# Transport properties of quinolinium (TCNQ)2 under pressure

C. Weyl, Denis Jerome, P.M. Chaikin, H. Strzelecka

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C. Weyl, Denis Jerome, P.M. Chaikin, H. Strzelecka. Transport properties of quinolinium (TCNQ)2 under pressure. Solid State Communications, 1981. hal-05000876

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TRANSPORT PROPERTIES OF QUINOLINIUM (TCNQ)₂ UNDER PRESSURE

C. Weyl, D. Jérôme and P.M. Chaikin*

Laboratoire de Physique des Solides
Université de Paris-Sud, 91405 Orsay (France)

and

H. Strzelecka

CNRS, Gr 12 Réactivité et Mechanisme eu Chimie Organique
2, rue Dunant, 94320 Thiais (France)

(Received 9 February by H. Suhl)

We have measured the electrical conductivity and thermoelectric power of quinolinium (TCNQ)₂ in the pressure range from atmospheric to 20 kilobars at room temperature and for 15 kilobars from 300 K to 30 K. The room temperature conductivity is enhanced by a factor of seven at 20 kilobars in a manner similar to TTF-TCNQ, whereas the thermopower is only slightly reduced (~ 30%). At 15 kilobars the low temperature behavior remains dominated by disorder and Coulomb correlations.

Quinolinium tetracyanoquinodimethane [QUIN(TCNQ)₂] is one of the most widely studied of the quasi-one-dimensional organic conductors.¹⁻⁶ It has a room temperature conductivity of ~ 80 (Ω cm)⁻¹ which is an order of magnitude less than the more popular tetrathiafulvalinium (TTF) - TCNQ. However, unlike TTF - TCNQ there is no metal insulation transition in the temperature region studied (350K - 5 mK). Rather the conductivity increases by several percent as temperature is lowered to ~ 250K and then falls in a manner which is not characterized by simple activation of carriers. The conductivity mechanism is still not well understood.

In order to further explore the properties of QUIN(TCNQ)₂, which also serves as a prototype for a number of similar linear charge transfer salts, we have performed a series of transport measurements as a function of pressure. Previous pressure studies have revealed a great deal about other quasi-one-dimensional conductors.⁷⁻⁸ The main effects of applied hydrostatic pressure are an increase in the longitudinal bandwidths and in the interchain coupling and possible changes in the electron-gas parameters. Charge transfer may also change with pressure.

The conductivity and thermoelectric power were measured in an apparatus described elsewhere.⁹ In figure 1 the room temperature conductivity and thermopower are plotted as a function of pressure up to 20 kbar. The conductivity is seen to increase linearly with pressure attaining a value ~ 7 times its ambient value at

20 kbar. The behavior is very similar to that observed in TTF-TCNQ.⁷ This is despite the fact that the conductivity mechanism is presumably quite different in the two cases and that TTF-TCNQ has two conducting chains rather than a single chain (TCNQ) in the present case.

The thermoelectric power is more indicative of the differences between these compounds. For QUIN(TCNQ)₂ the thermopower falls from a value of -65µV/k to -52µV/k at 20 kb, a change of 20%, while for TTC-TCNQ it falls from -30µV/k to -10µV/k for a change of 70% or a factor of three. The larger change in the TTF salt arises from a competition of two conducting chains with different signs to their thermopowers. Since the total conductivity exhibits a change of almost an order of magnitude it is reasonable that the relative conductivities change and hence the weighting of the thermopower from the two chains changes. In QUIN(TCNQ)₂ if both chains were conducting we would expect a similar change. However, since the change transfer is complete⁹ there is only one conducting chain (TCNQ) and no competition. Coupled with large correlation effects described below the result is a small change in thermopower.

In order to further discuss the conductivity it is necessary to consider its temperature dependence. In figure 2 we have shown the normalized conductivity as a function of temperature for 15 kbars and ambient pressure. The 15 kbar data is normalized at 250 K due to a small pressure change which accompanies the

<footnote>*Permanent address: Physics Department, University of California, Los Angeles, California 90024.
A.P. Sloan Foundation Fellow; work partially supported by NSF DMR 79-08560.</footnote>
<figure><img src="image_3.png" /><figcaption>FIG. 1. Normalized conductivity ($\sigma$) and absolute thermoelectric power at room temperature (295 K) as a function of pressure.</figcaption></figure>

freezing of the pressure medium (isopentane). In figure 3 we have plotted the absolute conductivity on an Arrenhius plot.

The ambient pressure conductivity has been interpreted in two conflicting ways.⁴,⁵ In one view the material is dominated by disorder which exists both because of random potentials which result from the asymmetric donors and from chain breaks along the TCNQ chains.⁴ In this model the low temperature conductivity results from hopping between localized sites and between the broken chains producing a non-Arrenhius behavior as well as large frequency dependent effects. The conductivity maximum results from dynamic phonon disorder eventually dominating over the static disorder as temperature is raised.

The contrary viewpoint suggests that $QUIN(TCNQ)_2$ is a band semiconductor with a well established gap at room temperature but also with a highly temperature dependent mobility.⁵ At low temperature the carrier activation dominates while at high temperature the decreasing mobility dominates producing a maximum conductivity at ~250 K.

