# Electrical conductivity of (perylene),M(MNT), (M = Pt, Au) under pressure

Rui T. Henriques, L. Alcacer, Denis Jerome, Claude Bourbonnais, C. Weyl

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Rui T. Henriques, L. Alcacer, Denis Jerome, Claude Bourbonnais, C. Weyl. Electrical conductivity of (perylene),M(MNT), (M = Pt, Au) under pressure. J. Phys.C:Solid State, 1986. hal-05000455

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# Electrical conductivity of (perylene)₂M(MNT)₂ (M = Pt, Au) under pressure

R T Henriques†‡, L Alcácer§, D Jérome†, C Bourbonnais†‖ and C Weyl†

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

‡ Laboratório de Engenharia e Tecnologia Industrial, Sector de Química, P 2685 Sacavém, Portugal

§ Instituto Superior Técnico, Departamento de Química, P 1096 Lisboa Codex, Portugal

‖ Centre de Recherche de Physique du Solide, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1

Received 19 November 1985

**Abstract.** We report electrical resistivity measurements along the high conductivity axis of the organic conductors (perylene)₂M(MNT)₂ (MNT = cis-2,3-dimercapto-2-butenedinitrile, commonly called maleonitriledithiolate, S₂C₂(CN)₂) and M = Pt, Au, at pressures up to 27 kbar. The variation of the resistivity minimum and of the metal-insulator transition temperatures are discussed and results are compared with corresponding data for other organic conductors. We also re-examine the analysis of possible mechanisms for the behaviour of these compounds at ambient pressure and discuss the non-zero pressure behaviour.

## 1. Introduction

The systematic studies of transport properties in quasi-one-dimensional organic conductors under pressure have revealed the most impressive features of these systems: tetrathiofulvalene-tetracyanoquinodimethane (TTF-TCNQ) has an unusual phase diagram (Friend et al 1978b); a metallic state at 0 K has been stabilised in hexamethyltetrathiofulvalene-tetracyanoquinodimethane (HMTTF-TCNQ) (Friend et al 1978a); superconductivity was observed for the first time in organic crystals in bis-tetramethyltetraselenofulvalene-hexafluorophosphate ((TMTSF)₂PF₆) (Jérome et al 1980), soon extended to other members of the family TMTSF₂X (Ribault et al 1980, Parkin et al 1981) and even to other family of organic conductors based on bis(ethylene-dithiolo)tetrathiofulvalene (BEDT-TTF) (Parkin et al 1983b) and, finally, an evolution of magnetic phases (spin-Peierls, antiferromagnetic) with pressure was found in the (TMTTF)₂X family (Coulon et al 1982b, Creuzet et al 1982, 1985b, Bourbonnais and Caron 1985).

In a previous paper (Henriques et al 1984), we have emphasised the unique place of Per₂M(MNT)₂ among the organic conductors: it is not a two-chain like TTF-TCNQ or derivatives and although it is a ¾ filled band like the Bechgaard salts, the planar counter-ion plays an important role in the magnetic properties of these compounds.
For the particular case of $Per_2Pt(MNT)_2$, the ambient pressure electrical conductivity exhibits a transition from a metal-like behaviour to an insulating state at 6.5 K with one-dimensional precursor effects detected by diffuse x-ray scattering up to 20 K. The one-dimensional character of the compound is also confirmed by a band width anisotropy of ~75 (Henriques *et al* 1984). The magnetic susceptibility at high temperature follows a Curie-Weiss behaviour with a spin $S = \frac{1}{2}$ per $Pt(MNT)_2$ unit, coherent with a complete charge transfer, undergoing a transition to a state of paired spins in the same temperature range of the transition detected by conductivity measurements (Alcácer *et al* 1979, 1980).

$Per_2Au(MNT)_2$ has also a metal-like resistivity at high temperatures but upon cooling it passes through a broad minimum at ~55 K. No sharp transition is defined and no one-dimensional x-ray diffuse scattering effects were detected. The anion $Au(MNT)_2$ is a closed shell anion and consequently $Per_2Au(MNT)_2$ has a Pauli type magnetic susceptibility due to the perylene conducting chains.

