A new organic low temperature conductor: HMTSF-TNAP

Solid State Communications,

Pergamon Press.

Vol. 25, pp. 875-879,1978.

A NEW ORGANIC LOW TEMPERATURE

CONDUCTOR:

Printed in Great Britain

HMTSF-TNAP

K. Bechgaard Chem. Lab. II, H.C. Oersted Institute,

DK-2100 Denmark

C.S. Jacobsen Phys. Lab. III, Technical University

of Denmark,

DK-2800 Denmark

and N. Hessel Andersen Phys. Lab. I, H.C. Oersted Institute,

DK-2100 Denmark

(Received 18 October 1977 by L. Hedin) Conductivity and optical data on a new organic, conducting charge transfer salt AZ J ‘-Bi-(4,5-trimethylene-1,3diselenole) 11 ,11’,12,12’-tetracyano26napthoquinodimethane (HMTSF-TNAP) are g&r. ~(300 K) = 2400 * 600 n2-’ cm-‘. A maximum in a(T) is found at TM = 47 K with a(T~)/a(300 K) = 6.0 + lO%, and ~(1.5 K) > 250 Cl-‘. u(T) is well defined in the high temperature region, but is sample dependent at low temperatures. The optical data indicate a bandwidth and carrier density comparable to that of HMTSF-TCNQ.

dimensionality results in a low temperature semimetallic state [9]. A related group of materials, halides of the organic donors TTT [lo] and TSeT [ 111 have recently been shown also to exhibit metallic conductivity at low temperatures. In this report we present conductivity and optical data for A2 *2‘-Bi-(4,5-trimethylene-1,3-diselenole) 11 ,I 1’,12,12’-tetracyano-2,6-napthoquinodimethane (HMTSF-TNAP, see Fig. l), a new system which at ambient pressure exhibits very high conductivity at low temperatures.

1. INTRODUCTION ORGANIC conducting materials, where the metallic character persists to very low temperatures, are of considerable interest in the experimental attempts to decide whether the superconducting state can occur in p,urely organic solids. Many organic conductors prepared so far are based on the prototype TTF-TCNQ [ 1,2]. The charge carriers move along uniform stacks of donor or acceptor molecules with n type valence orbitals, overlapping strongly along the stacks. Consequently the conductivity tensor is highly anisotropic, the crystals being electronically quasi-one-dimensional. Most of the materials undergo metal-insulator transitions at low temperatures, which are indeed commonly interpreted as arising from Peierls distortions [3], characteristic for a quasi-one-dimensional electron gas. In a few systems of the family, HMTTF-TCNQ [4] at moderate to high pressures, and HMTSF-TCNQ [5, 61 even at ambient pressure, the metallic conductivity persists to the lowest temperatures attainable. In both materials the actual low temperature conductivity is strongly pressure dependent, most dramatically for HMTTF-TCNQ for which a( 1.2 K) increases five orders of magnitude going from 1 bar to S-10 kbar. Hence pressure makes this material change from an insulating to a conducting state. It has been suggested that the two systems in question due to their particular crystal structures [7,8], are more two-dimensional than other members of the family and that the higher

2. EXPERIMENTAL

RESULTS

Crystals well suited for conductivity and optical measurements were prepared by slow cooling of a nitrobenzene solution of the purified constituents. Typical crystals had dimensions of 2 x 0.2 x 0.4 mm3 and exhibited a green shine in reflected light. Weissenberg photographs indicated a slight twinning as frequently found in systems based on TTF-TCNQ. Conductivity measurements were made using standard 4 probe techniques. About 15 samples from two different preparations have been investigated. Room temperature conductivities are in the range 1800-3000 Sz-’ cm-‘, th e h’gh 1 es t reported so far for an organic material. In Fig. 2 we show the temperature dependence of the conductivity normalized to the room temperature 875

876

A NEW ORGANIC LOW TEMPERATURE CONDUCTOR: HMTSF-TNAP ENERGY

HMTSF

Vol. 25, No. 11

(cV)

TNAP HMTSF - TNAP

Fig. 1. Molecular constituents.

T = 300 K

value. A well-defined maximum occurs near TM = 47 K with o(Tna)/a(300 K) = 6.0 f 10%. Above TM o(T)/o(300 K) is quite reproducible from sample to sample, but below - 35 K a pronounced sample dependence is found. In all crystals we have investigated, the conductivity falls monotonically at lower temperatures, but at for example 1.5 K the normalized conductivity varies from sample to sample in the range 0.08-l .O. This means that at 1.5 K the conductivity is in the range 250 52-r cm-’ -3000 n-r cm-r . In addition to the temperature dependence of the

-

.O’ HMTSF-TCNQ

$

I I 5000

I

I

I

I I 10000

FREQUENCY

I

I

I

I I 15000

(cm-‘1

Fig. 3. Polarized reflectance of HMTSF-TNAP for light polarized along (RI,), and perpendicular to (RI) the highly conducting direction, vs frequency from 5000-16,000 cm-‘.

