# Transport properties of (DMDCNQI)2Ag at normal and applied pressure

R. T. Henriques, Wuon. Kang, S. Tomić, Denis Jerome, P. Erk, S. Hünig, J.U.Von Schütz

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R. T. Henriques, Wuon. Kang, S. Tomić, Denis Jerome, P. Erk, et al. Transport properties of (DMDCNQI)2Ag at normal and applied pressure. Solid State Communications, 1988. hal-04999599

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TRANSPORT PROPERTIES OF (DMDCNQI)₂Ag AT NORMAL AND APPLIED PRESSURE

R. T. Henriques*, W. Kang, S. Tomic** and D. Jérôme

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

P. Erk and S. Hünig

Institut für Organische Chemie der Universität Würzburg, Am Hubland, D-8700 Würzburg
(Federal Republic of Germany)

and J. U. von Schütz

III. Physikalisches Institut der Universität Stuttgart, D-7000 Stuttgart 80 (Federal Republic of Germany)

(Received June 15, 1988 by J. Joffrin, revised form September 12, 1988)

We report results of the electrical resistivity of the silver salt of 2,5-dimethyl-N,N'-dicyanoquinonediimine, (DMDCNQI)₂Ag, at normal and applied pressure as well as of the thermoelectric power at ambient pressure. At room temperature, pressure induces a transition towards a more conductive state. The phase diagram of this compound includes the temperatures of resistivity minima and those of the metal-insulator transition. The first are shifted to lower temperature by pressure while the second are less sensitive to applied pressure. The present data together with previous results on ESR and X-ray diffuse scattering support the idea that the ground state is spin-Peierls. The phase diagram is discussed within the framework of a theory of competing interactions, elaborated for TMTSF and TMTTF salts.

## Introduction

In the field of organic conductors, the family (2,5-RR'-DCNQI)₂M (where DCNQI= N,N'-dicyanoquinonediimine, R,R'=CH₃, Cl, Br, I, OCH₃ and M=Li, Na, K, Cu, Ag, NH₄) has recently attracted a lot of attention¹⁻¹⁷. The copper salt of the dimethylated dicyanoquinonediimine has been so far the most studied of this series, due to its metallic-like conductivity down to 0.45 K. Since the earlier suggestion by Aumüller et al.¹ that the copper ions play a role in the conductivity, acting as bridges between the organic stacks, diversified experimental evidence has been provided in favour of such hypothesis, by XPS⁵,¹⁶, by Knight shift⁸ of ⁶³Cu and by optical reflectivity¹².

Moreover, the study of the electrical resistivity of (DMDCNQI)₂Cu under pressure revealed novel and unique features of this compound, namely the existence of pressure-induced phase transitions⁶,¹⁰ and the re-entrance of a metallic phase at low temperature and under very moderate pressure¹¹.

The silver analog, (DMDCNQI)₂Ag, although it has the same crystalline structure of the copper salt²,⁴, reveals quite a dissimilar behaviour, either in electrical properties, with an insulating ground state, or in magnetic properties where a sharp drop of the ESR signal intensity is observed at ca. 80 K, which is independent of crystal orientation². Although the Ag and Cu salts have identical static magnetic susceptibility at room temperature, respectively 5.4x10⁻⁴ and 5.8x10⁻⁴ emu/mole, the ESR signal is unobservable for the Cu compound, indicating a huge line broadening (> 1 Tesla) originating presumably from spin-orbit coupling, while for the Ag salt the linewidth varies from 0.5 to 1.3 mTesla in the entire temperature range²,⁹. The room temperature

<footnote>*Permanent address: ICEN-LNETI, Departamento de Química, P-2686 Sacavém Codex (Portugal).</footnote>

<footnote>**Present address: Institute of Physics of the University, P.O.B. 304, 41001 Zagreb (Yugoslavia).</footnote>
conductivity of the Ag compound (≈100 S.cm⁻¹) is smaller than that of the Cu salt by one order of magnitude. The X-ray diffuse scattering study performed by Moret¹⁷ et al. has revealed that (DMDCNQI)₂Ag exhibits structural fluctuations of one-dimensional character at c*/2 (corresponding to 4k<sub>F</sub> if one ignores electronic correlations), visible from room temperature and condensing at ~100 K, followed at lower temperature by a 2k<sub>F</sub> instability, which has also a 1-D character and condenses itself at a temperature (~83 K) that agrees reasonably well with that of the observed drop of the ESR signal intensity.