Both compounds have a positive thermopower indicative of a conduction by holes with room temperature values of 28 and $30 \ \mu\text{VK}^{-1}$, respectively for the Pt and Au compounds (Henriques *et al* 1985). A survey of the properties of these compounds is given by Alcácer (1985).

## 2. Experimental

The conductivity measurements were performed with the usual in-line four-probe configuration, using a low-frequency alternating current of 10 µA. The voltages were measured by a lock-in technique. First attempts to measure conductivity under pressure with silver, platinum or gold paste contacts painted directly on to the crystals failed, as contact problems arose when the crystals were immersed in isopentane, the pressure transmitting fluid. An improvement was made by using gold paint on previously evaporated gold contacts.

Crystals exhibiting high unnested/nested voltage ratios, as defined by Schafer *et al* (1974) were discarded. For the samples used the ratios ranged from 0.2 to 5%. Unnested voltages have also been checked under pressure at low temperature, because in a few runs that ratio became suddenly high during the cooling. At room temperature such ratios are much greater than for most two-chain one-dimensional conductors but identical to those found for $(TMSF)_2PF_6$ (Bouffard *et al* 1982).

The samples were placed within a Teflon cell containing isopentane, the pressure being achieved by the usual Orsay equipment. Temperatures were measured with a copper-constantan thermocouple directly placed within the pressure cell and below 4.2 K by the vapour pressure of helium.

The variation of thermopower with pressure up to 10 kbar was measured at room temperature for $Per_2Pt(MNT)_2$ using a technique described by Chaikin *et al* (1981).

## 3. Results

The room temperature needle axis conductivity of both compounds increases with pressure at an initial rate of $17 \pm 1\% \ \text{kbar}^{-1}$ and tends to saturate at a pressure above 9 kbar or so, the level being somewhat sample dependent. The deviation from linearity implies that over a range of 20 kbar, the mean increase of conductivity is only 11%
<figure><img src="image_2.png" /></figure>

**Figure 1.** Normalised conductivity $\sigma(P)/\sigma(1\,\text{bar})$ at room temperature as a function of applied pressure. Full circles, $\text{Per}_2\text{Pt(MNT)}_2$. Open inverted triangles, $\text{Per}_2\text{Au(MNT)}_2$. The broken curve represents the same ratio for TTF-TCNQ after Jérôme and Schulz (1982) and the chain curve for $(\text{TMTTF})_2\text{SCN}$ after Parkin *et al* (1983b). The absolute value of conductivity (left) is only valid for $\text{Per}_2\text{M(MNT)}_2$. Note that the ambient pressure conductivity of $(\text{TMTTF})_2\text{SCN}$ is an order of magnitude less than for $\text{Per}_2\text{M(MNT)}_2$ and TTF-TCNQ.

$\text{kbar}^{-1}$. This value is much smaller than the $30\% \text{kbar}^{-1}$ observed for TTF-TCNQ (Cooper *et al* 1975), $\text{QN(TCNQ)}_2$ (Weyl *et al* 1981) and $(\text{TMTTF})_2\text{Br}$ or the $62.5\% \text{kbar}^{-1}$ found for $(\text{TMTTF})_2\text{SCN}$ (Parkin *et al* 1983a) and the $70–80\% \text{kbar}^{-1}$ for $\beta-(\text{BEDT-TTF})_2\text{I}_3$ (Murata *et al* 1985, Creuzet *et al* 1985a) but much greater than the $3\% \text{kbar}^{-1}$ found by Carneiro *et al* (1985) for $\text{Li}_x\text{Pt(MNT)}_2$.

