) S(10) S(11) S(12) C(6) C(7) C(8) C(9) C(10) C(11) C(12) C(13) C(14) C(15) N(1) O(1) O(2) O(3)

x 1767(7) 2940(6) 1171(7) 4376(8) 250(30) 190(30) 2190(20) 3270(20) 910(30) 2121(7) 2413(7) 3501(7) 1126(7) 4170(7) 1201(8) 505(7) 5130(6) 799(19) 470(30) 40(40) 1880(20) 2860(20) 610(20) 3190(50) 880(30) 3880(30) 1620(40) 3730(20) 2450(16) 2720(20) 5894(17)

y

z

Beq.

4469(1) 4552(1) 3567(1) 3664(1) 4783(4) 4028(5) 4050(4) 3277(5) 3138(4) 5200(1) 5114(1) 4259(1) 4125(1) 5962(1) 6060(1) 3206(1) 3372(1) 4481(4) 4889(4) 5617(4) 5593(5) 3749(4) 3673(4) 6425(5) 6463(5) 2876(5) 2865(6) 2306(3) 2423(3) 2147(4) 2339(4)

5933(3) 4609(3) 6564(4) 4878(3) 5102(10) 5786(16) 5208(10) 5900(11) 6022(15) 906(4) 2348(4) 1581(4) 2867(3) 1861(4) 125(4) 3251(4) 1677(4) 2067(12) 1716(10) 956(12) 1611(13) 2062(12) 2663(10) 1111(15) 670(20) 2450(18) 2890(20) 4292(11) 4922(12) 3427(9) 4660(13)

41(1) 40(1) 51(1) 54(1) 61(5) 64(7) 32(4) 42(5) 66(7) 47(1) 55(2) 50(1) 43(1) 60(2) 63(2) 57(1) 45(1) 33(4) 59(5) 68(6) 50(5) 43(4) 29(4) 131(9) 105(10) 100(8) 194(14) 64(4) 88(5) 76(4) 97(6)

Table 2 Ê ) and angles (8) in the crystal a-(ET)3(NO3)2 Bond distances (A Anion N(1)±O(2) N(1)±O(1) O(2)±N(1)±O(3) O(3)±N(1)±O(1)

1.24(2) 1.28(2) 127.7(10) 115.4(10)

Cation A S(1)±C(1) S(2)±C(1) S(3)±C(5) S(4)±C(3) C(1)±C(1)a1 C(4)±C(5) C(1)±S(1)±C(2) C(5)±S(3)±C(2) C(1)a1±C(1)±S(1) S(1)±C(1)±S(2) C(3)±C(2)±S(3) C(2)±C(3)±S(2) S(2)±C(3)±S(4) C(4)±C(5)±S(3)

1.71(2) 1.72(2) 1.84(1) 1.74(1) 1.42(1) 1.43(2) 91.8(8) 100.9(7) 121(2) 116.3(7) 127.6(12) 114.9(11) 114.2(7) 111.6(10)

Cation B S(5)±C(7) S(6)±C(9) S(7)±C(10) S(8)±C(11) S(9)±C(9) S(10)±C(13) S(11)±C(15) S(12)±C(10) C(6)±C(7) C(10)±C(11) C(14)±C(15) C(7)±S(5)±C(8) C(10)±S(7)±C(6) C(9)±S(9)±C(12) C(15)±S(11)±C(11) C(7)±C(6)±S(7) S(7)±C(6)±S(8) C(6)±C(7)±S(6) C(9)±C(8)±S(5) S(5)±C(8)±S(10) C(8)±C(9)±S(6) C(11)±C(10)±S(7) S(7)±C(10)±S(12) C(10)±C(11)±S(8) C(13)±C(12)±S(9) C(15)±C(14)±S(12)

1.68(2) 1.76(2) 1.71(1) 1.73(1) 1.74(1) 1.78(1) 1.63(2) 1.77(1) 1.38(2) 1.39(2) 1.32(3) 96.2(8) 97.4(6) 107.1(9) 103.6(9) 116.0(11) 112.6(7) 112.8(11) 119.4(12) 112.1(10) 116.0(12) 117.6(10) 115.0(8) 113.7(9) 118(1) 118.9(14)

a

Symmetry codes are: 1

x,

N(1)±O(3) O(2)±N(1)±O(1)

S(1)±C(2) S(2)±C(3) S(3)±C(2) S(4)±C(4) C(2)±C(3) C(1)±S(2)±C(3) C(3)±S(4)±C(4) C(1)a1±C(1)±S(2) C(3)±C(2)±S(1) S(1)±C(2)±S(3) C(2)±C(3)±S(4) C(5)±C(4)±S(4)