All the previous reported results support the idea that in (DMDCNQI)₂Ag the mixed valence state does not exist and that this compound has a pronounced one-dimensional character in contrast with (DMDCNQI)₂Cu. We decided to perform a pressure study on the silver compound, aiming to understand from its behaviour what was peculiar to the Cu compound of this family and what can be attributed to the one-dimensional stacking of the DCNQI molecules.

## Experimental

The samples used in this work were needle-like single crystals, with typical dimensions 5x0.025x0.025 mm³, grown electrochemically¹. All resistivity data were obtained with the low frequency a.c. technique and lock-in detection. The pressure was achieved by the helium gas technique up to 8 kbar and with isopentane as transmitting medium for higher values.

The thermopower data were obtained by alternatively applying a thermal gradient to the sample through two copper sinks to which it was attached, and measuring the voltage at the sample/gold leads junctions. The system is computer-monitored and the data were averaged within 1K.

## Results

The normalized conductivity, at room temperature, is displayed in fig.1. It differs completely from the behaviour of the Cu analog where σ(P) passes through a maximum at ca. 5.5 kbar¹⁰, an unique behaviour in organic conductors. For the title compound, in spite of the fact that dσ/dP is always positive, an anomaly is observed centered at 10.4 kbar, with a sharp increase of conductivity. In the low pressure region below the anomaly, the increase of conductivity is 12.5% kbar⁻¹, whereas above the anomaly it is 23% kbar⁻¹ and the mean overall value for the 0-20 kbar range is 30% kbar⁻¹, a typical value for some TCNQ based compounds, e.g. (Quinolinium)TCNQ₂¹⁸ which has a

<figure><img src="image_3.png" /><figcaption>Fig.1 Plot of the normalized conductivity as a function of pressure at ambient temperature.</figcaption></figure>

similar value of σ at ambient temperature and pressure.

In fig.2 we present the ln R<sub>n</sub>(T) versus the reciprocal temperature, where R<sub>n</sub>(T)=R(T)/R(300K), at three different pressure values, namely, 1 bar, 7.5 kbar and 16 kbar. In several runs, drastic jumps of resistivity occurred upon cooling in the 100-150 K range, with no reversible behaviour on warming and so these runs were discarded. The ambient pressure results in the present study agree with those of refs. 9 and 15. Some discrepancy found earlier in different samples² arises probably from the occurrence of

<figure><img src="image_4.png" /><figcaption>Fig.2 The ln(R<sub>n</sub>) versus reciprocal temperature profiles for 1 bar, 7.5 kbar and 16 kbar.</figcaption></figure>
such jumps, commonly found in other one-dimensional systems.

The most relevant feature of the resistivity/temperature plots, either at normal and applied pressure, is the existence of broad minima ($\rho_m$, $T_p$) and a fairly large range of temperature where $dp/dT$ is negative, like for several members of the $(TMTTF)_2X$ family²⁰. At normal pressure, $T_p \approx 170 \pm 10$ K and no clear sign of a transition is detected at lower temperature from the derivative $d(\ln p)/d(1/T)$ that evolves gradually, remaining constant for $T \le 80$ K. With increasing pressure, a transition becomes noticeable (see the 7.5 and 16 kbar plots). These features are summarized in fig. 3, where are represented, as a function of pressure, $T_p$ (crosses), $T_{MI}$ (closed circles) or the highest temperature for which $d(\ln p)/d(1/T)$ is constant, for instance at 1 bar (open circles). Whereas the temperature of the resistivity minimum ($T_p$) decreases, the second characteristic temperature is less sensitive to pressure. As a consequence, the range of temperature for which the resistivity is metallic broadens with increasing pressure.

From the values of the resistivity at a given temperature for the runs at different pressures, we can see what would be the evolution of the conductivity with pressure at constant temperature (Fig. 4). This is an hypothetical experiment, as the application of pressure at different temperatures is limited by the freezing point of isopentane. It is only possible for $P<8$ kbar, for which we use the helium gas technique, unfortunately too low to detect the anomaly in the $\sigma(P)$ runs at constant temperature. From this plot, we infer that the magnitude of the anomaly increases with decreasing temperature.