The temperature dependence of the electrical resistivity of $\text{Per}_2\text{Pt(MNT)}_2$ at various pressures shows in all cases a change of $d\rho/dT$ from positive to negative upon cooling (figure 2). However the temperature of the transition as inferred from the maximum of $d\ln\rho/d(1/T)$ is not always very clear. At pressures of 17 kbar and higher, the anomaly of $\ln\rho$ plotted against $1/T$ is almost non-existent (figure 3), a feature already observed in other organic conductors (Friend *et al* 1978a). At lower pressure, the limitations for well defined and reproducible transition temperatures have two sources. The first is due to the measurement of the temperature within the pressure cell using a copper-constantan thermocouple with a small $dE/dT$ in the temperature range 4.2–10 K, a limitation discussed previously by Andrieux (1980), resulting from the inability of other thermocouples to bear pressure or to give reliable values under pressure. The second may arise from different crystal quality as, due to the number of samples used, they came from different batches. Bouffard *et al* (1982) showed that both the temperature of the metal-insulator transition and the activation energy in the semiconducting phase vary with induced defects. Therefore the tendency of variation of the metal-insulator transition temperature with pressure may be smeared if samples have different quality. So the experiments performed with the same crystal at two different pressures are important. In figure 4, we plot respectively the runs at 6 and 9 kbar for one sample and at 9 and 12 kbar for another one. We can conclude that the transition temperature increases with pressure, at least in this temperature range.

Below the transition temperature, the resistivity shows a tendency to level which accentuates below 2.5 K. This was also observed at ambient pressure and is commonly attributed to the presence of impurity levels in the energy gap $2\Delta(0)$ of the semiconductor.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Electrical resistivity (left) or conductivity (right) or $Per_2Pt(MNT)_2$ plotted against temperature at various pressures: open circles ○, 6 kbar; full circles ●, 9 kbar; open triangles △, 12 kbar; full squares ■, 17 kbar; full triangles ▲, 22 kbar. The broken curve is for ambient pressure from Henriques *et al* (1984).</figcaption></figure>

We cannot correlate the energy gap with pressure from the results of figures 2 and 4 but the sharpness of the transition, seen for instance by the ratio $\rho(2.5 \text{ K})/\rho_{\min}$ is always lower under pressure than at ambient pressure. From figure 4, however, it is possible to conclude that, in the range 6–12 kbar in the insulating phase, the resistivity increases

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Plots of normalised resistivity of $Per_2Pt(MNT)_2$ at (a) 17, (b) 22 and (c) 27 kbar, as a function of reciprocal temperature, showing no anomaly in the derivative and the evolution of the minimum $T_\rho$.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Plot of the resistivity of Per<sub>2</sub>Pt(MNT)<sub>2</sub> against the reciprocal temperature. For one sample S14 at 9 kbar (open squares) and 12 kbar (full squares); for another samples, S15, at 6 kbar (open circles) and 9 kbar (full circles).</figcaption></figure>

with pressure and so does the gap. At higher pressure, we notice that the transition becomes broader but *T*<sub>p</sub>, the temperature at which the resistivity passes through a minimum, increases with pressure (figure 5).

The behaviour of Per<sub>2</sub>Au(MNT)<sub>2</sub> at 6, 17 and 27 kbar is pictured in figure 6, where the inset displays the derivative of the normalised resistivity logarithm to the reciprocal temperature, showing that the almost non-existent anomalies at 1 bar and 6 kbar become sharper at 17 and 27 kbar, occurring at ~24 K. The resistivity minimum for this compound follows the behaviour of the Pt analog (figure 5).

<figure><img src="image_6.png" /><figcaption>**Figure 5.** The evolution of the resistivity minimum *T*<sub>p</sub> as a function of the pressure for Per<sub>2</sub>Pt(MNT)<sub>2</sub> (open triangles) and Per<sub>2</sub>Au(MNT)<sub>2</sub> (open circles). The full curves are merely eye-guides. The (×) indicates the transition temperatures for Per<sub>2</sub>Pt(MNT)<sub>2</sub> and the (+) for Per<sub>2</sub>Au(MNT)<sub>2</sub> at 17 and 27 kbar.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 6.** The normalised resistivity of $Per_2Au(MNT)_2$ at (a) 6, (b) 17, and (c) 27 kbar as a function of the reciprocal temperature. In the insets an anomaly in the derivative is observable at ~24 K.</figcaption></figure>