From the data shown in Figures 1, 2 and 3 we must account for three effects; the large increase of the conductivity with pressure, the fact that the temperature of the maximum changes only slightly and the less rapid decreases at low temperature. In the semiconductor picture the low temperature behavior must be attributed to a smaller gap. Unless truly drastic things happen to the temperature dependence of the mobility we would then expect a substantial lowering of the temperature of the conductivity maxima.

In both models the conductivity at room temperature is dominated by scattering from phonons. The similar conductivity enhancement

<figure><img src="image_4.png" /><figcaption>FIG. 2. Normalized conductivity as a function of temperature for atmospheric pressure and 15 kbar.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>FIG. 3. Logarithm of the conductivity as a function of inverse temperatures for 1 atmosphere and 15 kbar.</figcaption></figure>

of TTF-TCNQ with pressure has been the subject of considerable previous theoretical work.<sup>11-13</sup> Calculations of the change in the bandwidth or overlap integral and the stiffening have implied that a conductivity enhancement of the order of 12% kbar is reasonable for the TCNQ chain.<sup>12</sup> However this value is a factor of two smaller than what has been observed and often the blame is then placed on the TTF chain. For QUIN(TCNQ)₂ the single chain is all that contributes.

In the disorder model a large fraction of the resistivity is associated with simple chain breaks which must either be trasversed directly or by hopping to a neighboring chain. Experimental evidence for these breaks is found in the frequency dependence of the conductivity and dielectric constant as well as in radiation damage studies.<sup>14</sup> Indeed, it is very tempting to plot the present results along with the damage studies and conclude that the pressure acts as "negative damage." However, extrapolation of the damage studies indicate that if QUIN(TCNQ)₂ were made as perfect, break-free crystals, the room temperature conductivity would increase by a factor of two and not a factor of seven.

The most reasonable explanation for the conductivity change with pressure is a combination of several of the mechanisms cited above. The longitudinal bandwidth or overlap increases and the lattice excitations stiffen leading to a higher mobility. In addition the transverse overlap increases and the material becomes more two or three dimensional. This effect is particularly important in QUIN(TCNQ)₂ because of its extreme anisotropy. The interchain transfer facilitates the avoidance of chain breaks at high temperature and allows three dimensional hopping between localized states at low tempera-

<figure><img src="image_6.png" /><figcaption>FIG. 4. Absolute thermopower as a function of temperature for 1 atmosphere and 15 kbar.</figcaption></figure>
ture producing a less rapid exponential variation than expected for samples with lower dimensionality.

The thermoelectric power is shown in figure 4 for a sample at 15 kbar and compared with previous results at ambient pressure. The interpretation of the thermopower of QUIN(TCNQ)₂ and similar 1/4 filled TCNQ salts has centered around the observation that they all tend to a value of ~-60µV/k at room temperature.¹,¹⁵ this is what is expected from the spin entropy of unpaired electrons and suggests that the Coulomb correlations are dominant and that two electrons cannot occupy the same site. Using this model and assuming that the TCNQ chain is the only one which conducts, we would not expect large changes in the thermopower of QUIN(TCNQ)₂ unless the bandwidth were made sufficiently large that it overcame the Coulomb interaction. The observation that the thermopower changes only slightly while the conductivity changes dramatically supports this model in which the orbital and spin degrees of freedom are decoupled.

There has not been tremendous success in explaining the deviation of the thermopower of this class of materials from the spin value of $\frac{\kappa\ln^2 k}{kT} \approx -59\mu\text{V/k}$. The room temperature thermopower deviation of QUIN(TCNQ)₂ at ambient pressure with its value of -65 V/k could imply incomplete charge transfer. This would necessitate more than complete charge transfer for the sample under pressure, an untenable supposition. X-ray experiments imply that charge transfer is complete at ambient.

Below 100 K the gradual decrease of the ambient pressure thermopower suggests that the

spin entropy is being quenched by antiferromagnetic exchange. In a strong coupling Hubbard model this exchange varies as $t^2/U$ where t is the hopping integral and U the on-site Coulomb repulsion. Under a pressure of 15 kbar the thermopower quenches at higher temperatures which may result from an increase in exchange by a factor of two or a 40% increase in t.

It is somewhat surprising that with the increase in three dimensional coupling the material does not undergo a phase transition to a good insulating state at low temperature. Such a transition has been observed under pressure in KCP,¹⁶ a similar disordered extremely anisotropic conductor which has no transition at ambient pressure. The differences probably lie in the form of the interchain coupling which is probably electrostatic in the case of KCP and increased transfer in the case of QUIN(TCNQ)₂. The disorder potential is still too strong to allow a phase locking between the chains in QUIN(TCNQ)₂ despite the formation of charge density wave distortions on the chains (as seen in X-ray studies).¹⁰ Perhaps such a transition can be observed at higher pressures.

In conclusion we have found that pressure increases both the longitudinal and transverse transfer integrals in QUIN(TCNQ)₂. The low temperature state remains a disordered semiconductor with strong correlations. The room temperature conductivity of this single conducting chain compound changes very rapidly with pressure in a manner very similar to TTF-TCNQ but the mechanism in neither case is well understood.

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