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Field-orientation dependence of magnetoresistance of a double-column organic conductor, (DMET)2ClO4
ELSEVIER
Synthetic
Field-orientation
Metals
82 (1996)
83-88
dependence of magnetoresistance of a double-column organic conductor, (DMET) &lo4
Harukazu Yoshino ‘,‘, Kazuya Saito b7*, Koichi Kikuchi a, Hiroyuki Nishikawa ‘, Keiji Kobayashi ‘, Isao Ikemoto a of
a Department Chemistry, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan b Microcalorimetry Research Center, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan ’ Department of Chemistry, College of Aris and Sciences, The University of Tokyo, Meguro, Tokyo 153, Japan
Received 11 March 1996; revised 26 April 1996; accepted 21 May 1996
Abstract An organic conductor, (DMET) ,ClO, (DMET = dimethyl( ethylenedithio) diselenadithiafulvalene), has two types of DMET columns along the CI- and b-axes of the crystal lattice, respectively. The salt undergoes a metal-to-semiconductor transition at 32 K. Its resistivity and thermopower both along the a- and b-axes show anomalies at 18 and 7 K in addition to that at 32 K. Magnetoresistance along the c*-axis was measured while rotating the magnetic field within the ab plane above and below temperatures of the anomalies. The identification of the column that is mainly relatedto eachanomalyis tried using the change in the peak position of the field-orientation dependence of the magnetoresistance. The resistivity along the @-axis and thermopower along the c-axis show anomalies at 70 and 110K in additionto those at low temperatures. Keywords:
Metal-insulator
transition;
Dimethyl(ethylenedithio)diselenadithiafulvalene;
1. Introduction DMET (dimethyl(ethylenedithio)diselenadithiafulvalene) is the first unsymmetrical organic donor that gives an organic superconductor [ 1,2]. Conducting DMET salts are 2: 1 radical saltswith monovalent inorganic anionssuch asI,- or AuBr, - [ 341. Two types of crystal structureshave been found for the superconducting DMET salts, i.e., onedimensional (1D) columnar structure and two-dimensional (2D) K-type structure [3-6]. Another type of crystal structure has been found for conducting DMET salts, (DMET),BF,
and (DIHET)2C104
[3-81,
which
are not
superconducting. The crystal structure of (DMET),BF, is shown in Fig. 1 [7]. (DMET)J104 is isostructural to the BF, salt [ 81. The salts have two types of donor columns along the a- and b-axes of the crystal lattice, respectively. This type of crystal structure will be called ‘double-column’ structure in this paper. The crystal structure suggeststhat the interaction between columns is small. The structure can be * Corresponding author. Fax: -I- 81 6 850 @chem.sci.osaka-u.ac.jp. ’ Present address: ElecUotechnical Laboratory, Japan. 0379-6779/96/$15,00 PIISO379-6779(96)03770-a
0 1996 Elsevier
5526;
e-mail:
Tsukuba,
Ibaraki
kazuya 305,
Science S.A. All rights reserved
Magnetoresistance
understoodasthe modification of the 1D columnar structure. It is interesting to consider whether electrical properties of the double-column saltscan bedescribedasthe superposition of two quasi-1D salts whose column directions are almost perpendicularto each other. The DMET molecules in the two types of columns are crystallographically inequivalent and, consequently, considered as being in different oxidation states [S-7]. In other words, if both columnsgive two independentquasi-1Dbands in the double-column salts, both bandsare not l/Cfilled as seenin typical 2:l saltsbut have different band-filling. The authors have estimated [9] the band-filling and bandwidth of (DMET) ,BF, assumingsuch independent quasi-1D columns, both from Raman spectroscopy and anisotropiesof thermopower and plasma frequency. A band structure of (DMET)&fO, is expected to be similar to that of (DMET) *BF4, becausenot only the crystal structure but also the physical properties are very similar. It has been known for the BF, salt since early times that its resistivity is metallic below room temperatureand the salt undergoesa metal-to-insulator (or semiconductor) (M-I) transition around 35 K [4]. The recent measurementsof resistivity and thermopower along the a- and b-axes of (DMET) 2BF4 and ( DMET)2C104 have, however, revealed
84
H. Yoshino et al. /Synthetic
Fig. I. Crystal structure of (DMET)IBF, viewed along the (a) a- and (b) b-axes, respectively [7]. (DMET),C104 is iso$uchml to $DhKET),Bc,. Crystal datafor (DMET)2C101: spacegroupPl, a = 7.041A, b = 7.839 A, c=27.515 A, cu=88.67”, p=90.49’, y= 105.79”andZ=2 [S].
three anomalies, probably corresponding to changes in electronic states [ lo], The temperatures of the anomalies are 32, 19 and 7 K for the BF4 salt and 32, 18 and 7 K for the C104 salt. The first anomaly at 32 K (TM-,) is the M-I transition for both salts. The step-like increase in the resistivity suggests that other bandgaps also open at 19 ( 18) and 7 K. The absence of any significant anomaly in the spin susceptibility, Xspin, and linewidth of EPR, AH, around 32 K [ 11-131 implies that the M-I transition at 32 K is caused by strong correlation between conduction electrons [ lo]. The behavior Of Xspinand AH suggests that the change in electronic state at 19 (or 18) K (T,,) has antiferromagnetic nature. The nature of the change at 7 K ( Tc2) is not clear at present. As for DMET and its analogs, five isostructural doublecolumn salts with tetrahedral anions have been reported [ 14181. The double-column system has commonly shown interesting properties hardly seen in other organic conductors. The characteristics
are, for example, the coexistence of inequi-
valent quasi-1D donor columns in different oxidation states (or inequivalent conduction columns composed of one kind of molecule) in one material, the occurrence of several changes in the electronic state at ambient pressure only by varying temperature, and having almost the same M-I transition temperature of about 30 K in spite of difference in
Metals
82 (1996) 83-88
donors and/or anions [ 10,17,18]. Investigation of these points probably gives further knowledge of the low-dimensional physics. Among the questions about the properties of the doublecolumn salts, one of the most primitive and important is whether both types of columns are affected at the temperatures of the three anomalies in the resistivity and thermopower simultaneously or separately. When one measures the resistivity or thermopower along the u-axis, the electrical or heat current runs both through the u-column and between the bcolumns, though the former path probably gives a larger contribution to the conduction. The resistivity and thermopower measured along the a-axis, therefore, reflect the electronic states of both a- and b-columns. This is the reason that it is difficult to observe the influence of the changes such as the M-I transition on each column separately. Recently, Tajima et al. [ 191 have identified the column affected by the phase transition in OL-(EDT-TI’F) 2Ni (dmit) 2 (TTF = tetrathiafulvalene, dmit = 1,3-dithiol-2-thione-45 dithiolate) , which has EDT-TTF and Ni( dmit), columns and shows a transition due to a formation of a kind of density wave only in one column. They have measured the magnetoresistance perpendicular to both EDT-m and Ni( dmit) 2 columns by rotating the magnetic field in the plane that contains both columns. In this study, the magnetoresistance along the c*-axis (A p,*( 0) ) was measured for (DMET) &lo4 at low temperatures while rotating the magnetic field of 7.0 T in the ab plane. The change in Ap,*( S) with varying temperature is used to identify the column affected at the temperatures of the three anomalies in the resistivity and thermopower. During the measurement of pc* (7’) , anomalous behaviors have been found at higher temperatures. Thermopower along the c-axis (S,) was also measured to obtain some information on the anomalies.