The room temperature dependence of the thermopower of $Per_2Pt(MNT)_2$ with pressure is shown in figure 7. We have found a discrepancy between the absolute value at P = 1 bar in this experiment using the pressure cell and the previous one in vacuum (Henriques *et al* 1985); the value in the right scale of the figure comes from this reference. The normalised thermopower reveals a drop of 26% at 10 kbar relative to 1 bar. This value is intermediate between that for TTF-TCNQ which drops 52% (Weyl *et al* 1982) and for (TMTSF)₂ClO₄, 15% for the same pressure (Weyl 1985). That drop corresponds roughly, in a tight-binding approximation, to an increase of ~35% in $t_‖$, and assuming $σ_‖αt_†^2$ would give $σ(10 kbar)/σ(1 bar) \sim 1.8$, smaller than the experimental value of 2.2 of figure 1, indicating that there is another source besides the band effect for the conductivity enhancement.

<figure><img src="image_8.png" /><figcaption>**Figure 7.** The normalised thermopower of $Per_2Pt(MNT)_2$ at room temperature. The absolute value at ambient pressure on the right-hand scale comes from Henriques *et al* (1985).</figcaption></figure>
**4. Discussion and concluding remarks**

**4.1. Re-examination of the ambient pressure physical properties**

Per₂Pt(MNT)₂ and Per₂Au(MNT)₂ should be compared with other one-chain conductors like the (TMTSF)₂X and (TMTTF)₂X series rather than two-chain conductors like TTF-TCNQ or derivatives. This is mainly shown by only one chain contribution to the conductivity, namely the perylene stack, and also from the complete charge transfer between perylene and dithiolate molecules. In contrast, two-chain conductors like TTF-TCNQ have incomplete charge transfer with concomitant incommensurability features which evolve under pressure (Jérôme and Schulz 1982).

Recent theoretical works for the analysis of the physical properties of the above mentioned one-chain conductors (Emery *et al* 1982, Bourbonnais and Caron 1985) have shown the important role played by electron-electron Umklapp scattering processes for the electronic properties under pressure. These electronic processes in (TMTTF)₂X and (TMTSF)₂X came mainly from a charge redistribution in the unit cell (Barisic and Brazowskii 1981) which is the result of the 4k<sub>F</sub> anion (X<sup>-</sup>) potential. In fact, for complete charge transfer, the 2:1 stoichiometry of these compounds would impose an excess of 1/2 hole per each TMTSF or TMTTF organic molecule, making the conduction band one-quarter filled. Following Barisic and Brazowskii (1981), the electrostatic potential produced by the neighbouring and non-conducting anion (X<sup>-</sup>) chains is not small, however, and this tends to impose a 4k<sub>F</sub> modulation in the hole gas of the conducting stacks. As a result, the positive charge is expected to be redistributed between the two organic molecules of the unit cell and the periodicity of this potential will open a small gap Δ in the middle (±2k<sub>F</sub>) of the conduction band. This makes the conduction band effectively 'half-filled' at low temperature, thereby producing an activation of the charge degrees of freedom. This was the physical explanation proposed for the resistivity minimum observed at T<sub>ρ</sub> = 100...230 K in almost all of these compounds (Coulon *et al* 1982a). It is therefore very interesting to observe that at ambient pressure all the Per₂M(MNT)₂ compounds present a similar profile in the resistivity, with T<sub>ρ</sub> = 100 K (Pd), 20 K (Pt) and 55 K (Au) (Henriques *et al* 1984; see also figure 2), with however no apparent anomaly for the perylene susceptibility contribution in the same range of temperature (Alcácer and Maki 1976, Flandrois 1980, Henriques *et al* 1985) indicating that the broad minimum in resistivity concerns only charge degrees of freedom and does not mark the onset of a phase transition. It would be rather associated, as for (TMTTF)₂X compounds with a 1D charge localisation due to electron-electron Umklapp processes. As reported in a previous work, x-ray measurements showed that the minimum at T<sub>ρ</sub> for Per₂Pt(MNT)₂ and Per₂Pd(MNT)₂ compounds is followed by a 2k<sub>F</sub> lattice fluctuation and finally a 2k<sub>F</sub> distortion on the dithiolate stacks at T<sub>c</sub> = 28 K for M = Pd and presumably at 6.5 K for M = Pt (the minimum temperature attained in the x-ray diffuse scattering experiment was 10 K). Such a lattice instability was not found however in Per₂Au(MNT)₂.