Finally, in fig. 5 the thermopower is represented as a function of temperature, at ambient pressure. The thermopower is negative, with a room temperature value of $-65 \ \mu\text{V.K}^{-1}$, it becomes smoothly more negative upon cooling and around 80 K drops steeply and changes signal in a similar way to other $(RR'-\text{DCNQI})_2\text{M}$ compounds of the so called group I in the recent work of Mori et al.¹⁵. The thermopower profile also recalls much of the behaviour of the $(TMTTF)_2X$ series²⁰.

## Discussion

Among the variety of behaviours found in organic conductors, $(DMDCNQI)_2Ag$ exhibits some peculiar features that will emphasize by comparison either with $(DMDCNQI)_2Cu$ or with compounds belonging to different families with some similarities of behaviour. They concern the dimensionality, the existence of structural instabilities

<figure><img src="image_5.png" /><figcaption>Fig. 3 Phase diagram of $(DMDCNQI)_2Ag$; Crosses indicate the temperature of the resistivity minimum, closed circles the $T_{MI}$ and open circles that temperature defined with a different criterion (see text).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4 Evolution of the normalized conductivity as a function of pressure for different temperatures.</figcaption></figure>

and the nature and the importance of coulombic correlations.

First, $(DMDCNQI)_2Ag$, as mentioned above, is much more one-dimensional than its copper analog. The interstack bridging role played by copper ions in a mixed-valence state is not present in the silver compound. A direct observation of its one-dimensionality is given by the X-ray diffuse sheets observed along the stacking axis, even at room temperature¹⁷. Both $4k_F$ and $2k_F$ satellite condensations have one-dimensional precursor effects. The copper
<figure><img src="image_7.png" /><figcaption>Fig. 5 The absolute thermopower as a function of temperature.</figcaption></figure>

compound (CIMDCNQI)₂Cu, that has a structural transition at 210 K, does not present this one-dimensional features¹⁹.

In the transport properties, the similarities with the (TMTTF)₂X series (mainly the PF₆ salt) are easily noticeable: existence of a minimum of resistivity (Tρ)²⁰, not directly connected with a phase transition, with a similar pressure dependence²¹ and a thermopower profile close to a semiconductive behaviour in a temperature range (T>Tρ) where the resistivity is still metallic²².

As far as structural instabilities are concerned, MEM(TCNQ)₂ is an evident model compound as it also exhibits both the 4kF and 2kF instabilities²³.

The large U limit throughout (U/4t=∞) adopted for the latter compound, ascribing the 4kF(335 K) and 2kF(19 K) instabilities to Peierls and spin-Peierls transitions²⁴, respectively, cannot be applied satisfactorily to our compound. In fact, within that model, instead of being a quarter-filled band as the 2:1 stoichiometry implies (1/2 electron per molecule), our compound would be a half-filled band due to coulombic repulsions for two electrons in the same site. In such situation the condensation of satellites at c*/2 (the 4kF transition observed at 100 K)¹⁷ corresponds to a 2K'F Peierls transition (K'F=2kF previously defined in the U=0 limit), with the opening of a gap at the Fermi level. However, as there is no anomaly in the lnρ versus 1/T plot at 100 K, we must conclude that the 4kF condensation occurs without the opening of a gap at the Fermi level. So the simple "big U" model does not explain correctly (DMDCNQI)₂Ag.

What is said above does not mean that coulombic correlations are irrelevant. On the contrary, there is evidence for them, through the enhancement of the magnetic

susceptibility⁹. However, in a comparison with MEM(TCNQ)₂ such enhancement at ambient temperature is similar, as for the same electronic density (1/2 electron per site), the susceptibility is about one half of the corresponding value for MEM(TCNQ)₂²⁴ and the bandwidth must not exceed 0.4 eV, the value used in the estimation of U/4t for the latter compound²⁵, and also found in band calculations for (DMDCNQI)₂Cu¹⁴. Besides, the magnetic susceptibility is already decreasing with decreasing temperature for (DMDCNQI)₂Ag, like in (TMTTF)₂PF₆²⁰, in contrast with the opposite behaviour of MEM(TCNQ)₂²⁴. So we conclude that at ambient temperature, U/4t must be smaller for our compound.

For intermediate strength of coulombic correlations, we must appeal to the theories of the one-dimensional electron gas with competing interactions, that have been developed for TMTTF and TMTSF salts²⁶⁻²⁸. Within that framework, the existence of Tρ in (TMTTF)₂X was attributed to electron-electron Umklapp processes, relevant when the band is half-filled, what happens for that series due to the small dimerization existing already at room temperature²⁹.