2. Experimental
Sample crystals were obtained by the usual electrochemical oxidation method described elsewhere [ l] . The samples were single crystals of black rhombic plate with typical size of 0.10 X 0.15 X 0.02 mm. The orientation of the crystal to the magnetic field was checked by an optical measurement. For the four-probe measurement of resistivity, two pairs of annealed gold wires (10 km in diameter) were contacted with gold paste on the surfaces of the sample crystal corresponding to the ab plane, on which gold had been deposited.
The magnetoresistance was measured by applying the magnetic field of 7.0 T in the ab plane. A block-like single crystal, whose thickness along the caxis was 0.28 mm, was used for the measurement of the thermopower along the c-axis. The method is described in previous work [ 20,211.
H. Yoshblo et al. /Synthetic
MetaZs 82 (1996) 83-88
85
3.2. Field-orientation dependenceof the magnetoresistance
-0
100
200
300
T/K Fig. 2. Resistivity of (DMET),ClO., normalized at 300 K. Open circles, closed circles and open squares represent the data measured along the a-, band P-axes, respectively. The inset shows the resistivity along the c*-axis in an enlarged scale.
3. Results and discussion
3. I. Temperature dependenceof resistivity
The field-orientation dependenceof the magnetoresistance along the c*-axis under the field of 7.0 T rotated in the ab plane hasbeen measuredat 40,25, 12 and 4.2 K. The result is shown in Fig. 4. All the data in Fig. 4 have a period of 180” reflecting the inversion symmetry of the crystal structure. The extraction of the information about the changesin the electronic statesof the a- and b-columns from the changesin Ap,*( 6) is basedon two assumptions. First Ap,*( 0) is supposedto be divided into the elements arising from the a- and b-columns.When one measuresmagnetoresistanceperpendicular to the conducting plane of a quasi-1D metal by rotating the magnetic field in the plane, it is usual that the magnetoresistancebecomesmaximum for the field perpendicular to the highest conducting direction. Each ‘highest conducting direction’ for each donor column correspondsto its column direction in the double-column salts. Thus, it is reasonableto express Ap,*( 0) under the magnetic field rotated in the ab plane by Ap,*( 19)=A sin2(B- 0,) +B sin2(8- 19,)+C
The temperature dependenceof pc* of (DMET)&104 is shownin Fig. 2. The resistivities along the a- and b-axes (pa and pb) presentedtogether were measuredby the Montgomery method 122,231 for another samplecrystal. The roomtemperature conductivities along the a-, b- and c*-axes are about 30, 30 and 5 X lo-’ S cm-‘, respectively. The results for paand pbagreewell with thosepreviously reported [ IO]. Below 300 K pa and pb are metallic and showrapid increase below 30 K due to the M-I transition. On the other hand, pc* shows non-metallic behavior even around room temperature. It can beclearly seenin the inset in Fig. 2. With decreasing temperaturepC*turns to decreasearound 140 K and has a local minimum at about 70 K. Below 70 K pC* increases again with decreasing temperature. The increase in pC* is more rapid below about 30 K. Although the dominant conduction mechanism along the c*-axis is unclear, the rapid increasein pC*below 30 K suggeststhe temperaturedependenceof pC*reflects that of pnand pb, andtherefore the change in the electronic statesof the columns along the a- and b-axes at least below TMsI of 32 K. To show that one can obtain the information about the changein the electronic statesof the a- and b-columnsfrom p+, the numerical derivative of the Arrhenius plot of pC*is presentedin Fig. 3. Peakscorrespondingto the M-I transition and antiferromagnetic changeareclearly seenat about30 and 18 K (arrows a and b), respectively, though an anomaly corresponding to the third change around 7 K is very slight (arrow c) . Besidesthe peaks, one can find the minimum at 100 K (arrow d) anda shoulder-like anomaly at 70 K (arrow e) . No anomalieshave been pointed out around thesetemperaturesin previous studies.This point will be discussedin a later section,
(1)
where 0, ( = 74”) and 19,( = 0”) represent the directions of the a- and b-axes when 0 = O“, andA, B and.C are constants. The secondassumptionis that the field direction that gives the maximum magnetoresistancechangeslittle when a bandgap opens, though the absolute value of the magnetoresistante becomesmuch larger than that in the metallic state. These points have been confirmed by measuringp,*( 0) for a quasi-ID organic conductor, ( DMET)2A~12 [ 241, that undergoesan SDW transition at 16 K [21,2.5]. This may be curious, since a possibleimperfect nestingof a pair of Fermi surfacesin a ‘quasi-1D’ metal gives a closed Fermi pocket that hasquasi-2D nature and the principal axis of the magnetoresistancecanbe turned. The carriersin the Fermi pocket, however, govern the electrical conduction only at very low temperaturesandthe resistivity usually showsactivation-type behavior below the M-I transition temperature [ 261. Thus, the small changein the principal axis of pC*( 0) of the quasi-
Fig. 3. Derivative of the Arrhenius plot of the resistivity along the c*-axis of (DMET)&104, d(log&p,*)/d(llT). Arrows show the anomalies discussed in the text.