S(5)±C(8) S(6)±C(7) S(7)±C(6) S(8)±C(6) S(9)±C(12) S(10)±C(8) S(11)±C(11) S(12)±C(14) C(8)±C(9) C(12)±C(13) C(9)±S(6)±C(7) C(11)±S(8)±C(6) C(13)±S(10)±C(8) C(10)±S(12)±C(14) C(7)±C(6)±S(8) C(6)±C(7)±S(5) S(5)±C(7)±S(6) C(9)±C(8)±S(10) C(8)±C(9)±S(9) S(9)±C(9)±S(6) C(11)±C(10)±S(12) C(10)±C(11)±S(11) S(11)±C(11)±S(8) C(12)±C(13)±S(10) C(14)±C(15)±S(11) y ‡ 1,

1.26(2) 116.6(10)

1.77(1) 1.75(1) 1.83(2) 1.79(1) 1.25(2) 95.2(6) 99.1(6) 123(2) 121.7(10) 110.7(8) 130.9(12) 118.0(11)

1.77(2) 1.85(2) 1.72(1) 1.74(1) 1.76(2) 1.78(1) 1.70(1) 1.89(2) 1.25(2) 1.35(3) 92.9(6) 98.3(6) 99.8(9) 102.4(7) 131.2(10) 132(1) 115.1(8) 128(1) 128.9(14) 115.0(8) 127.3(10) 128.0(10) 118.3(7) 129.7(13) 138(2)

z ‡ 1.

The resistance of a-(ET)3(NO3)2 crystals was measured in the ac conducting plane along the a-axis by a standard dcfour-probe method in the range 1.4±295 K at different pressures. A special cell of the ``piston-cylinder'' type made of beryllium bronze was used to create a quasi-hydrostatic pressure up to 15 kbar. 3. Results and discussion The crystal of a-(ET)3(NO3)2 has a layered structure. The layers of the ET radical cations alternate with the anion

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537

Fig. 1. The structure of the a-(ET)3(NO3)2 radical cation salt projected on to the ab-plane.

layers in the b-axis direction of the unit cell (Fig. 1). In the independent part of the unit cell there is one ET radical cation in the local central symmetric position (cation A) and another in the general position (cation B) and one NO3 anion. In the conducting layers the radical cations form nonequidistant stacks in the c-axis direction of unit cell and ribbons, approximately, in the h2 0 1i direction (Fig. 2). Radical cations in the stacks are shifted to each other so that the stacks axis forms an angle of 308 with the normal to the middle plane of radical cations. Overlapping of the ET molecules in the stacks is shown in Fig. 3a. There are not short contacts in the stacks. The speci®c feature of the stacks structure is different shifting of the radical cations in them in the direction of the long cation axis. Three radical cations (A, B and B0 ) lie almost strictly over each other (Fig. 3b). The next three radical cations (A, B and (B0 )) are shifted

relative to the ®rst three ones approximately at a length of the central C=C bond. It is noted that the distances between the middle planes of the B radical cations (B


B0 ) belonging to the neighbouring ``triads'' within one stacks are similar Ê . The distances between the middle and equal to 3.71 A planes of radical cations within one ``triad'' differ signi®Ê (A


B) and 4.22 A Ê (A


B0 ). cantly: 3.31 A The angle between the middle planes of the A and B radical cations is 1.78. The A cation has 8 S


S contacts shorter than the sum of van-der-Waals radii of sulphur atoms Ê [9]) and one S


O short contact less than the sum (1.85 A Ê of van-der-Waals radii of sulphur and oxygen ones (1.40 A [9]). The A radical cation has short contacts only with the ones from the same ribbon. The B radical cation is connected with the cations from two different ribbons by the short contacts. As a result, it has 11 (S


S) and 2 (S


O) short

Fig. 2. The view of the packing of the ET radical cations in the a-(ET)3(NO3)2 radical cation salt.

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N.D. Kushch et al. / Synthetic Metals 123 (2001) 535±539

Fig. 4. The temperature dependence of resistance for several crystals of a(ET)3(NO3)2 along the a-axis.

Fig. 3. Overlapping (a) and the alternation (b) of ET radical cations in the stacks.

contacts. The distribution of the bond lengths and bond angles in the ET radical cations allow to suggest that A is a radical cation with a charge of (‡1), and B is a radical cation with a formal charge of (‡0.5). It is primarily con®rmed by the elongation of the central C=C bond in the A cation up to Ê . According to the data of [10,11] the ET radical 1.42(1) A cations with a charge of (‡1) have such a length of the central C=C bond. Thus, every ``triad'' of the cations has a charge of (‡2). As was mentioned before, the structure of the salt a-(ET)3(NO3)2 was also studied by Weber et al [5]. However, the central C=C bonds in the A and B radical Ê and 1.332(15) A Ê , respeccations found in [5] (1.358(12) A tively) did not allow to say de®nitely about the charge of ET molecules in the conducting radical cation layers. Based on the composition of the salt and symmetry considerations, the authors [5] assumed that the radical cation layers are formed of two stacks consisting of the ET cation with a charge of (‡1) and one stack construct from the neutral ET molecules. This supposition, as the authors [5] themselves stress, contradicts to the conducting properties of the crystals a(ET)3(NO3)2. Our conclusion of the charge distribution on the ET molecules is in good agreement with the conductive properties of the salt.

