The structure, conductivity, and thermopower of HMTTF-TCNQ

Solid State Coemunications, Vol. 20,

~.

943—946, 1976.

Pergamon Press.

Printed in Great Britain.

ThE STRUCTURE, CONDUCTIVITY, AND THERMOPOWER OF HMTTF-TCNQ R. L. Greene, 3. 3. Mayerle, R. Schumaker, and C. Castro IBM Research Laboratory San Jose, California 95193 and P. M. Chaikint Department of Physics, University of California, Los Angeles, California 90024 and S. Etemad Department of Physics, Aria—Mehr Univbrsity, P. 0. Box 3406, Tehran, Iran and S. 3. LaPlaca IBM Thomas 3. Watson Research Center Yorktown Heights, New York 10598

-

The structure and some transport properties of a new organic conductor, HMTTF—TCNQ, are reported. A metallic conductivity and thermopower are found between room temperature and 50°K,with insulating behavior at lower temperatures. The conductivity data suggest that two phase transitions occur, one at 43±1Kand the other at 50±1K. Comparison with the

properties of TTF—TCNQ and HMTSeP—TCNQ is made.

The past few years have seen a dramatic increase in research on metallic—like charge—transfer compounds, in particular the organic salt TTF—TCNQ (tetrathiofulvalenium 1 and its selenium tearacyanoquinodiaethanide) analog TSeF—TCNQ,2,3 These materials, anisotropic metals at room temperature, upon cooling undergo a Peierls transition to a low temperature insulating state. Since the origin

trithiocarbonate, which in turn was formed by cyclo—sulfurization of a xsnthate ester. Single crystals of its 1:1 salt with TCNQ were grown over a ofperiod of several days by slow evaporation a saturated acetonitrile—tetrachloroethylene solution. Black lustrous crystals were obtained with typical dimensions of 2x0.lxO.OSimn3. The quality of the best crystals was not as high as desired for crystallographic work, a co~on problem in dealing with TCNQ salts. For the structure determination a crystal of }DffTF—TCNQ was mounted on an Enraf—Nonius CAD4 automatic diffractometer. Computer centering and indexing of 15 reflections led to an orthorhomic cr11 with unit cell parameters a—l2,470A, b—3,9061, c—2l,6021, and 2.2.6 Analysis of the intensities after correction for background and Lp effects indicated the space grout to be Pmoa (orthorhombic, No. 53, D~h).7 The structure was solved and refined by the usual Fourier and least—squares methods. A difference map

of this behavior is subtle and not yet predictable for a given donor—acceptor complex, it has been important to study a variety of

such salts. We report here the crystal structure and some electrical properties4 of, an unusual derivative of TTF—TCNQ, hexamethylenetetrathiofulvalenium tetracyanoquinodimethanide (HMTTF—TCNQ). The selenium analog of this new compound, (WITSeF—TCNQ), was recently found to be the first organic material to show appreciable conductivity throughout the temperature range 0.007 — 3000K.5 Our conductivity measurements show that, in striking contrast to RMTSeF—TCNQ, HMTTF—TCNQ undergoes a metal—insulator

transition below approximately 50K.

computed after refinement of the entire

Since

structure showed electron density in positions

the constituents of these two compounds are structurally similar, this large difference in low temperature conductivity behavior is of considerable interest. In addition, we find evidence for two phase transitions in HMTTF—TCNQ, making it important to compare it with TTF—TCNQ, in which at least two phase

corresponding to reflection of the constituent ions through the cc plane. These mirror—related molecules were included in further refi.nement and shown, by variation of occupancy factors, to account for approximately 8% of the structure. The R value at this stage of the refinement is 0.08. Figure 1

transitions have also been observed.3 RMTTF was prepared by oxidative coupling of the perchlorate salt of the corresponding

shows the [010] projection of the structure and Figure 2 shows the [100] projection. The packing arrangement is one of independent 943

944

THE STRUCTURE, CONDUCTIVITY, AND THERMOPOWER OF MHTTF—TCNQ

100

Vol. 20, No. 10

T(K) 5040 30

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Figure 1 A projection of the structure of HM’rTF—TCNQ onto the ac place (010]. The HMTTY radical cations are centered at 0,0,0 and the TCNQ radical anions

_______________________ T(K) ‘

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are centered at 0,1/2,1/2. I

0

10

20

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40

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1000 Figure 3 Logarithm of resistance (R) versus inverse Solid line is resistance of ~ffSeF—TCNQ taken temperature (l/T) for a typical )3NTTF—TCNQ 1cm~) in the b direction. crystal (a 300~4O0

S

from Ref. 5 and normalized to our HMTTF data Figure 2 Structure projected onto the ho plane (100].