H. Yoshino et al. /Synthetic Metals 82 (1996) 83-88
86
ID conductors suggests that the magnitude of the bandgap opened upon the M-I transition is smaller than that at the boundary of the Brillouin zone. This implication is valid in most casesbecause the bandgap, resulting fromthetransition, is of the order of the transition temperature. For a comparison among relative magnitudes, Appc*is normalized by pc* without magnetic field. Then the experimental result in Fig. 4 was fitted to Ap,Jpc*(O
T) =a sin2( 19- 19,)+b sin2( d- 6,) fc
II Jd
// b
II a / * I (a)40 K :
I
(d) 4.2 K _
I / , 120 180 240 300 360 0 i degree Fig. 4. Field-orientation dependence of the c*-axis resistivity of (DMET),ClO, for the magnetic field of 7.0 Tat (a) 40, (b) 25, (c) 12 and (d) 4.2 K. Data are plotted by open circles. Solid curves are the results of fits using Eq. (2). Broken and dotted curves represent the contribution from the a- and b-columns, respectively. 0
60
T(K)
103a
lo36
103c
40 25 12 4.2
0.761 0.357 3.20 5.15
0.886 4.51 11.7 0.739
1.51 20.3 28.7 48.5
(2)
where a, b and c are fitted parameters for the least-squares calculation. The first and second terms of the right-hand side represent the angle-dependent contribution of the a- and b-. columns, respectively. Solid curves in Fig. 4 represent the 11b
Table 1 Fitted parameters, a, b and c of Eq. (2) for the field-orientation dependence of magnetoresistance of (DMET)&lO,, at 40,25, 12 and 4.2 K
results of fits. Dashed and dotted curves correspond to the contribution from the u-columns (the sum of the first and third terms) and b-columns (the sum of the second and third terms), respectively. The values of the fitted parameters are summarized in Table 1. The result at 40 K is shown in Fig. 4 (a), The enhancement of the magnetoresistance has the maximum around 120” (300”)) though its magnitude is less than 1% of p,*(O T) even at this angle. The contributions from the a- and b-columns are comparable with each other as shown by the dashed and dotted curves. This is reasonable because (DMET) 2C104 is in the metallic state at 40 K and the conductivities along both a- and b-axesare almost the same. Fig. 4(b) shows the data at 25 K, below the TMaI=32 K and above Tcl= 18 K. Data points in Fig. 4(b) and (c) are scattered because of small fluctuation in temperature, as the resistivity rapidly increases around these temperatures with decreasing temperature ( Id(log pc*) /dT] =5.3 and 17% at 25 and 12 K, respectively). One can, however, find that the position of the maximum of the magnetoresistance shifts from 120’ at 40 K to about 90” at 25 K. This is caused by the drastic increase in the contribution of the b-column. This result shows that the M-I transition at 32 K is mainly related to the b-column. Between 12 and 25 K, Ap,*( 0) does not change its maximum (or minimum) position as seen in Fig. 4(c). The contribution to Ap,*( 0) of the b-column is also dominant at 12 K and its normalized value becomes more than twice of that at 25 K. The u-column, however, gives a relatively larger contribution to Ap,*( S) at 12 K than at 25 K. This implies that the anomaly at 18 K is related to the a-column. Below 18 K the spin susceptibility rapidly vanishes and the linewidth of the EPR spectra is broadened with decreasing temperature, though no significant anomaly can be seen at 32 K, where the M-I transition occurs [ 1 l-131. These observations show that the change at 18 K affects both a- and b-columns, because the spin degree of freedom disappears in both columns at 18 K. The last result shown in Fig. 4(d) was obtained at 4.2 K, below the third anomaly at Tc2= 7 K. Since the sample crystal was immersed in liquid helium at 4.2 K, the data in Fig. 4(d) are less scattered than those in Fig. 4(a)-(c) . The peak position of Apc*( 0) becomes 160” at 4.2 K from about 90” at higher temperature. As shown in the figure the contribution from the a-column is much larger than that from the b-column at 4.2 K, whereas the former is always smaller than the latter
H. Yoshino ef al. /Synthetic Metals 82 (1996) 83-88 Table 2 Changes in the electronic state in (DMET),ClO,
and column(s) relatedto
thechanges T(K)
Related column(s)
Note
32 i = TM-,) 18 (=Tc,) 7 (=Tc,)
b aandb a
antiferromagnetic
Mot&f&e
81
or another M-I transition in such low-dimensional systems must be discussed,taking also the effects of dimensionality into account. To investigate the influence of the changein the bandwidth and/or the dimensionality, further studiessuchas themeasurementof the transportpropertiesunderhydrostatic pressureare necessary.
unclear
at and above 12 K. The remarkable changein the electronic state in the u-column is, therefore, consideredto occur at the third anomaly at 7 K. The characteristic temperaturesand the related columnsare summarizedin Table 2. 3.3. Relation to the other double-column salts It is consideredthat a similar situation must be found for the changes in the electronic state of isostructural (DMET) $F4 becauseits transport properties are very similar to thoseof (DMET)aC104 between4.2 and300 K. Thus, the M-I transitions at 32 K observed for both DMET salts are related to the b-column and the nature of the M-I transition is Mott transition-like due to the rather strong electron correlation and the narrow bandwidth. This implies that the bandwidth of each salt is narrower in the b-column than in the a-column. For (DMET) 2BF4, the bandwidth and band-filling of the a- and b-bands have been determined experimentally [9]. According to Ref. [93, the bandwidth is 0.72 and 0.88 eV and the band-filling 0.58 and 0.42 holes/molecule for the aand b-bands,respectively. Namely, the a-band is ‘narrower’ than the b-band. The tight-binding calculation, however, gives the opposite result for (DMET)J3F4 [ 271 that the aband is wider than the b-band reflecting the packing of the DhGT moleculesin columns.The tight-binding result seems consistent with the fact that the M-I transition occursin the b-column at a higher temperaturethan the temperaturewhere the other anomaliesconcerning the a-column are seen. On the other hand, it has been previously pointed out for another double-column salt, (DIMET),BF, (DIMET = dimethyl (ethylenedithio) tetrathiafulvalene), that the M-I transition mainly related to the b-column occurs first with decreasingtemperatureat 30 K IlO]. Thus, the occurrence of the M-I transition in the b-column at a higher temperature seems common for the double-column salts, though the nature of the M-I transition in the DIMET salt is not clear at present. One should, however, consider the ‘effective bandwidth’ when discussing the effect of the bandwidth on the M-I transitions becausethe effective bandwidth can be almost half of the bandwidth expected from the average transfer integral among the moleculeswithin a column if the dimerization of the donor moleculesis strong and a mid-gap opens around the middle of the band. Indeed, the tight-binding calculation gives a rather large mid-gap of the a-band for (DMET) ZBFd [ 27 1. Furthermore, the tendency to the Mott
3.4. Anomaliesat IO and 110 K in (DMET),ClO, As seenin the inset in Fig. 2, pC* is non-metallic below 300 K and gradually increaseswith decreasingtemperature down to 140 K, though the transport properties along the aand b-axesaremetallic in this temperatureregion. Below 140 K pC* turns to decreaseand has a local ‘minimum at 70 K. While the salt still remains in the metallic state above TM-, = 32 K, pC* rapidly increaseswith decreasingtemperature below 70 K. Namely, pc* shows ‘metallic’ temperature dependencebetween70andllO K. The derivative of the Arrhenius plot of pC*, d(log p,*) / d( l/T), shows anomaliesat 70 and 100 K as indicated by the arrows d and e in Fig. 3. The shoulder-shapeanomaly at 70 K correspondsto the local minimum of PC*. The minimum of d(log p,*) /d( l/T) around 100 K corresponds to the relatively large slopeof pC*. Thermopower along the c-axis (S,) is measured for (DMET),ClO, to see possibleanomaliesat 70 and 110 K. The result is shown in Fig. 5. The thermopowersalong the aand b-axes are alsopresentedwith S, in the inset of Fig. 5 for a wider region. The value of S, at 300 K is 6.3 p,V K-t. Although the positive sign of S, is consistent with the hole conduction revealed by S, and S,, S, decreaseswith decreasing temperature, passesthrough zero at about 210 K and changesits sign from positive to negative. Changeof sign in S, occurs again at about 60 K after having a local minimum around 150 K. Although the reasonof the changesin its sign above TM-, is not clear, the non-linear temperature dependencebelow 300 K seemsto be consistentwith the non-metallic temperaturedependenceof p+ The changein S, should be observedat the three tempera@res( TMsI,T,, and r,,) . The
I 0
I
I
100
200
I
300
T/K Fig. 5. Thermopower of (DMET),CIO, along the c-axis. The linear solid line is drawn to emphasize changes in slope at 70 and 100 K. The inset also shows thermopower along the a- andb-axes of (DMET),ClO,together with
thec-axisfor comparison.