The anion layers are formed of practically ¯at NO3 anions. The N±O distances and bond angles in the anion have the values closed to usual (Table 2). The crystal conductivity measured along the a-axis at room temperature is 200±660 O 1 cm 1. It is in good agreement with the data of [5]. The temperature dependence of resistance for several single crystals is shown in Fig. 4. In the range 27±295 K all crystals show metal behaviour. Below 27 K there is a sharp rise of the resistance (for same crystals by a factor up to 3000) with a saturation reached at about 20 K. At the liquid helium temperature the conductivity is still high, of the order of 5±20 O 1 cm 1. It should be noted that at temperature decreasing to 27 K (the temperature of the metal±insulator transition) resistance of the major part of the crystals decrease with by a factor of 10±50. However, some crystals demonstrate an resistance decreasing by a factor of approximately 100. In this case, in the range 27±45 K the resistance of such crystals decreases in

Fig. 5. The temperature dependence of resistance for the single crystal of a-(ET)3(NO3)2 along the a-axis at different pressure.

N.D. Kushch et al. / Synthetic Metals 123 (2001) 535±539

8±10 times and conductivity reaches 60 000 O 1 cm 1. The reason of this acceleration in the resistance decreasing is unclear. As our investigation shows, the minimum of the resistance of these crystals does not shift under magnetic ®eld of 10 T. There is no superconducting ¯uctuation in the salt a-(ET)3(NO3)2. Thus, to elucidate the reason of the observed phenomena, future investigations are needed. The temperature dependence of resistance for one single crystal at different pressures is shown in Fig. 5. Under pressure 5 kbar the metal±insulator transition in partially suppressed. It is expressed in a less signi®cant resistance rise. Nevertheless, no signi®cant shift of the resistance minimum to the low-temperature region is observed. A further pressure increasing to 10 kbar completely suppressed the metal±insulator transition. 4. Conclusion The high-quality crystals of the radical cation salt a-(ET)3(NO3)2 were synthesised and their structure was veri®ed. Conducting properties of the salt were studied under different pressures. It is shown that the ET molecules in the stacks have a charge of (‡0.5) and (‡1), which is in good agreement with the conducting properties. The salt is a metal to 27 K. In the range 20±27 K there is a sharp (by a factor up to 3000) rise of the resistivity with a saturation reached. The pressure of 10 kbar completely suppressed the metal±insulator transition.

539

Acknowledgements N.D. Kushch is grateful to DFG Grant 436Rus 17/52/99 for the supporting of this work. The work was also supported by the RFBR (Project no. 99-02-16119). References [1] J.M. Williams, J.R. Ferraro, R.I. Thorn, K.D. Carlson, U. Geiser, H.H. Wang, A.M. Kini, M.-H. Whangbo, Organic Superconductors, Prentice-Hall, Englewood Cliffs, NJ, 1992. [2] H. Yamochi, T. Nakamura, T. Komatsu, N. Matsukawa, T. Inoue, G. Saito, Solid State Commun. 79 (1991) 101. [3] N.D. Kushch, L.I. Buravov, A.G. Khomenko, E.B. Yagubskii, L.P. Rozenberg, R.P. Shibaeva, Synth. Met. 53 (1993) 155. [4] J. Schlueter, K.D. Carlson, U. Geiser, H.H. Wang, J.M. Williams, W.K. Kwok, J.A. Fendrich, U. Welp, P.M. Keane, J.D. Dudek, A.S. Komosa, D. Naumann, T. Roy, J.E. Schurber, W.R. Bayless, B. Dobrill, Phys. C 233 (1994) 379. [5] A. Weber, H. Endres, H.J. Keller, E. Gogu, I. Heinen, K. Bender, D. Schweitzer, Z. Naturforsch 40b (1985) 1658. [6] W. Lortz, G. Schin, J. Chem. Soc., Dalton Trans. (1987) 623. [7] M. Sheldrick, SHELX 86, Program for crystal structure determination, University of Gottingen, Germany, 1986. [8] M. Sheldrick, SHELXL 93, Program for the Refinement of Crystal Structures, University of Gottingen, Germany, 1993. [9] L. Pauling, The Nature of the Chemical Bond, University Press, Cornell, 1960. [10] J.M. Williams, H.H. Wang, T.J. Emge, U. Geiser, M.A. Beno, P.C.W. Leung, K.D. Karlson, R.J. Thorn, A.J. Schultz, Prog. Inorg. Chem. 35 (1987) 54. [11] V.E. Korotkov, N.D. Kushch, M.K. Makova, R.P. Shibaeva, E.B. Yagubskii, Izv. Acad. Sci. USSR Ser. Chim. (1988) 1687.