Pressure dependence of the resistivity of
(BEDO–TTF)ReO4 H2O)

Denis Jerome, S. Kahlich, D. Schweitzer, Pascale Auban-Senzier

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Denis Jerome, S. Kahlich, D. Schweitzer, Pascale Auban-Senzier. Pressure dependence of the resistivity of (BEDO–TTF)ReO4 H2O). Solid State Communications, 1992. hal-05003062

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259

# PRESSURE DEPENDENCE OF THE RESISTIVITY
OF (BEDO-TTF)₂ReO₄·(H₂O)

S. Kahlich and D. Schweitzer

3. Physikalisches Institut der Universität Stuttgart, Pfaffenwaldring 57,
7000 Stuttgart 80, Germany.

P. Auban-Senzier and D. Jérome

Laboratoire de Physique des Solides (associé au C.N.R.S.), Université Paris-Sud,
91405 Orsay Cedex, France.

H. J. Keller

Anorganisch Chemisches Institut der Universität Heidelberg,
Im Neuenheimer Feld 270, 6900 Heidelberg, Germany

(Received 10 April 1992 by M. Cardona)

(revised May 1, 1992)

The temperature dependence of the resistivity under pressure (up to 6kbar) of the organic metal (BEDO-TTF)₂ReO₄·(H₂O) was investigated. The increase of the resistivity below 35K, which is observed at ambient pressure, is already totally suppressed at 1kbar. In addition at this pressure the superconducting transition sharpens drastically and the onset temperature of 2.3K is nearly the same as at ambient pressure.

## Introduction

Recently [1] the observation of superconductivity below 2.5K in single crystals of (BEDO-TTF)₂ReO₄·(H₂O) was reported. (BEDO-TTF)₂ReO₄·(H₂O) is the second crystal system in the large family of the radical salts of the electron donor BEDO-TTF (bisethylenedithiolotetrathiafulvalene [2]) in which superconductivity in the bulk was observed. Before only in single crystals of (BEDO-TFF)₃·Cu₂(NCS)₃ [3] superconductivity (below 1K) was found.

In (BEDO-TTF)₂ReO₄·(H₂O) a first order metal-metal phase transition at 213 K in resistivity (see figure 1) and thermopower measurements was observed [1]. Additional phase transitions exist at around 90 K and 35 K. The most striking fact in the resistivity characteristic of the crystals at ambient pressure is the increase in resistivity below 35 K (see figure 1) and the strong decrease below 2.5 K. Nevertheless, the superconducting transition at ambient pressure is rather broad and the onset temperature is much lower in the ac-susceptibility with respect to the resistivity data [1]. Therefore it was assumed that structural disorder might be responsible for the increase in resistivity below 35 K and the rather broad superconducting transition.

In order to clarify the nature of the phase transitions and the question whether the superconducting transition sharpens under pressure we have investigated the resistivity under pressure up to 6 kbar.

## Experimental

Crystals of (BEDO-TTF)₂ReO₄·(H₂O) were prepared as described in reference 1. The resistivity data have been obtained by the usual four contact methods on needle shaped crystals of a typical length of 2-3 mm. Gold contacts were evaporated on these crystals and thin gold wires were attached using gold paint. The resistance has been measured with a low frequency lock-in technique passing a current of 10-100 µA through the sample. The high pressure was provided by either a helium gas hydrostatic medium using a Cu-Be pressure vessel, or a clamped pressure cell with isopentane as a pressure fluid. In the first cell, the measurements were performed at temperatures between 130 K and room temperature, up to 6kbar with a good accuracy for the pressure determination as the helium gas pressure can be regulated when the temperature is decreased. The measurements in the second type of
<figure><img src="image_3.png" /></figure>

Figure 1: Temperature dependence of the resistivity parallel to the c*-axes of single crystals of (BEDO-TTF)₂ReO₄(H₂O). The insert shows the temperature region of the resistivity characteristic where the metal-metal phase transition occurs in the cooling down and warming up cycle.

cell were done between 350 mK and 300 K using a He³ system, with applied pressure up to 5 kbar but without any pressure regulation at low temperature.

## Results

The typical resistivity of single crystals of (BEDO-TTF)₂ReO₄·(H₂O) at room temperature and ambient pressure parallel to the c*-axes is of the order ρc* = 0.005 Ωcm and the resistivity ratio ρc*:ρa:ρb ≈ 1:3:1000. The pressure (p) dependence of the conductivity σc* at room temperature in the investigated region up to 6 kbar is linear and the slope Δσc*/Δp = 28 (Ωcm·kbar)⁻¹. Figure 2 shows the temperature dependence of the re-

<figure><img src="image_4.png" /></figure>

Figure 3: Resistivity parallel to the c*-axes of a single crystal of (BEDO-TTF)₂ReO₄(H₂O) below 10 K at an isotropic pressure of 1 kbar.

sistivity of single crystals of (BEDO-TTF)₂ReO₄(H₂O) at ambient pressure, about 300 bar and 1 kbar in the temperature range below 30 K. Since the experiments were done on different crystals the resistivity in fig. 2 is normalized to the resistivity values at 30 K. It can be seen that already a pressure of 1 kbar is sufficient in order to suppress the increase in resistivity below 30 K and the crystals show a metallic like behaviour down to about 2.3 K where the onset to superconductivity is observed. Figure 3 shows again the low temperature part (below 10 K) of the resistivity characteristic of (BEDO-TTF)₂ReO₄(H₂O) under an isotropic pressure of 1 kbar.