88
H. Yoshino et al, /Synthetic
data are, however, scattereddue to the large resistanceof the sample.Thus, it is rather difficult to find the changesin S, at TM-, and T,,, while the change in S, is very large below T,, of7 K. Corresponding well to the anomalies in d(log p,*) I d( l/T), S, also seemsto change its slope at 70 and 100 K. A close inspection of S, and S, in the metallic region also reveals small anomalies.As seenin the inset in Fig. 5, the slopesof S, and S, between 70 and 100 K are steeperthan thoseabove 100 K and below 70 K. Furthermore, the slopes above 100 K and below 70 K are different from each other both for S, and S,. Thus, the electronic statesare considered to be slightly different from each other above 100 K and below 70 K. Consequently, the electronic statejust above 32 K will be different from that aroundroom temperature. 4. Conclusions The variation of the dependenceof the magnetoresistance on the orientation of the magnetic field within the ab plane of the double-columnorganic conductor (DMET) &lo, was studied with varying temperature.The experimental results show that the M-I transition at 32 K, the antiferromagnetic change at 18 K and a third anomaly at 7 K are related to the b-, the a- and b-, and the a-column( s) ?respectively. Although the transport properties along the a- and b-axesever studied show anomaliesat each transition’ temperature,the present study succeededin identifying the relation to each column. Two additional anomalieshave been found in d(log p,*) I d( 1/T) and S, at 70 and 100 K in the temperatureregion where the transport properties of (DMET),ClO, along the a- and b-axes are metallic. Acknowledgements This work was supported in part by a Grant-in-Aid for Specially Promoted Research (No. 63060004) from the Ministry of Education, Scienceand Culture.
References [l] K. Kikuchi, I. Jkemoto and K. Kobayashi, Synf/l. Met., 19 (1987) 551.
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[2] K. Kikuchi, M. Kikuchi, T. Namiki, K. Saito, I. Ikemoto, K. Murata, T. Ishiguro and K. Kobayashi, Clzem.Left., (1987) 931. [3] K. Murata, K. Kikuchi, T. Takahashi, K. Kobayashi, Y. Honda, K. Saito, K. Kanoda, T. Tokiwa, H. Anzai, T. Ishiguro and I. Ikemoto, J. Mol. Electron., 4 (1988) 173. [4] K. Kikuchi, K. Saito, I. Ikemoto, K. Murata, T. Ishiguro and K. Kobayashi, Synth. Met., 27 (1988) B269. [S] K. Kikuchi, Y. Ishikawa, K. Saito, I. Ikemoto and K. Kobayashi, Synth. Met., 27 (1988) B391, [6] I. Ikemoto, K. Kikuchi, K. Saito, K. Murata and K. Kobayashi, Mol. ctyt. Liq. crysr., 181 (1990) 185. [7] Y. Ishikawa, K. Salto, K. Kikuchi, K. Kobayashi and I. Ikemoto, &II. Chem. Sot. Jpn., 64 (1991) 212. [8] T.G. Takhiiov, O.N. Karasochka,O.A.D’Yachenko,L.O. Atovmyan, M.L. Petrov, I.K. Pubtsovaand R.N. Lyubovskaya, Zh. Strukr. Khim., 30 (1989) 114. [9] K. Saito, H. Yoshino, K. Nerishi, K. Kikuchi, I. Ikemoto and K. Kobayashi, Synth. Met., 55-57 (1993) 1756. [lo] H. Yoshino, K. Saito, K. Kikuchi, I. Ikemoto and K. Kobayashi, Synth. Met., 72 (1995) 141. [ 1I] K. Kanoda, T. Takahashi, K. Kikuchi, K. Saito, I. Ikemoto and K. Kobayashl, Synth. Met., 27 (1988) B385. [ 121 K. Kanoda, T. Takahashi, T. Tokiwa, K. Kikuchi, K. Saito, I. Ikemoto and K. Kobayashi, Phys. Rev. B, 38 (1988) 39. [ 131 K. Kanoda and T. Takahashi, personal communication. [14] H. Saltoh, H. Itoh, K. Saito, K. Kikuchi and I. Ikemoto, Acta Crystallogr., Sect. C, 51 (1995) 1656. [ 151 H. Endres, R. Heid, H.J. Keller, I. Heinen and D. Schweitzer, Acta Crystallogr., Sect. C, 43 (1987) 115. [16] K. Bender, H. Endres, S. GXrtner, E. Gogu, R. Heid, I. Heinen, H.J. Keller, A. Kraatz and D. Schweitzer, Synth. Met., 19 (1987) 559. [17] T.G. Takhirov, O.N. Krasochka, O.A. D’Yachenko, L.O. Atovmyan, M.Z. Aldoshina, L.M. Gol’Denberg, R.N. Lyubovskaya, V.A. Merzhanov and R.B. Lyubovskik, Sov.J. Chem. Phys., 7 (1990) 209. [ 181 R. Kato, S. Aonuma, Y. Okano, H. Sawa, A. Kobayashi and H. Kobayashi, Synth. Met., 55-57 (1993) 2084. [19] H. Tajima, S. Ikeda, A. Kobayashi, H. Kuroda, R. Kato and H. Kobayashi, Solid State Cornman., 86 (1993) 7. [20] K. Saito, H. Kamio, Y. Honda, K. Kikuchi, K. Kobayashi and I. Ikemoto, J. Phys. Sot. Jpn., 58 (1989) 4093. [21] K. Saito, H. Yoshino, K. Kikuchi, K. Kobayashi and I. Ikemoto, J. Phys. Sot. Jpn., 62 (1993) 1001. [22] H.C. Montgomery, J. Appl. Phys., 42 (1971) 2971. [23] B.F. Logan, SO. Rice and R.F. Wick, J. Appl. Phys., 42 (1971) 2975. [24] H. Yoshino, K. Saito, H. Nishikawa, K. Kikuchi, K. Kobayashi and I. Ikemoto, in preparation. [25] Y. Nogarni, M.Tanaka, S. Kagoshima, K. Kikuchi, K. Saito, I. Ikemoto and K. Kobayashi, J. Phys. Sot. Jpn., 56 (1987) 3783. [26] Y. Honda, K. Murata, K. Kiuchi, K. Saito, I. Ikemoto and K. Kobayashi, SolidState Commrm., 71 (1989) 1087. [27] Y. Ishikawa, M.Sc. Thesis, Department of Chemistry, Tokyo Metropolitan University, 1989.