In the following we want to clarify in more detail the possible mechanisms behind these transitions at ambient pressure in order to better locate the context of interpretation for the data under pressure. These compounds are characterised by a complete charge transfer with twice as many perylene as dithiolate stacks by unit cell. This gives for each conducting stack 1/2 hole per perylene and for each localised spin or insulating stack one electron per dithiolate molecule. This situation bears some similarity with (TMTTF)₂X and (TMTSF)₂X series. Consequently, under the influence of the dithiolate (anion) stack, the hole gas of perylene chains is likely to undergo a 4k<sub>F</sub> modulation in order to minimise
its electrostatic energy. This redistribution of charge for the conducting stack costs a little in repulsion energy due to the presence of only ½ hole for each perylene molecule. It follows that at sufficiently low temperature, the conduction is likely to be effectively half filled. For example, one can expect that, under the influence of the environment, the positive charge tends to be redistributed between two molecules of perylene, making an effective half-filled band at low temperature. In such a case, repulsive electron-electron interactions lead inevitably to an activation of charge degrees of freedom in the 1D regime which is expected to be large in temperature (Henriques et al 1984) as for (TMTTF)₂X series. This mechanism is probably at the origin of the observed minimum in resistivity.

X-ray measurements on Per₂Pt(MNT)₂ and Per₂Pd(MNT)₂ compounds have shown evidence for 1D $2k_F$ lattice fluctuations on the dithiolate stacks ($2k_F^d$) at $T_c^0 = 20$ and 100 K for the Pt and Pd compounds respectively (Henriques et al 1984). Because of the presence of strongly localised spins on the dithiolate stacks, the real phase transition occurring at $T_c = 28$ K (Pd) and $T_c = 6.5$ K (Pt) is of the spin–Peierls type. However one should note that as we approach the temperature range $T_c^0$, where 1D structural fluctuations occur, the uniform magnetic susceptibility is still of the Curie–Weiss type (Alcácer and Maki 1976) and it only reaches a maximum at $T \sim T_c^0$, as a result of non-magnetic (singlet) fluctuations always associated with 1D $2k_F^d$ lattice (with spins) fluctuations. Therefore it follows that the dithiolate stacks have no $2k_F^d$ electronic instability developed at $T \gg T_c^0$. From the 1D spin–Peierls theory (Bourbonnais and Caron 1985), the 1D staggered electronic instability is a necessary ingredient with electron–phonon coupling for lattice fluctuations. The latter are possible only if the chains have already developed strong antiferromagnetic correlations at $T \gg T_c^0$, with a ‘Pauli’ type of magnetic uniform susceptibility which is nearly temperature independent near $T \gg T_c^0$. Such requirements are effectively met for the two spin–Peierls compounds ((TMTTF)₂PF₂, (TMTTF)₂AsF₆) (Coulon et al 1982a, Maaroufi et al 1985, Bourbonnais and Caron 1985).

