Spin resonance of an organic conductor: Tetramethyltetrathiafulvalene tetracyanoquinodimethane

Solid State Communications, Vol. 20, pp. 767—770, 1976.

Pergamon Press.

Printed in Great Britain

SPIN RESONANCE OF AN ORGANIC CONDUCTOR: TETRAMETHYLTETRATHIAFULVALENE TETRACYANOQU1NODLMETHANE Y. Tomkiewicz, A.R. Taranko and D.C. Green IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598, U.S.A. (Received 11 June 1976 by G. Bums) Evaluation of the magnetic gaps of the (TMTTF) and (TCNQ) stacks with the help of the temperature dependence of the measured g-values leads to the conclusion that the (TCNQ) stack is driving the metal—insulator transition. The relative sizes of the magnetic gaps on the (TMTTF) and (TCNQ) stacks are compatible with the existence of only one phase transition. A comparison with (TTF)(TCNQ) is made and the lower transition temperature observed for (TMTFF)(TCNQ) is in agreement with the smaller gap observed In this compound for the (TCNQ) stack. THE UNIQUE FEATURE of (TTF)(TCNQ) [tetrathiafulvalene tetracyanoquinodimethane] in comparison to other conducting (TCNQ) salts is that both of the stacks, the donor and acceptor, are potentially conducting. The phase transitions of (TTF)(TCNQ) are related to different ordering temperatures of the two kinds of stacks, the (TCNQ) stack1 ordering at 53 K while the (TTF) stack ordering2 at 47 K. Therefore it is of great interest to modify only one of the stacks and study the effects of this modification on the physical properties of interest, in particular the phase transitions. One of the earliest modifications performed on the donor chain was replacement ofthe 4 protons of (TTF) by methyl groups.3 The salt of the modified donor (TMTTF) [tetramethyltetrathiafulvalenej with (TCNQ) exists4 in at least two different stoichiometries (TMTTF) (TCNQ) and (TMTTF) 1 3(TCNQ)2. (TMTFF)(TCNQ) has conducat least 5The two different crystallographic structures.~ tivity in the metallic regime of both of these phases has6 a very similar temperature dependence to that of (TTF) (TCNQ). However the temperature corresponding to the metal—insulator transition is lower and the temperature dependence of the conductivity inidcates6 existence of only one phase transition, In the present report we will show the spin resonance results for (TMTTF)(TCNQ). The single crystals of (TMTTF)(TCNQ) were prepared in the following way: Equimolar portions of (TMTTF) and (TCNQ) were dissolved separately in acetomtrile at 338 K. The solutions were combined and allowed to cool to 278 K. The (TMTTF)(TCNQ) was filtered off and recrystallized by redissolving in acetonitrile at 353 K and cooling over 48 h. The EPR spectrum of (TMTTF)(TCNQ) was measured at X band frequencies (~-10 GHz). It consists of two lines of equal intensity shown in the insert of —

Fig. 1. The presence of the two lines is quite surprising in view of the fact that the spectrum of (TI’F)(TCNQ) consists of a single line.7 An additional Striking difference between the magnetic properties of these two organic metals is the factthat in the metallic regime the linewidth of (TMTTF)(TCNQ) decreases8~9with decreasing temperature while the linewidth of (TTF)(TCNQ) increases.7 One might be tempted to relate the existence of the two lines of (TMTFF)(TCNQ) to a better resolution made possible by the different temperature dependence of linewidths. This different temperature dependence makes the linewidth of (TMTTF)(TCNQ) at 60K about an order of magnitude smaller than the linewidth of (TTF)(TCNQ). However, even when the linewidth of (TTF)(TCNQ) is comparable to the linewidth of (TMTTF)TCNQ), in the regime one canThereresolve only one absorption line inT<40K, (TTF)(TCNQ). fore the difference between the number of observed absorption lines is not related to the better resolution in (TMTTF)(TCNQ) caused by its narrower EPR linewidth. If the two signals would correspond to the respective EPR absorptions of the (TCNQ) and (TMTTF) stacks, the appropriate g-tensors would have different principal values. However, evaluation of the g-tensors of the two signals yielded for both of them the same principal values. The fact that the g-tensors of the two signals are the same can be seen not only by direct evaluation but also from the result that the g-values of the two signals are symmetrical around the crystallographic axes as is shown in Fig. 1. Therefore the origin of the two signals are the two non-equivalent constituents of the unit cell. Since the g-values of the measured signals are intermediate between the (TMTTF) and (TCNQ) values, as will be shown later, we conclude that each constituent corresponds to a pair of (TMTTF) and (TCNQ)