Synthetic
Field-orientation
Metals
82 (1996)
83-88
dependence of magnetoresistance of a double-column organic conductor, (DMET) &lo4
Harukazu Yoshino ‘,‘, Kazuya Saito b7*, Koichi Kikuchi a, Hiroyuki Nishikawa ‘, Keiji Kobayashi ‘, Isao Ikemoto a of
a Department Chemistry, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan b Microcalorimetry Research Center, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan ’ Department of Chemistry, College of Aris and Sciences, The University of Tokyo, Meguro, Tokyo 153, Japan
Received 11 March 1996; revised 26 April 1996; accepted 21 May 1996
Abstract An organic conductor, (DMET) ,ClO, (DMET = dimethyl( ethylenedithio) diselenadithiafulvalene), has two types of DMET columns along the CI- and b-axes of the crystal lattice, respectively. The salt undergoes a metal-to-semiconductor transition at 32 K. Its resistivity and thermopower both along the a- and b-axes show anomalies at 18 and 7 K in addition to that at 32 K. Magnetoresistance along the c*-axis was measured while rotating the magnetic field within the ab plane above and below temperatures of the anomalies. The identification of the column that is mainly relatedto eachanomalyis tried using the change in the peak position of the field-orientation dependence of the magnetoresistance. The resistivity along the @-axis and thermopower along the c-axis show anomalies at 70 and 110K in additionto those at low temperatures. Keywords:
Metal-insulator
transition;
Dimethyl(ethylenedithio)diselenadithiafulvalene;
1. Introduction DMET (dimethyl(ethylenedithio)diselenadithiafulvalene) is the first unsymmetrical organic donor that gives an organic superconductor [ 1,2]. Conducting DMET salts are 2: 1 radical saltswith monovalent inorganic anionssuch asI,- or AuBr, - [ 341. Two types of crystal structureshave been found for the superconducting DMET salts, i.e., onedimensional (1D) columnar structure and two-dimensional (2D) K-type structure [3-6]. Another type of crystal structure has been found for conducting DMET salts, (DMET),BF,
and (DIHET)2C104
[3-81,
which
are not
superconducting. The crystal structure of (DMET),BF, is shown in Fig. 1 [7]. (DMET)J104 is isostructural to the BF, salt [ 81. The salts have two types of donor columns along the a- and b-axes of the crystal lattice, respectively. This type of crystal structure will be called ‘double-column’ structure in this paper. The crystal structure suggeststhat the interaction between columns is small. The structure can be * Corresponding author. Fax: -I- 81 6 850 @chem.sci.osaka-u.ac.jp. ’ Present address: ElecUotechnical Laboratory, Japan. 0379-6779/96/$15,00 PIISO379-6779(96)03770-a
0 1996 Elsevier
5526;
e-mail:
Tsukuba,
Ibaraki
kazuya 305,
Science S.A. All rights reserved
Magnetoresistance
understoodasthe modification of the 1D columnar structure. It is interesting to consider whether electrical properties of the double-column saltscan bedescribedasthe superposition of two quasi-1D salts whose column directions are almost perpendicularto each other. The DMET molecules in the two types of columns are crystallographically inequivalent and, consequently, considered as being in different oxidation states [S-7]. In other words, if both columnsgive two independentquasi-1Dbands in the double-column salts, both bandsare not l/Cfilled as seenin typical 2:l saltsbut have different band-filling. The authors have estimated [9] the band-filling and bandwidth of (DMET) ,BF, assumingsuch independent quasi-1D columns, both from Raman spectroscopy and anisotropiesof thermopower and plasma frequency. A band structure of (DMET)&fO, is expected to be similar to that of (DMET) *BF4, becausenot only the crystal structure but also the physical properties are very similar. It has been known for the BF, salt since early times that its resistivity is metallic below room temperatureand the salt undergoesa metal-to-insulator (or semiconductor) (M-I) transition around 35 K [4]. The recent measurementsof resistivity and thermopower along the a- and b-axes of (DMET) 2BF4 and ( DMET)2C104 have, however, revealed
84
H. Yoshino et al. /Synthetic
Fig. I. Crystal structure of (DMET)IBF, viewed along the (a) a- and (b) b-axes, respectively [7]. (DMET),C104 is iso$uchml to $DhKET),Bc,. Crystal datafor (DMET)2C101: spacegroupPl, a = 7.041A, b = 7.839 A, c=27.515 A, cu=88.67”, p=90.49’, y= 105.79”andZ=2 [S].
three anomalies, probably corresponding to changes in electronic states [ lo], The temperatures of the anomalies are 32, 19 and 7 K for the BF4 salt and 32, 18 and 7 K for the C104 salt. The first anomaly at 32 K (TM-,) is the M-I transition for both salts. The step-like increase in the resistivity suggests that other bandgaps also open at 19 ( 18) and 7 K. The absence of any significant anomaly in the spin susceptibility, Xspin, and linewidth of EPR, AH, around 32 K [ 11-131 implies that the M-I transition at 32 K is caused by strong correlation between conduction electrons [ lo]. The behavior Of Xspinand AH suggests that the change in electronic state at 19 (or 18) K (T,,) has antiferromagnetic nature. The nature of the change at 7 K ( Tc2) is not clear at present. As for DMET and its analogs, five isostructural doublecolumn salts with tetrahedral anions have been reported [ 14181. The double-column system has commonly shown interesting properties hardly seen in other organic conductors. The characteristics
are, for example, the coexistence of inequi-
valent quasi-1D donor columns in different oxidation states (or inequivalent conduction columns composed of one kind of molecule) in one material, the occurrence of several changes in the electronic state at ambient pressure only by varying temperature, and having almost the same M-I transition temperature of about 30 K in spite of difference in
Metals
82 (1996) 83-88
donors and/or anions [ 10,17,18]. Investigation of these points probably gives further knowledge of the low-dimensional physics. Among the questions about the properties of the doublecolumn salts, one of the most primitive and important is whether both types of columns are affected at the temperatures of the three anomalies in the resistivity and thermopower simultaneously or separately. When one measures the resistivity or thermopower along the u-axis, the electrical or heat current runs both through the u-column and between the bcolumns, though the former path probably gives a larger contribution to the conduction. The resistivity and thermopower measured along the a-axis, therefore, reflect the electronic states of both a- and b-columns. This is the reason that it is difficult to observe the influence of the changes such as the M-I transition on each column separately. Recently, Tajima et al. [ 191 have identified the column affected by the phase transition in OL-(EDT-TI’F) 2Ni (dmit) 2 (TTF = tetrathiafulvalene, dmit = 1,3-dithiol-2-thione-45 dithiolate) , which has EDT-TTF and Ni( dmit), columns and shows a transition due to a formation of a kind of density wave only in one column. They have measured the magnetoresistance perpendicular to both EDT-m and Ni( dmit) 2 columns by rotating the magnetic field in the plane that contains both columns. In this study, the magnetoresistance along the c*-axis (A p,*( 0) ) was measured for (DMET) &lo4 at low temperatures while rotating the magnetic field of 7.0 T in the ab plane. The change in Ap,*( S) with varying temperature is used to identify the column affected at the temperatures of the three anomalies in the resistivity and thermopower. During the measurement of pc* (7’) , anomalous behaviors have been found at higher temperatures. Thermopower along the c-axis (S,) was also measured to obtain some information on the anomalies.