In this aspect, (DMDCNQI)₂Ag is different as Tρ occurs in a temperature range where the stacks are still regular (T>100K) and only precursor effects of the dimerization are observed. The origin of electronic localization is probably different from the (TMTTF)₂X series. The condensation of the 4kF satellites, at 100 K, can be seen as the opening of a dimerisation gap, ΔD, in the energy dispersion of electrons, i.e. the band becomes truly half-filled. In the temperature domain between 100 and 83 K, the resistivity becomes more activated, reaching at the latter temperature 5xρm. This means that when the 2kF structural transition occurs, the electronic degrees of freedom, unaffected at Tρ, are drastically reduced as indicated by the ESR intensity results²,⁹. So this transition acquires a spin-Peierls character.

The thermopower results present more difficulties to be understood. The existing models cannot account for a d|S|/dT<0 in a temperature range (T>Tρ) where dp/dT is positive. Similar profile is also found for several members of the (TMTTF)₂X series (X=PF₆, AsF₆)²².

The possibility of extrinsic behaviour cannot be neglected as it was also suggested for (TMTTF)₂PF₆²²,²⁹. The drop observed below 80 K can also have an extrinsic origin, in spite of occuring at a temperature range where there is a transition. Such drops occur by alloying (TMTTF)₂PF₆ with (TMTSF)₂PF₆²⁹ or can be also been observed in a different sense, as abrupt upturns, e.g. in
(TMTTF)₂ClO₄, depending on the batch preparation³⁰. In fact, it is not very plausible that the 2k_F transition, involving the spin degrees of freedom, could be seen in such a drastic way by the thermopower and not by resistivity.

The evolution of the resistivity behaviour with pressure presents two important features: the decrease of T_p with increasing pressure and the appearance of an anomaly in the lnP vs. 1/T plots. The transition temperature reveals only a slight tendency to increase that seems to be interrupted in the 10 kbar range where an anomaly in σ(P) is found at ambient temperature. Although the evolution of T_p with pressure is the same as for (TMTTF)₂PF₆²¹, the T_Mi evolves differently. For the latter compound, the spin-Peierls transition occuring at 19 K at ambient pressure³¹, with 1-D precursor effects below 60K³² becomes an antiferromagnetic transition under pressure, as shown by NMR experiments³¹.
In (DMDCNQI)₂Ag the evolution is probably towards a different ground state as the pressure dependence of T_Mi is not the same. It is presumably towards a Peierls transition. The crossover from a Peierls to a spin-Peierls transition is continuous as it was shown recently²⁷,²⁸,³³, depending on the relative strength of the Hubbard gap Δ_H, the dimerization gap Δ_D and the electron-phonon coupling constant. The decrease of T_p with pressure is an indication of a decrease of Δ_H²⁸, that can be responsible for pushing the system towards the Peierls side, for the pressure range 4-7.5 kbar. For higher pressures, as there is an anomaly in σ(P), which is not yet understood, the nature of the transition can change. Only structural and pressure electron dynamics studies under pressure will clarify this point.

## Conclusions

(DMDCNQI)₂Ag is a particular case among the diversity of DCNQI based conductors, as it exhibits both 4k_F and 2k_F instabilities. Its physical properties cannot be described without taking into account coulombic correlations, that we suppose to be of intermediate strength. The existence of electronic localization at T_p, gives to the 2k_F transition a spin-Peierls character, that seems to evolve with applied pressure towards a Peierls transition.

The theories developed for the (TMTTF)₂X series are helpful for (DMDCNQI)₂Ag but further theoretical work is needed to obtain a comprehensive description of the transport properties when coulombic interactions are important but not in the U/4t=∞ limit.

## Acknowledgements

We are grateful to R. Moret for the knowledge of his X-ray diffuse scattering results prior to publication and for many discussions. C.Bourbonnais, P.Batail and D.Köngeter are acknowledged for useful comments. This work would not be possible without the expertise and collaboration of J. C. Ameline in the pressure experiments. We thank the skilful technical assistance of T. Pourthié. The stays in France of RTH and ST were possible through the EEC allocation research contract ST2*0284 and an European Cooperation contract, respectively.

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