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T(°K) Fig. 1. g-anisotropy of (TMTTF)(TCNQ) in the (c*b) crystallographic plane. The insert represents the EPR absorption of a (TMFFT)(TCNQ) smgle crystal at 27 K. molecules. Therefore the existence of 2 non-equivalent pairs of (TMTFF) and (TCNQ) in the unit cell implies that the coupling between chemically alike species, i.e. (TCNQ}—(TCNQ) and (TMTTF)—(TMTTF) is significantly lowered in comparison to that existing in (TTF) (TCNQ), where the two non-equivalent molecules of (TTF)(TCNQ) in the unit cell give rise to only one EPR signal. A common feature of the magnetic properties of (TTF)(TCNQ) and (TMTTF)(TCNQ) is the temperature dependence of the measured g-values. Figure 2, curves a and b, show this dependence for HDC II b and HDC .1. b, where b corresponds to the stacking axis. The very weak temperature dependence of the measured g in the metallic regime of the conductivity (T> 60K) is to be contrasted with the very pronounced temperature dependence in the semiconducting regime (T <60K). As in (TTF)(TCNQ), the g-values shift (for 20 K < T < 60 K) towards the g-values of the (TMTTF) stack with decreasing temperature. In the temperature range T< 20 K the g-value shifts towards its value in the metallic temperature range. However, since the susceptibiity in this temperature range is dominated by a Curie tail, indicating the presence of crystalline imperfections, this behavior of the measured g-values is probably extrinsic. At 20 K the isotropic g-value of the measured signal is comparable to the value measured for (TMTTF) (C 1)07. The physical meaning of this is that the magnetic excitations at this temperature are localized on the donor stack. The temperature dependence of the g-values for 20K < T <60K therefore signifies a gradual shift of the magnetic excitations towards the donor stack with decreasing temperature. This could be due to opening of different gaps in the magnetic excitation spectra on the different stacks, the gap on the donor stack being smaller than the gap on the acceptor stack. The temperature

Fig. 2. Curves a and b respectively show the temperature dependence of the measured g-values in the HDC II b, and HDC .1. b orientations. dependence of the measured g.value is related17 to the fraction of the susceptibility a on the (TCNQ) stack by the relationship ~0, 7) = a(r)gQ(O) + [1 a(7’)]gp{O), (1) where gQ(O) is the g-value of the (TCNQ) stack and gp(8) is the g-value of the (TTF) stack. The evaluation of a was done for the particular orientation HDC II c”. The values of gQ and g~for this orientation were taken as = 2.0032 and g~. = 2.0096. The value of g~. at this orientation is the g-value measured at 20K when the magnetic excitations are localized on the donor stack. The value of gQ was chosen as one of the typical g-values measured1°for (NMP)(TCNQ) [NMP = N-methylphenazimum] in a similar orientation of the (TCNQ) molecular plane with regard to the magnetic field. However, since the g-anisotropy in this plane is 6 x 1O~,the actual value of a might be slightly different. Combining the value of a and the spin susceptibility1 one can determine the susceptibilities of the mdividual stacks XQ and XF as a function of temperature. The spin susceptibility was determined by subtracting from the value of the measured static susceptibility1’ a diamagnetic contribution of —2.57 x 1 o-~ emu/mole. An additional correction for the observed Curie tail, corresponding to about 0.1% free spins per site, was also made. The diamagnetic contribution and the Curie tail correction were obtained by a least squares fitting to the susceptibility in the temperature range 7.2K ~ T ~ 20 K with the following expression: B x = A + ~. (2) —

Curves a, b and c of Fig. 3 give respectively the total susceptibility and its decomposition to the (TCNQ) and

Vol. 20, No.8

SPIN RESONANCE OF AN ORGANIC CONDUCTOR

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Fig. 3. Curve a is the total measured static susceptibility while curves b and c are the (TMTTF) and (TCNQ) contributions respectively, (TMTTF) contributions as a function of temperature. It is clearly seen that both stacks have a temperature dependent susceptibility in the temperature range mvestigated. This behavior is to be contrasted with the temperature dependence of the (TCNQ)- and (TTF)-stack susceptibilities found’ for (TTF)(TCNQ). The 53 K transition there causes a sharp drop in the (TCNQ) susceptibility while the (TTF) susceptibility stays practically temperature independent. Therefore one could conclude that the 54K transition of (TTF)(TCNQ) is driven by the (TCNQ) stack and that it has only a small effect, if any, on the (TTF) stack. However for the case of (TMTTF)(TCNQ), since both susceptibilities are strongly temperature dependent over the measured regime, it seems as if both stacks are coupled and undergo a joint transition. Because of the similarities between the (TTF)(TCNQ) and (TMTTF)(TCNQ) systems we will assume in the following discussion that this is a Peierls transition. However no experimental evidence exists to support this view. As to which stacks are driving the transition, this can be determined by evaluation of the respective magnetic activation energies. The magnetic activation energies are 250 K for the TCNQ stack and 160K for the TMTTF stack as determined from the fit of the respective susceptibilities to the expression