2. Experimental
Sample crystals were obtained by the usual electrochemical oxidation method described elsewhere [ l] . The samples were single crystals of black rhombic plate with typical size of 0.10 X 0.15 X 0.02 mm. The orientation of the crystal to the magnetic field was checked by an optical measurement. For the four-probe measurement of resistivity, two pairs of annealed gold wires (10 km in diameter) were contacted with gold paste on the surfaces of the sample crystal corresponding to the ab plane, on which gold had been deposited.
The magnetoresistance was measured by applying the magnetic field of 7.0 T in the ab plane. A block-like single crystal, whose thickness along the caxis was 0.28 mm, was used for the measurement of the thermopower along the c-axis. The method is described in previous work [ 20,211.
H. Yoshblo et al. /Synthetic
MetaZs 82 (1996) 83-88
85
3.2. Field-orientation dependenceof the magnetoresistance
-0
100
200
300
T/K Fig. 2. Resistivity of (DMET),ClO., normalized at 300 K. Open circles, closed circles and open squares represent the data measured along the a-, band P-axes, respectively. The inset shows the resistivity along the c*-axis in an enlarged scale.
3. Results and discussion
3. I. Temperature dependenceof resistivity
The field-orientation dependenceof the magnetoresistance along the c*-axis under the field of 7.0 T rotated in the ab plane hasbeen measuredat 40,25, 12 and 4.2 K. The result is shown in Fig. 4. All the data in Fig. 4 have a period of 180” reflecting the inversion symmetry of the crystal structure. The extraction of the information about the changesin the electronic statesof the a- and b-columns from the changesin Ap,*( 6) is basedon two assumptions. First Ap,*( 0) is supposedto be divided into the elements arising from the a- and b-columns.When one measuresmagnetoresistanceperpendicular to the conducting plane of a quasi-1D metal by rotating the magnetic field in the plane, it is usual that the magnetoresistancebecomesmaximum for the field perpendicular to the highest conducting direction. Each ‘highest conducting direction’ for each donor column correspondsto its column direction in the double-column salts. Thus, it is reasonableto express Ap,*( 0) under the magnetic field rotated in the ab plane by Ap,*( 19)=A sin2(B- 0,) +B sin2(8- 19,)+C
The temperature dependenceof pc* of (DMET)&104 is shownin Fig. 2. The resistivities along the a- and b-axes (pa and pb) presentedtogether were measuredby the Montgomery method 122,231 for another samplecrystal. The roomtemperature conductivities along the a-, b- and c*-axes are about 30, 30 and 5 X lo-’ S cm-‘, respectively. The results for paand pbagreewell with thosepreviously reported [ IO]. Below 300 K pa and pb are metallic and showrapid increase below 30 K due to the M-I transition. On the other hand, pc* shows non-metallic behavior even around room temperature. It can beclearly seenin the inset in Fig. 2. With decreasing temperaturepC*turns to decreasearound 140 K and has a local minimum at about 70 K. Below 70 K pC* increases again with decreasing temperature. The increase in pC* is more rapid below about 30 K. Although the dominant conduction mechanism along the c*-axis is unclear, the rapid increasein pC*below 30 K suggeststhe temperaturedependenceof pC*reflects that of pnand pb, andtherefore the change in the electronic statesof the columns along the a- and b-axes at least below TMsI of 32 K. To show that one can obtain the information about the changein the electronic statesof the a- and b-columnsfrom p+, the numerical derivative of the Arrhenius plot of pC*is presentedin Fig. 3. Peakscorrespondingto the M-I transition and antiferromagnetic changeareclearly seenat about30 and 18 K (arrows a and b), respectively, though an anomaly corresponding to the third change around 7 K is very slight (arrow c) . Besidesthe peaks, one can find the minimum at 100 K (arrow d) anda shoulder-like anomaly at 70 K (arrow e) . No anomalieshave been pointed out around thesetemperaturesin previous studies.This point will be discussedin a later section,
(1)
where 0, ( = 74”) and 19,( = 0”) represent the directions of the a- and b-axes when 0 = O“, andA, B and.C are constants. The secondassumptionis that the field direction that gives the maximum magnetoresistancechangeslittle when a bandgap opens, though the absolute value of the magnetoresistante becomesmuch larger than that in the metallic state. These points have been confirmed by measuringp,*( 0) for a quasi-ID organic conductor, ( DMET)2A~12 [ 241, that undergoesan SDW transition at 16 K [21,2.5]. This may be curious, since a possibleimperfect nestingof a pair of Fermi surfacesin a ‘quasi-1D’ metal gives a closed Fermi pocket that hasquasi-2D nature and the principal axis of the magnetoresistancecanbe turned. The carriersin the Fermi pocket, however, govern the electrical conduction only at very low temperaturesandthe resistivity usually showsactivation-type behavior below the M-I transition temperature [ 261. Thus, the small changein the principal axis of pC*( 0) of the quasi-
Fig. 3. Derivative of the Arrhenius plot of the resistivity along the c*-axis of (DMET)&104, d(log&p,*)/d(llT). Arrows show the anomalies discussed in the text.