at 300°K. Inset

shows

(~-~) versus T

metal—insulator transition stacks of cations and anions parallel to the short (b) axis, with the molecular planes being tilted 23.8° (ffl4TTP) and 34.2° (TCNQ) relative to the ac plane. Intrachain m~lecular overlaps and interplanar spacings (3.57A for HMTTF and 3.231 for TCNQ) are typical of conducting TCNQ salts.8 The dc conductivity along the b axis was measured by a standard four—probe technique using silver paint contacts. In Figure 3 we show the data by plotting the logarithm of the measured resistance versus the inverse temperature for a representative sample. The typical room temperature (300°K)conductivity (a) is 400±5004cm1. Between 300°Kand 80°K a increases by a factor of about 3.5 with a(T)/a(300°K)having the same temperature

data are shown as a function of temperature in Figure 4. As can be seen, S is small and positive at high temperatures. After crossing +3

-3

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dependence exhibited by BMrSeF-TCt~Q.5 Below about 80°Ka broad metal—insulator transition begins. The actual transition temperature is defined by the peak position of the logarithmic derivative of the resistance, (l/R)(dR/dT), versus temperature shown in the inset of Figure 3. From this we see that apparently two transitions occur in HMTTPTCNQ with Tcl50±l°K and Tc2043±l°K. The sharpness of the upper transition (Tcl) is quite sample dependent. Below 43°Kwe find an activation energy (2~) for ~ of approximately 480°Kwith a tailing off at much lower T, probably due to the

in the

region.

-12

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—15

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204 i 0

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120 T(K)

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Figure 4 Absolute b axis thermopower (S) of BMTTF—TCNQ versus temperature. Inset shows the low temperature data.

Vol. 20, No. 10

THE STRUCTURE, CONDUCTIVITY, AND THERNOPOWER OF MHTTF—TCNQ

zero near l50°KS goes sharply negative below 50°Kand peaks at approximately —26Oijv/°Knear 25°K. As in TTF—TCNQ and TSeF—TCNQ, the sharp break in S suggests the onset of a metal—insulator transition. Both in magnitude and temperature dependence the thermopower of HMTTF—TCNQ is qualitatively similar but quantitatively different than HMTSeF—TCNQ.5 This is in contrast to isostructural TTF—TCNQ and TSeF—TCNQ ip which the thermopovers are very different.~ The positive value of the room temperature thermopover in }IMTTF is unique among the TTF derivative TCNQ salts. Many features of our data are interesting and clearly warrant future study. We believe that the differences between the properties of RMTTF—TCNQ and HMTSeF—TCNQ will prove to be largely steric, rather than electronic, in origin. A structural comparison of HMTSeF—TCNQ and HMTTF—TCNQ qn be made from the work of Phillips et al.1-” and Figures 1 and 2 of this paper. We find that the structures are quite similar but have some crucial differences in directions perpendicular to the stacking axis. In HMTSeF—TCNQ all cation and anion stacks have their molecular planes tilted in the same direction relative to the b axis whereas in BMTTF—TCNQ the tilt angle alternates from stack to stack along the c axis in a herringbone fashion. In other words in the Se compound crystallographically equivalent molecules are related by translation whereas in the S compound they are related by a glide plane. There is disorder along the c axis in both materials but it is greater in the selenium compound (20%) than in the sulfur compound (8%). Finally, and perhaps most • significantly, the short Se———N contact (3.lOA) suggests considerably stronger a axis coupling in HMTSeF—TCNQ than i~HMTTF—TCNQ, where the S——N contact is 3.25A (the same as in TTF—TCNQ). As Phillips et ~ suggest, this may make HMTSeF—TCNQ the most two—dimensional conductor in the TTF—TCNQ family. Our a and S results (Figures 3 and 4), when compared to those of TTF—TCNQ, suggest that HMTTF—TCNQ undergoes a Peierls transition with three—dimensional (3D) ordering and insulating behavior below 50°K. In contrast the high disorder. Note that 2tC/Tc’lO in agreement with values found for other quasi—iD conductors.3 The thermopower (S) of several crystals of RMTTF—TCNQ was measured along the b axis by a technique described previously.9 The conductivity observed in RMTSeF—TCNQ at low temperature could arise from either increased dimensionality, as has been found in (SN)x,l~~