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The fact that the difference between the activation energies of the donor and acceptor stacks is reduced in (TMTFF)(TCNQ), in comparison to (TTF)(TCNQ), by about a factor of 3 Indicates that the coherence lengths on both kinds of stacks and much more similar in (TMTFF)(TCNQ) than in (rrF)(TCNQ). This might be the reason that both kinds of stacks order in (TMTTF) (TCNQ) at the same temperature. As to the lowering of the metal—insulator transition temperature in ~M~F) (TCNQ). The occurence of the 3D phase transition in (TTF)(TCNQ) is related to the locking of the phases of the charge density waves on the (TCNQ) stacks. The transition temperature in (TMTI’F) (TCNQ) will occur at lower temperature because of the diminished coupling between (TCNQ) molecules on adjacent stacks as Indicated by electron—phonon the presence of two EPR lines. In addition, reduced coupling on the (TCNQ) the stack, as indicated by the smaller magnetic activation energy on this stack, will tend to reduce the transition temperature. The reduction of the electron—phonon coupling is related to two different phenomena: (1) The smaller bandwidth resulting from the increased spacing between adjacent (TCNQ) molecules caused by the steric hindrance introduced4 by the four methy groups on the (TTF) molecule. (2) The lower density of states at the Fermi level as indicated by the lower magnetic susceptibility measured’2 for (TMTTF)TCNQ) in comparison to (TTF)(TCNQ). The lowering of the density of states most probably results from the existence of a bigger effective hybridization gap in (TMTTF)(TCNQ). This fmding might seem surprising in view of the fact that the normalized Iinewidth to the square of the spin—orbit coupling indicates’3 that the anisotropy of the transfer integrals is increased in (TMTTF)(TCNQ) in comparison to (TTF)(TCNQ) by at least factor of two. The increase of the anisotropy might indicate that the hybridization gap in (TMTTF)(TCNQ) is smaller than in (TTF) (TCNQ). However the coupling between chemically alike species, which in (TTF)(TCNQ) is responsible for a partial smearing’4 of the hybridization gap, is also reduced in (TMTTF)(TCNQ). Therefore it is quite likely that the effective hybridization gap will be bigger in (TMTTF)(TCNQ) than in (TTF)(TCNQ). A lower limit for the gap can be determined from the fact thathybridization the susceptibility is temperature dependent’2 even in the temperature regime of 300 K.

Since the activation energy gap of the TCNQ stack is bigger it stifi seems, as in the case of (TTF)(TCNQ),that the In conclusion, we have shown that the (TCNQ) TCNQ stack drives the transition. However, the much stack drives the metal—insulator transition in (TMTTF) smaller value of the magnetic activation energy (the (TCNQ). The lower transition temperature, in comparicorresponding value in (TTF)(TCNQ) was 400K) would son to (TTF)(TCNQ),is related to:

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SPIN RESONANCE OF AN ORGANIC CONDUCTOR

(1) Reduced interaction between alike stacks as indicated by the presence of two EPR absorption lines and their smaller linewidth (2) Reduced electron—phonon coupling on the stack which dnves the transition as mdicated by the reduced value of its magnetic activation energy.

Vol. 20, No.8

Acknowledgements The authors are grateful to T.O. Poehier for his conductivity measurements on IBM samples, Scott B.A. to Si.for LaPlaca his invaluable for X-ray help measurements in material preparation. and to We are very grateful for static susceptibility data provided to us by Li. Tao. Helpful discussions with A.N. Bloch, F.Mehran, J.B. Torrance and T.D. Schultz are greatly appreciated. —

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TOMKIEWICZ Y., TARANKO A.R. & TORRANCE J.B., Phys. Rev. Lett. 36,751(1976).

2.

BAK P. & EMERY VJ.,Phys. Rev. Lett. 36,978(1976).

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FERRARIS J.P., POEHLER T.O., BLOCH A.N. & COWAN D.O., Tet. Lett. 27,2553(1973). PHILLIPS T.E., KISTENMACHER TJ., COWAN D.O., BECHGAARD K., BLOCH A.N. & POEHLER T.O. (to be published). LAPLACA SJ. (private communication).

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POEHLER T.O. & BLOCH A.N. (private communication). TOMKIEWICZ Y., SCOTT B.A., TAO Li. & TITLE R.S., Phys. Rev. Lett. 32,1363(1974).

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TOMKIEWICZ Y., MEHRAN F., SCOTT B.A. & GREEN D.C. Bull. Am. Phys. Soc. 19,296(1974).

9.

BERTHIER C., JEROME D., SODA G., WEYL C., ZUPPIROLU L, FA]3RE JM. & GIRAL L. (submitted for publication). TOMKIEWICZ Y. (unpublished results).

10. 11.

The static susceptibility data were kindly provided to us by TAO Li. [TOALi., TORRANCE J.B. & GREEN D.C., Bull. Am. Phys. Soc. 19,223 (1974).J.

12.

SCOTT J.C., GARITO A.F. & HEEGER Ai.,Phys. Rev. BlO, 3131 (1974).

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TOMKIEWICZ Y., ENGLER E.M. & SCHULTZ T.D., Phys. Rev. Lett. 35,456(1975).

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SCHULTZ T.D. & BLOCH A.N. (private communication).