H. Yoshino et al. /Synthetic Metals 82 (1996) 83-88
86
ID conductors suggests that the magnitude of the bandgap opened upon the M-I transition is smaller than that at the boundary of the Brillouin zone. This implication is valid in most casesbecause the bandgap, resulting fromthetransition, is of the order of the transition temperature. For a comparison among relative magnitudes, Appc*is normalized by pc* without magnetic field. Then the experimental result in Fig. 4 was fitted to Ap,Jpc*(O
T) =a sin2( 19- 19,)+b sin2( d- 6,) fc
II Jd
// b
II a / * I (a)40 K :
I
(d) 4.2 K _
I / , 120 180 240 300 360 0 i degree Fig. 4. Field-orientation dependence of the c*-axis resistivity of (DMET),ClO, for the magnetic field of 7.0 Tat (a) 40, (b) 25, (c) 12 and (d) 4.2 K. Data are plotted by open circles. Solid curves are the results of fits using Eq. (2). Broken and dotted curves represent the contribution from the a- and b-columns, respectively. 0
60
T(K)
103a
lo36
103c
40 25 12 4.2
0.761 0.357 3.20 5.15
0.886 4.51 11.7 0.739
1.51 20.3 28.7 48.5
(2)
where a, b and c are fitted parameters for the least-squares calculation. The first and second terms of the right-hand side represent the angle-dependent contribution of the a- and b-. columns, respectively. Solid curves in Fig. 4 represent the 11b
Table 1 Fitted parameters, a, b and c of Eq. (2) for the field-orientation dependence of magnetoresistance of (DMET)&lO,, at 40,25, 12 and 4.2 K
results of fits. Dashed and dotted curves correspond to the contribution from the u-columns (the sum of the first and third terms) and b-columns (the sum of the second and third terms), respectively. The values of the fitted parameters are summarized in Table 1. The result at 40 K is shown in Fig. 4 (a), The enhancement of the magnetoresistance has the maximum around 120” (300”)) though its magnitude is less than 1% of p,*(O T) even at this angle. The contributions from the a- and b-columns are comparable with each other as shown by the dashed and dotted curves. This is reasonable because (DMET) 2C104 is in the metallic state at 40 K and the conductivities along both a- and b-axesare almost the same. Fig. 4(b) shows the data at 25 K, below the TMaI=32 K and above Tcl= 18 K. Data points in Fig. 4(b) and (c) are scattered because of small fluctuation in temperature, as the resistivity rapidly increases around these temperatures with decreasing temperature ( Id(log pc*) /dT] =5.3 and 17% at 25 and 12 K, respectively). One can, however, find that the position of the maximum of the magnetoresistance shifts from 120’ at 40 K to about 90” at 25 K. This is caused by the drastic increase in the contribution of the b-column. This result shows that the M-I transition at 32 K is mainly related to the b-column. Between 12 and 25 K, Ap,*( 0) does not change its maximum (or minimum) position as seen in Fig. 4(c). The contribution to Ap,*( 0) of the b-column is also dominant at 12 K and its normalized value becomes more than twice of that at 25 K. The u-column, however, gives a relatively larger contribution to Ap,*( S) at 12 K than at 25 K. This implies that the anomaly at 18 K is related to the a-column. Below 18 K the spin susceptibility rapidly vanishes and the linewidth of the EPR spectra is broadened with decreasing temperature, though no significant anomaly can be seen at 32 K, where the M-I transition occurs [ 1 l-131. These observations show that the change at 18 K affects both a- and b-columns, because the spin degree of freedom disappears in both columns at 18 K. The last result shown in Fig. 4(d) was obtained at 4.2 K, below the third anomaly at Tc2= 7 K. Since the sample crystal was immersed in liquid helium at 4.2 K, the data in Fig. 4(d) are less scattered than those in Fig. 4(a)-(c) . The peak position of Apc*( 0) becomes 160” at 4.2 K from about 90” at higher temperature. As shown in the figure the contribution from the a-column is much larger than that from the b-column at 4.2 K, whereas the former is always smaller than the latter
H. Yoshino ef al. /Synthetic Metals 82 (1996) 83-88 Table 2 Changes in the electronic state in (DMET),ClO,
and column(s) relatedto
thechanges T(K)
Related column(s)
Note
32 i = TM-,) 18 (=Tc,) 7 (=Tc,)
b aandb a
antiferromagnetic
Mot&f&e
81
or another M-I transition in such low-dimensional systems must be discussed,taking also the effects of dimensionality into account. To investigate the influence of the changein the bandwidth and/or the dimensionality, further studiessuchas themeasurementof the transportpropertiesunderhydrostatic pressureare necessary.
unclear
at and above 12 K. The remarkable changein the electronic state in the u-column is, therefore, consideredto occur at the third anomaly at 7 K. The characteristic temperaturesand the related columnsare summarizedin Table 2. 3.3. Relation to the other double-column salts It is consideredthat a similar situation must be found for the changes in the electronic state of isostructural (DMET) $F4 becauseits transport properties are very similar to thoseof (DMET)aC104 between4.2 and300 K. Thus, the M-I transitions at 32 K observed for both DMET salts are related to the b-column and the nature of the M-I transition is Mott transition-like due to the rather strong electron correlation and the narrow bandwidth. This implies that the bandwidth of each salt is narrower in the b-column than in the a-column. For (DMET) 2BF4, the bandwidth and band-filling of the a- and b-bands have been determined experimentally [9]. According to Ref. [93, the bandwidth is 0.72 and 0.88 eV and the band-filling 0.58 and 0.42 holes/molecule for the aand b-bands,respectively. Namely, the a-band is ‘narrower’ than the b-band. The tight-binding calculation, however, gives the opposite result for (DMET)J3F4 [ 271 that the aband is wider than the b-band reflecting the packing of the DhGT moleculesin columns.The tight-binding result seems consistent with the fact that the M-I transition occursin the b-column at a higher temperaturethan the temperaturewhere the other anomaliesconcerning the a-column are seen. On the other hand, it has been previously pointed out for another double-column salt, (DIMET),BF, (DIMET = dimethyl (ethylenedithio) tetrathiafulvalene), that the M-I transition mainly related to the b-column occurs first with decreasingtemperatureat 30 K IlO]. Thus, the occurrence of the M-I transition in the b-column at a higher temperature seems common for the double-column salts, though the nature of the M-I transition in the DIMET salt is not clear at present. One should, however, consider the ‘effective bandwidth’ when discussing the effect of the bandwidth on the M-I transitions becausethe effective bandwidth can be almost half of the bandwidth expected from the average transfer integral among the moleculeswithin a column if the dimerization of the donor moleculesis strong and a mid-gap opens around the middle of the band. Indeed, the tight-binding calculation gives a rather large mid-gap of the a-band for (DMET) ZBFd [ 27 1. Furthermore, the tendency to the Mott
3.4. Anomaliesat IO and 110 K in (DMET),ClO, As seenin the inset in Fig. 2, pC* is non-metallic below 300 K and gradually increaseswith decreasingtemperature down to 140 K, though the transport properties along the aand b-axesaremetallic in this temperatureregion. Below 140 K pC* turns to decreaseand has a local ‘minimum at 70 K. While the salt still remains in the metallic state above TM-, = 32 K, pC* rapidly increaseswith decreasingtemperature below 70 K. Namely, pc* shows ‘metallic’ temperature dependencebetween70andllO K. The derivative of the Arrhenius plot of pC*, d(log p,*) / d( l/T), shows anomaliesat 70 and 100 K as indicated by the arrows d and e in Fig. 3. The shoulder-shapeanomaly at 70 K correspondsto the local minimum of PC*. The minimum of d(log p,*) /d( l/T) around 100 K corresponds to the relatively large slopeof pC*. Thermopower along the c-axis (S,) is measured for (DMET),ClO, to see possibleanomaliesat 70 and 110 K. The result is shown in Fig. 5. The thermopowersalong the aand b-axes are alsopresentedwith S, in the inset of Fig. 5 for a wider region. The value of S, at 300 K is 6.3 p,V K-t. Although the positive sign of S, is consistent with the hole conduction revealed by S, and S,, S, decreaseswith decreasing temperature, passesthrough zero at about 210 K and changesits sign from positive to negative. Changeof sign in S, occurs again at about 60 K after having a local minimum around 150 K. Although the reasonof the changesin its sign above TM-, is not clear, the non-linear temperature dependencebelow 300 K seemsto be consistentwith the non-metallic temperaturedependenceof p+ The changein S, should be observedat the three tempera@res( TMsI,T,, and r,,) . The
I 0
I
I
100
200
I
300
T/K Fig. 5. Thermopower of (DMET),CIO, along the c-axis. The linear solid line is drawn to emphasize changes in slope at 70 and 100 K. The inset also shows thermopower along the a- andb-axes of (DMET),ClO,together with
thec-axisfor comparison.