For Per₂Pd(MNT)₂ and Per₂Pt(MNT)₂, this is obviously not the case and one has to give another explanation for the electronic 'driving force' for the lattice instability at $2k_F^d$. We think that the most natural source of 1D staggered electronic instability would come from the neighbouring chains of perylene. Indeed, as discussed above, the charge distribution in the unit cell is such that a $4k_F^p$ (perylene) charge instability giving rise to an observable $T_ρ$ will be seen as a $2k_F^d$ electronic instability for the dithiolate stacks. This is specially interesting when we look at the values of $T_ρ$ which coincide with $T_c^0$, found by x-ray diffuse scattering (see figures 1, 3 and 6 of Henriques et al 1984). Another interesting remark concerns the case of Per₂ Au(MNT)₂. Besides the presence of the closed-shell anion Au(MNT)₂⁻, which implies the absence of localised spins, resistivity data at ambient pressure still present a non-zero $T_ρ$ with no lattice fluctuations, showing that the existence of $T_ρ$ is effectively an intrinsic effect of the conducting chains. However the 1D $2k_F^d$ lattice instability, when it exists, sharpens the resistivity minimum at $T_ρ$, because it reinforces the $4k_F^p$ anomaly on perylene. Such a reinforcement due to the environment stacks has been already suggested for the (TMTTF)₂SCN compound (Emery 1983) which presents an anion ordering with a $4k_F$ wave-vector along the chains and a sharp increase of the resistivity at the transition (Coulon et al 1982a, b). For compounds which present a lattice instability, the precise nature of the coupling between the two different stacks is not yet clear. However, EPR experiments by Alcácer and Maki (1976) on these compounds have shown the existence of important spin-exchange interactions between localised spins of the dithiolate stack and spins of the conduction electrons of
the perylene chains. It is therefore not impossible that the $2k_F^d$ dithiolate lattice mode is coupled to this spin dynamics. This should deserve a more detailed study in the future.

## 4.2. Discussion of the results under pressure

If we now look at the variation of the room temperature conductivity under pressure in figure 1, we note that the pressure coefficient is small compared with the case of (TMTTF)₂SCN in the same conditions (Parkin *et al* 1983a). As mentioned at the end of § 3, this small coefficient is compatible with the decrease of the thermoelectric power which is itself attributable to an increase of the band width. In (TMTTF)₂SCN, the pressure acts to destroy the anion ordering (Parkin *et al* 1983b) and, for all compounds in this series, to decrease the $4k_F$ stack dimerisation (Emery *et al* 1982) which are the two main sources for the decrease of the conductivity. Such a variation of the conductivity under pressure is not apparent in the perylene compounds suggesting that the charge distribution within the unit cell is not affected in the same way. This becomes even more apparent if one looks that the variation of $T_ρ$ with pressure (figure 5). For $Per_2Pt(MNT)_2$ and $Per_2Au(MNT)_2$ we first observe a decrease of $T_ρ$ and $T_c$ (Pt) at $P < 6$ kbar followed by an increase of $T_ρ$ at $P > 6$ kbar. Such a profile contrasts with all existing pressure data on (TMTTF)₂X where a monotonic decrease of $T_ρ$ with pressure is always observed (Creuzet *et al* 1983, Emery *et al* 1982).

For the $Per_2Au(MNT)_2$ compound, another interesting physical phenomenon seems to emerge for $P \ge 17$ kbar. As shown in figure 6, an anomaly in $d \log(R/R_{300})/d(1/T)$ is detected at $T = 24$ K when $T_ρ \sim 50$ K (see also figure 5). A very similar profile of $ρ$ against $T$ for (TMTTF)₂X compounds with no anion ordering is observed at moderate pressure (Creuzet *et al* 1983) where the low-temperature anomaly is associated with real antiferromagnetic phase transition characterised by $T_ρ \sim 70$ K and $T_N \sim 15$ K. In order to establish the existence as well as the nature of the transition for $Per_2Au(MNT)_2$ at high pressure, more experimental work, specially spectroscopic studies, is needed.

In conclusion, the $Per_2M(MNT)_2$ series of organic materials ($M = Pd, Pt, Au, etc$) is a very special class of compounds which has some marked differences with the (TMTTF)₂X series in its physical properties, and it shows a very rich variety of new physical phenomena. Among them, one has to emphasise the occurrence of a spin–Peierls transition on the localised spin chains with low-dimensional features which are apparently induced by electronic correlations taking place in the neighbouring conducting chains. The evolution of electronic correlations with pressure is also quite unusual.

## Acknowledgments

We are grateful to J-P Pouget for many discussions and penetrating remarks and to F Creuzet, S S P Parkin and C Coulon for useful comments. Thanks to H Novais for chemical aid to sample preparation and to A Andrieux and C Andrzejewski for skilful technical assistance. The French Embassy at Lisbon is acknowledged for a grant supporting RTH's stay at Orsay. This work was partly supported by Instituto Nacional de Investigação Científica contract 83/CEN/3.
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