. . .

t

1

:

I

3

10

\

30

TEMPERATURE

I

I

100

300

(Kl

Fig. 2. Normalized conductivity of four different samples of HMTSF-TNAP, vs temperature. For comparison is also shown the conductivity of HMTSF-TCNQ with typical spread at low temperatures [ 51 (dashed lines), and the conductivity of TMTSF-TCNQ (dotted line). Note the logarithmic axes.

metallic conductivity component, we have measured the anisotropies at room temperature using the Montgomery technique [ 121. We find comparable anisotropy ratios in the two directions perpendicular to the highly conducting direction: utt/ulr and u,~/u~~both of order 100 + 20%. It should be mentioned that another group [ 131 has reported the same overall results for HMTSF-TNAP above TM as those reported here, but as the temperature is lowered their crystal goes insulating. A preliminary report on the structure of HMTSF-TNAP has appeared from the same group [ 141. HMTSF-TNAP crystallizes in a triclinic structure and exhibits very short Se-N and Se-Se interstack distances. From Weissenberg photographs of crystals used in the present work we infer isostructurality. Therefore we argue that the discrepancy in conductivity is not a result of different structures, but rather results from differences in defect and impurity levels. It has been shown that in HMTSF-TCNQ whose overall behaviour resembles that of HMTSF-TNAP very much, that purposely induced defects from fission fragment bombardment or substantial acceptor stack doping reduces the low temperature conductivity dramatically [ 151. This could indicate that the crystals used in our study are of higher crystalline perfection than the

vol. 25, No. 11

A NEW ORGANIC LOW TEMPERATURE CONDUCTOR: HMTSF-TNAP

crystals used in the above mentioned work. On the other hand preliminary measurements of the static magnetic susceptibility of our material do indicate a certain amount of paramagnetic impurities, so the materials problem is not finally resolved. In order to clarify whether band structure and carrier density have changed appreciably in this TNAP salt as compared to the more well known HMTSF-TCNQ salt, we have performed an initial study of the optical properties. The polarized single crystal reflectance in the frequency range from 5000 to 16,000 cm-’ is shown in Fig. 3. It is similar to what is found in HMTSF-TCNQ [ 161: In the highly conducting direction the stacking axis reflectance, R/I, rapidly falls off from the infrared, but around 8000 cm-’ a transition interrupts the plasma edge so no clean plasma minimum is found. Above 10,000 cm-’ the reflectance rises to a new transition centered at 15,000 cm-’ . This transition is also seen perpendicular to the chains (RI) while the 8000 cm-’ transition in RI is rather weak. RI is almost flat and the absolute value low down to 5000 cm-‘. We tentatively assign the 8000 cm-’ absorption as being dominated by the lowest lying intramolecular excitation in TNAP-, which in solution is found at 9000 cm-’ [ 171. The analogous spectral feature in TCNQ conductors is found at 10-l 2,000 cm-’ (in solution spectra the lowest monomer absorption is near 12,000 cm-‘). The corresponding shifts in solid state and in solution suggest that the position of this transition is related to the monomer transition. However, mixing with charge transfer excitations of the type 2TNAP- + TNAP2- + TNAPc cannot be excluded. The spectral features in the vicinity of the plasma edge are conventionally analysed by fitting to a Drude model for the dielectric function: 2 cll(a)

=

eb

-

0, w(0

+

ir)

where op = ~ne2/e0mf is the plasma frequency, I’ is the relaxation rate, and eb is the background dielectric constant. n is the carrier density, and m* is the optical effective mass for the conduction band. Such an analysis is quite inaccurate in the present case, since ea must show important spectral dependence due to the 8000 cm-’ transition. We feel that a two or three oscillator fit on the rather limited data is not meaningful, and just state that the plasma edge in HMTSF-TNAP is red shifted some 12% as compared to the edge in the TCNQ compound [ 161. Some of this shift may be due to eb being effectively higher and the conclusion is that n/m* is not very different in the two cases. This is reasonable since the crystal structures show molecular stacking with comparable overlap in the two cases [S, 141.