In addition the pressure dependence of the first order phase transition temperature T<sub>MM</sub> (213 K at ambient pressure) was studied. Figure 4 shows the dependence of the phase transition temperature with respect to the applied pressure in the cooling down and warming up cycle.

## Discussion

The resistivity ratio at room temperature of ρc*:ρa:ρb ≈ 1:3:1000 (t<sub>c</sub>:t<sub>a</sub> ≈ 2.5; where t is the transfer integral) indicates that the electronic properties of the (BEDO-TTF)₂ReO₄(H₂O) crystals are not so ideal two dimensional as observed in the radical salts of the BEDT-TTF family where the typical ratio of the resistivities in the conducting plane is about 1:1.5 (t<sub>a</sub>:t<sub>b</sub> ≈ 1). On the other hand the electronic properties of the (BEDO-TTF)₂ReO₄(H₂O) crystals are certainly less anisotropic than those of the TMTSF radical salts where the resistivity ratio is about 1:25:400 (t<sub>a</sub>:t<sub>b</sub> ≈ 10) [4].

Since the structural data indicate a slight disorder in the ReO₄⁻-anions at room temperature [1] it was assumed that the ReO₄⁻-anions do order at the phase transition at 213 K (see fig. 1) similar as in (TMTSF)₂ReO₄ where the ordering of the ReO₄⁻-anions appears at 180 K and leads to a metal-insulator transition [5]. Here in (BEDO-TTF)₂ReO₄(H₂O) the observed transition

<figure><img src="image_5.png" /></figure>

Figure 2: Temperature dependence of the resistivity (normalized at 30 K) parallel to the c*-axes of single crystals of (BEDO-TTF)₂ReO₄(H₂O) below 30 K at ambient pressure and an isotropic pressure of about 300 bar and 1 kbar.
<figure><img src="image_6.png" /><figcaption>Figure 4: Pressure dependence of the metal-metal phase transition temperature $T_{MM}$ of single crystals of $(BEDO-TTF)_2ReO_4(H_2O)$ in the cooling down and warming up cycle. The drawn curves represent the equation</figcaption></figure>

$$T_{MM} = T_{MM}(0) + a \cdot p^2$$

where $T_{MM}(0)$ is the phase transition temperature at ambiet pressure and $p$ the applied isotropic pressure. The constant $a = \Delta T_{MM}/\Delta p^2 = -0.55 \text{ K(kbar)}^{-2}$ in the cooling down cycle and $a = -0.40 \text{ K(kbar)}^{-2}$ in the warming up cycle.

is a metal-metal phase transition ($T_{MM}$). Therefore it was expected that pressure might increase the transition temperature. In contrast to this expectation the phase transition temperature decreases as can be seen from fig. 4. Recent structural investigations at a temperature below 200 K [6] seem to indicate that in fact a structural

phase transition into a phase with lower symmetry occurs. Therefore it can be understood that the phase transition temperature lowers by applying pressure to the system. Nevertheless, the nature of this structural phase transition seems to be of first order which can be seen from the hysteresis in the resistivity (and thermopower) data (see inset in fig. 1) and the data shown in fig. 4 in the cooling down and warming up cycle. The data in fig. 4 show that by increasing hydrostatic pressure the phase transition temperature $T_{MM}$ shifts to lower values. Hereby the change in phase transition temperature $\Delta T_{MM}$ is almost proportional to the square of the applied pressure. This indicates that a pressure of about 19 kbar would be nessecary in order to suppress this structural phase transition.

In reference 1 it was assumed that disorder phenomena either in the anionic or (and) cationic sublattice might drive the phase transition at around 35 K and that such disorder could lead to the increase in resistivity below 35 K. From figure 2 it can be seen that in fact already an isotropic pressure of 1 kbar is sufficient in order to suppress the increase in resistivity below 35 K. In addition at this pressure the superconducting transition sharpens strongly (see fig. 3) and the onset temperature for superconductivity is shifted to lower temperatures only about 0.2 K. At a pressure of 5 kbar the onset for superconductivity is shifted to 1.4 K. Nevetheless, in order to clear up the nature of the phase transition at 35 K further experiments are in progress.

## Acknowledgement

We gratefully acknowledge financial support of this work by the Forschungsschwerpunkt Supraleiter des Landes Baden-Württemberg and by the Commission of the European Communities [contract CI-CT90-0863 (CD)].

## References

[1] S. Kahlich, D. Schweitzer, I. Heinen, Song En Lan, B. Nuber, H. J. Keller, K. Winzer and H. W. Helberg; Solid State Comm. **80**, 191, 1991.

[2] T. Suzuki, H. Yamochi, G. Srdanov, K. Hinkelmann and F. Wudl; J. Am. Chem. Soc. **111**, 3108, 1989.

[3] M. A. Beno, H. H. Wang, A. M. Kini, K. D. Carlson, U. Geiser, W. K. Kwok, J. E. Thompson, J. M. Williams, J. Ren and M. H. Whangbo; Inorg. Chem. **29**, 1599, 1990.

[4] R. L. Greene, P. Haen, S. Z. Huang, E. M. Engler, M. Y. Choi and P. M. Chaikin; Mol. Cryst. Liq. Cryst. **79**, 183, 1982.

[5] S. S. P. Parkin, D. Jerome and K. Bechgaard, Mol. Cryst. Liq. Cryst. **79**, 213, 1982.

[6] K. Boubekeur, private communication.