or from the disorder along the weakly coupled e axis. It is clear that disorder can smear a Peierls transition either by introducing states into the gap or by disrupting the 3D coupling between chains. However, the large pressure induced increase in low 12 temperature suggests that conductivity observed recently higher dimensionality is playing the dominant role. The cause of the two transitions (at 50°K and 43°K)in HMTTF—TCNQ is also of interest. The possibility that two transitions occur in TTF—TCNQ was first noted by Chu et al.,13 who suggested that these night arise from TTF and TCNQ chains undergoing Peierls transitions at different temperatures. Recent work344 has more definitively shown that at least two transitions15 occur (at 54°Kand 38°K)and has further shown that the 54°Ktransition is associated with the TCNQ chain apd ~he 38°K transition with the TTF chain.3’~’4°A detailed understanding of these transitions awaits further experimental and theoretical work but it seems likely that the two transitions in HMTTP—TCNQ are of a similar origin. In TTF—TCNQ a temperature—variable modulation of the lattice in the a direction is observed between 54°K(2a) and 38°K (4a).14’15 It has been suggestedl547 that this arises from a coulomb coupling of charge density waves (CDW), first between the TCNQ stacks (54°K)and then between the TTF stacks (38°X). We anticipate that the fine details of this coupling will be different in HMTTF—TCNQ because the cation and anions have a different relative arrangement. In TTF—TCNQ8 sheets (in the be plane) of TTF and TCNQ alternate along the a axis. In contrast, Figure 3 shows that in HMTTF—TCNQ each stack of TCNQ (HMTTF) has nearest neighbor stacks of HMTTF (TCNQ). In addition, the tilting of the molecules with respect to the ac plane and their relative positions along the b axis are different in the two compounds. A theory which can explain the transitions in these two salts in a unified manner is clearly needed. Ack.~owled,~gments— We have benefited from many discussions with A. N. Bloch and thank him for communicating the results of the Johns Hopkins group prior to publication. We thank Robert Craven for his help with the conductivity measurements and Max Ebenhahn and Robert Bingham for their invaluable technical assistance in sample preparation and measurements. One of us (S.E.) would like to thank Torn Penny for the use of his equipment and the IBM Watson Research Center for their hospitality and support during the time of these experiments.

REFERENCES t 1. 2. 3. 4.

945

Research at UCLA supported by the National Science Foundation, Grant #NSF DMR 73—75l8AOl. For a review see, Low Dimensional Cooperative Phenomena, edited by H. A. KELLER (Plenum, New York 1975). S. ETEMAD, T. PENNY, E. H. ENGLER, B. A. SCOTT, and P. E. SEIDEN, Phys. Rev. Lett. 34, 741 (1975). S. ETEMAD, Phys. Rev. Bl3, 2254 (1976). Our results were first presented in Bull. An. Phy. Soc. 20, 495 (1975).

946

THE STRUCTURE, CONDUCTIVITY, AND THERNOPOWER OF MHTTF—TCNQ 5.

6.

7. 8. 9. 10. 11. 12. 13. 14. 15.

16. 17.

Vol.

20, No. 10

A. N. BLOCH, D. 0. COWAN, K. BECHGAARD, R. E. PYLE, R. H. BANKS, and T. 0. POERLER, Phys. Rev. Lett. 34, 1561 (1975). The conductivity below l°Kwas measured by R. L. Greene and W. A. Little, to be published. Slight deviations of the interaxial angles from 90°indicated the possibility that the crystal system was monoclinic with 8r~9O°.Thus, enough data were collected to give a complete unique monoclinic dataset. Analysis of this data showed several slight violations of the systematic absences req~Uredby space group Pmna. Indeed, averaging of equivalent reflections in both the o~thorhombicand monoclinic systems yielded slightly better results in the latter (the 12,47OA axis being unique). Thus the space groups considered were Pmna (orthorhombic), p2/n (monoclinic), and their noncentrosyminetric subgroups. The structure was initially solved and refined in Pmna. Since refinement in the various nonoclinic space groups did not produce results as satisfactory as did refinement in Pmna, the results reported here are for the orthorhonbic system. Further work is necessary to clarify this troubling point. In any case, the results would be essentially unaffected should the crystal system ultimately be proven to be monoclinic. “International Tables For X—Ray Cystallography,” Vol. I, 3rd ed., Kynoch Press, Birmingham, England, 1969, p. 141. T. 3. KISTENMACHER, T. E. PHILLIPS, and D. 0. COWAN, Acta Cryst. B30, 763 (1974). P. M. CRAIKIN, R. L. GREENE, S. ETDIAD, and E. ENGLER, Phys. Rev. 313, 1627 (1976). T. E. PHILLIPS, T. 3. KISTENMACHER, A. N. BLOCH, and D. 0. COWAN, preprint. W. E. RUDGE and P. M. GRANT,Phys. Rev. Lett. 35, 1799 (1975) and references therein. 3. R. COOPER, H. WEGER, D. JEROME, D. LEFUR, K. BECRGAARD, and D. 0. COWAN, preprint. C. W. çHTJ, 3. N. E. HARPER, T. H. GEBALLE and R. L. GREENE, Phys. Rev. Lett, 31, 1491 (1973). R. COMES, S. M. SHAPIRO, G, SHIRANE, A. F. GARITO, and A. 3. HEEGER, Phys. Rev. Lett. 35, 1518 (1975) and references therein. P. BAR and V. J. EMERY, Phys. Rev. Lett. 36, 978 (1976), have given experimental and theoretical evidence for three transitions in TTF-TCNQ. They find that one type of chain (either TTF or TCNQ) orders at 54°K,the other type at 47°Kwith the modulation in the a direction constant and equal to 2a. Below 47°Kthe a nodulation increases and locks into a period of 4a at 38°K. Y. TOMXIEWICZ, A. R. TARANKO, and 3. B. TORRANCE, Phys. Rev. Lett. 36, 751 (1976) T. D. SCHULTZ and S. ETEMAD, preprint.