877

3. DISCUSSION OF o(T) The TNAP molecule (Fig. 1) differs from TCNQ in several aspects apart from its size and lower symmetry. Its electron affinity as judged from solution reduction potentials is slightly higher, it certainly has a higher polarizability, and due to the size the effective Coulomb repulsion at double occupancy might be lower. These features make it attractive in the design of organic metals [ 181. One other TNAP conductor has been reported in the literature: TTF-TNAP [ 191, which has a room temperature conductivity of order 40 S2-’ cm-’ , and virtually no temperature dependence down to 185 K, where a sharp MI transition takes place. This tells us immediately that HMTSF-TNAP cannot be understood by discussing the behaviour of the TNAP stacks alone. Clearly the details in crystal structure and interactions between constituent molecules are important. In the whole class of materials, there is now little doubt that the one-dimensional electron-lattice instability at wave-vector 2kF plays a major role, normally leading to a 3-dimensionally ordered distortion at low temperatures, with semiconducting behaviour due to a static energy gap at the Fermi surface (the Peierls state). At higher temperatures the instability results in precursor effects: More or less pronounced 2kF scattering in X-ray and neutron diffraction. The question of the electrical transport mechanism in the materials is still an open one, but it is hard to believe that the transport is unaffected by the strong 2kF backwards scattering. One of the fascinating aspects of the whole field is the possibility that 2kF charge density waves [20] or other collective states like quantum solitons [21] contribute directly to the conductivity. Now turning to the data on HMTSF-TNAP let us first discuss the high temperature part in the range 50 K < T < 300 K. Here the conductivity rises from approximately 2400 to 15,000 a-’ cm-’ . The resistivity can be fitted reasonably well to a form p(T) = p,, +AT” with Aclose to 2.3. No physical significance is attached to this formula, but it is generally used in the literature [ I] for the purpose of comparing, and indeed for most members of the TTF-TCNQ family, h falls in the range 2.2-2.4. Therefore the high temperature transport mechanism presumably is the same in all the compounds including HMTSF-TNAP. We now proceed to discuss the overall behaviour and want in particular to compare with two other systems, HMTSF-TCNQ because the low temperature behaviour is similar [S] (see dashed lines in Fig. 2) and TMTSFTCNQ (see dotted line in Fig. 2) because its crystal structure [22] is qualitatively similar to that of HMTSF-TNAP.

A NEW ORGANIC LOW TEMPERATURE CONDUCTOR: HMTSF-TNAP

878

A few comments on crystal structure are in order at this point: In ail systems one finds separated, uniform stacks of slightly tilted donor and acceptor molecules. However, the arrangement of the stacks differs. In HMTSF-TCNQ [8] the donor and acceptor stacks are arranged in a chessboard pattern with four short Se-N distances per molecule. In TMTSF-TCNQ [22] and HMTSF-TNAP [ 141 the two kinds of stacks are arranged in alternating rows, but also here with relatively short Se-N distances (two per molecule). If this distance is taken as measure for the interchain coupling, it increases from TMTSF-TCNQ through HMTSF-TCNQ to HMTSF-TNAP. However, if the anisotropy is us,ed as a measure, one would assign a somewhat lower interchain coupling to HMTSF-TNAP than to HMTSF-TCNQ, where the anisotropy ratio for the direction of the short N-Se distances is as low as ull/uli N 33 + 30%, and in the other direction all/uLz N 450 + 30% [23]. Returning to Fig. 2 it is interesting to note that u~~x/u(300 K) is comparable for TMTSF-TCNQ and HMTSF-TNAP. Below Tni the details of the interchain coupling presumably become important. TMTSF-TCNQ which is more one-dimensional has a sharp MI transition below 60 K. HMTSF-TNAP also has a distinct drop in conductivity, but clearly the low temperature state is not semiconducting. In spite of sample dependence the extrapolated zero temperature conductivity is at least 250 Sl-’ cm-‘. Here we will compare with HMTSFTCNQ [5] Diffuse X-ray scattering experiments have revealed pronounced 2kp scattering in this compound at room temperature [ 241, so apparently the tendency towards a Peierls transition is present. It is reasonable to assume that the same is the case for HMTSF-TNAP. Then as the temperature is lowered there seems to be two possibilities: (1) The 2kF instability is suppressed

Vol. 25, No. 11

by band structure effects, and the low temperature semimetallic behaviour is explained by a hybridization gap covering most of the Fermi surface, merely leaving small pockets of electrons and holes [9]. (2) A Peierls transition does take place, but due to the interchain coupling induced curvature of the Fermi surface, the Peierls gap only destroys part of it. Again a semimetallic state results. To distinguish between these two situations, low temperature structural studies are needed. There is, however, some support for the second interpretation in the presence of anomalies in specific heat and susceptibility of HMTSF-TCNQ near 30 K [ 11. These have been interpreted as arising from a phase transition. After having discussed HMTSF-TCNQ and -TNAP in the same qualitative picture, we would like to stress the important differences in anisotropies and normalized maximum conductivities. Preliminary reports also indicate that the low temperature conductivity of the TNAP-salt is less sensitive to pressure than that of the TCNQ-salt [25]. This may mean that the low temperature state is more truly metallic in HMTSF-TNAP than in HMTSF-TCNQ, but further speculations seem premature , until more experimental data is available. Measurements of the conductivity in the mK region, of thermopower, magnetic susceptibility and microwave properties are in progress in our laboratories and should help clarifying to what degree the TCNQ and the TNAP compounds are comparable. - We thank Dr. H. Soling for crystallographic assistance and Dr. E. Pedersen for making preliminary magnetic measurements. We are grateful to Dr. T.J. Kistenmacher for communicating the full crystal structure to us. Acknowledgements

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