88
H. Yoshino et al, /Synthetic
data are, however, scattereddue to the large resistanceof the sample.Thus, it is rather difficult to find the changesin S, at TM-, and T,,, while the change in S, is very large below T,, of7 K. Corresponding well to the anomalies in d(log p,*) I d( l/T), S, also seemsto change its slope at 70 and 100 K. A close inspection of S, and S, in the metallic region also reveals small anomalies.As seenin the inset in Fig. 5, the slopesof S, and S, between 70 and 100 K are steeperthan thoseabove 100 K and below 70 K. Furthermore, the slopes above 100 K and below 70 K are different from each other both for S, and S,. Thus, the electronic statesare considered to be slightly different from each other above 100 K and below 70 K. Consequently, the electronic statejust above 32 K will be different from that aroundroom temperature. 4. Conclusions The variation of the dependenceof the magnetoresistance on the orientation of the magnetic field within the ab plane of the double-columnorganic conductor (DMET) &lo, was studied with varying temperature.The experimental results show that the M-I transition at 32 K, the antiferromagnetic change at 18 K and a third anomaly at 7 K are related to the b-, the a- and b-, and the a-column( s) ?respectively. Although the transport properties along the a- and b-axesever studied show anomaliesat each transition’ temperature,the present study succeededin identifying the relation to each column. Two additional anomalieshave been found in d(log p,*) I d( 1/T) and S, at 70 and 100 K in the temperatureregion where the transport properties of (DMET),ClO, along the a- and b-axes are metallic. Acknowledgements This work was supported in part by a Grant-in-Aid for Specially Promoted Research (No. 63060004) from the Ministry of Education, Scienceand Culture.
References [l] K. Kikuchi, I. Jkemoto and K. Kobayashi, Synf/l. Met., 19 (1987) 551.
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[2] K. Kikuchi, M. Kikuchi, T. Namiki, K. Saito, I. Ikemoto, K. Murata, T. Ishiguro and K. Kobayashi, Clzem.Left., (1987) 931. [3] K. Murata, K. Kikuchi, T. Takahashi, K. Kobayashi, Y. Honda, K. Saito, K. Kanoda, T. Tokiwa, H. Anzai, T. Ishiguro and I. Ikemoto, J. Mol. Electron., 4 (1988) 173. [4] K. Kikuchi, K. Saito, I. Ikemoto, K. Murata, T. Ishiguro and K. Kobayashi, Synth. Met., 27 (1988) B269. [S] K. Kikuchi, Y. Ishikawa, K. Saito, I. Ikemoto and K. Kobayashi, Synth. Met., 27 (1988) B391, [6] I. Ikemoto, K. Kikuchi, K. Saito, K. Murata and K. Kobayashi, Mol. ctyt. Liq. crysr., 181 (1990) 185. [7] Y. Ishikawa, K. Salto, K. Kikuchi, K. Kobayashi and I. Ikemoto, &II. Chem. Sot. Jpn., 64 (1991) 212. [8] T.G. Takhiiov, O.N. Karasochka,O.A.D’Yachenko,L.O. Atovmyan, M.L. Petrov, I.K. Pubtsovaand R.N. Lyubovskaya, Zh. Strukr. Khim., 30 (1989) 114. [9] K. Saito, H. Yoshino, K. Nerishi, K. Kikuchi, I. Ikemoto and K. Kobayashi, Synth. Met., 55-57 (1993) 1756. [lo] H. Yoshino, K. Saito, K. Kikuchi, I. Ikemoto and K. Kobayashi, Synth. Met., 72 (1995) 141. [ 1I] K. Kanoda, T. Takahashi, K. Kikuchi, K. Saito, I. Ikemoto and K. Kobayashl, Synth. Met., 27 (1988) B385. [ 121 K. Kanoda, T. Takahashi, T. Tokiwa, K. Kikuchi, K. Saito, I. Ikemoto and K. Kobayashi, Phys. Rev. B, 38 (1988) 39. [ 131 K. Kanoda and T. Takahashi, personal communication. [14] H. Saltoh, H. Itoh, K. Saito, K. Kikuchi and I. Ikemoto, Acta Crystallogr., Sect. C, 51 (1995) 1656. [ 151 H. Endres, R. Heid, H.J. Keller, I. Heinen and D. Schweitzer, Acta Crystallogr., Sect. C, 43 (1987) 115. [16] K. Bender, H. Endres, S. GXrtner, E. Gogu, R. Heid, I. Heinen, H.J. Keller, A. Kraatz and D. Schweitzer, Synth. Met., 19 (1987) 559. [17] T.G. Takhirov, O.N. Krasochka, O.A. D’Yachenko, L.O. Atovmyan, M.Z. Aldoshina, L.M. Gol’Denberg, R.N. Lyubovskaya, V.A. Merzhanov and R.B. Lyubovskik, Sov.J. Chem. Phys., 7 (1990) 209. [ 181 R. Kato, S. Aonuma, Y. Okano, H. Sawa, A. Kobayashi and H. Kobayashi, Synth. Met., 55-57 (1993) 2084. [19] H. Tajima, S. Ikeda, A. Kobayashi, H. Kuroda, R. Kato and H. Kobayashi, Solid State Cornman., 86 (1993) 7. [20] K. Saito, H. Kamio, Y. Honda, K. Kikuchi, K. Kobayashi and I. Ikemoto, J. Phys. Sot. Jpn., 58 (1989) 4093. [21] K. Saito, H. Yoshino, K. Kikuchi, K. Kobayashi and I. Ikemoto, J. Phys. Sot. Jpn., 62 (1993) 1001. [22] H.C. Montgomery, J. Appl. Phys., 42 (1971) 2971. [23] B.F. Logan, SO. Rice and R.F. Wick, J. Appl. Phys., 42 (1971) 2975. [24] H. Yoshino, K. Saito, H. Nishikawa, K. Kikuchi, K. Kobayashi and I. Ikemoto, in preparation. [25] Y. Nogarni, M.Tanaka, S. Kagoshima, K. Kikuchi, K. Saito, I. Ikemoto and K. Kobayashi, J. Phys. Sot. Jpn., 56 (1987) 3783. [26] Y. Honda, K. Murata, K. Kiuchi, K. Saito, I. Ikemoto and K. Kobayashi, SolidState Commrm., 71 (1989) 1087. [27] Y. Ishikawa, M.Sc. Thesis, Department of Chemistry, Tokyo Metropolitan University, 1989.