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# Pressure-induced metal to insulator phase transitions in the organic conductor 2,5(DM-DCNQ)2Cu

Denis Jerome, A. Aumüller, P. Erk, S. Hünig, J.U.Von Schütz, S. Tomić

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Denis Jerome, A. Aumüller, P. Erk, S. Hünig, J.U.Von Schütz, et al. Pressure-induced metal to insulator phase transitions in the organic conductor 2,5(DM-DCNQ)2Cu. EPL - Europhysics Letters, 1988. hal-04982919

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# Pressure-Induced Metal-to-Insulator Phase Transitions in the Organic Conductor (2,5 DM-DCNQI)₂Cu.

S. TOMIĆ (*) (§), D. JÉROME (*), A. AUMÜLLER (**), P. ERK (**), S. HÜNIG (**)
and J. U. VON SCHÜTZ (***)

(*) Laboratoire de Physique des Solides, Bâtiment 510, 91405 Orsay Cedex, France

(**) Institut für Organische Chemie der Universität Würzburg Am Hubland
D-8700 Würzburg, W. Germany

(***) III. Physikalisches Institut der Universität Stuttgart
D-7000 Stuttgart 80, W. Germany

(received 19 October 1987; accepted in final form 11 January 1988)

PACS. 71.30 - Metal-insulator transitions.

PACS. 72.80L - Organic semiconductors.

PACS. 72.90 - Other topics in electronic transport in condensed matter.

**Abstract.** - The electrical resistivity along the high-conductivity axis has been measured at pressures up to 9 kbar in the novel organic conductor (2,5 DM-DCNQI)₂Cu. Deviations from the usual linear behaviour of the conductivity under pressure at room temperature have been observed. Moreover, at lower temperatures metal-to-insulator phase transitions have been induced with values of transition temperatures depending on the applied pressure. The obtained results are discussed and compared with corresponding data for other organic compounds. A tentative interpretation of this novel and unusual phase diagram is proposed in terms of a mixed valence state for copper ions.

*Introduction.* - The vast domain of organic quasi-one-dimensional compounds is currently one of the most attractive research fields connecting organic chemistry and solid-state physics. In particular, the stabilization of the metallic state in the large temperature domain appears to be quite a difficult task. Recently, Aumüller *et al*. [1] have reported new radical anion salts based on the organic acceptor N,N'-dicyanoquinonediimines (DCNQI). The investigated compounds with Li, Ag and Cu as counterions are all isomorphous. The large cation-cation distances along the stacking axis prevent electron conduction along metal-ion chains. Consequently, these materials could be considered as single-chain organic conductors in which the 1/4 conduction band is formed by the overlap of pπ DCNQI orbitals on the organic chains. In this regard, one would expect that the one-dimensional nature of (2,5 DM-DCNQI)₂X compounds is quite important and consequently that the instability towards a dielectric ground state is likely to arise at low temperatures. Indeed, two of them (X = Li, Ag) exhibit a metal-to-insulator transition at about 100 K [2].

<footnote>(§) Permanent address: Institute of Physics of the University, POB 304, 41001 Zagreb, Yugoslavia.</footnote>
However, the compound with Cu as a counterion differs from the others by having a much higher room temperature (RT) conductivity (~800 (Ω cm)⁻¹ in contrast to ~20 (Ω cm)⁻¹ for the Ag compound). Furthermore, it displays a metallic behaviour down to the lowest accessible temperature (450 mK) [2]. A close examination of the crystal structure reveals that the unit cell parameter along the stacking axis is hardly affected by the choice of cation, while those along the axes perpendicular to the stacking one vary with the counterion. It is important to point out that each metal-ion stack is surrounded by four anion stacks and consequently, the cations are four-fold coordinated with nitrogen atoms in a tetrahedral symmetry. Moreover, it is only in the case of the Cu compound where Cu-N distance (1.98 Å) is of the order of chemical bond and, therefore, an additional conduction path from stack to stack via the N-Cu-N bridge is possible.

*Experimental measurements and results.* – Hereafter we report about measurements of the electrical conductivity under pressure of (2,5 DM-DCNQI)₂Cu [3]. Twelve single crystals with typical length of about 2.5 mm and cross-section of about 2·10⁻⁴ mm² were mounted in a four-probe configuration for the measurements along the high-conductivity axis. The standard low-frequency a.c. technique was used.

The RT conductivity starts to increase with pressure at an initial rate of about 15% kbar⁻¹. While for the Ag compound it continues to increase up to the highest applied pressure of 8.5 kbar [4], in the case of the Cu compound it begins to deviate already at about (2÷3) kbar, reaches a maximum value at about (4÷5) kbar and then decreases again (fig. 1).

<figure><img src="image_3.png" /><figcaption>Fig. 1. – Normalized conductivity σ(p)/σ(1 bar) of (2,5 DM-DCNQI)₂X (X = Cu (○), Ag (●)) at room temperature as a function of applied pressure.</figcaption></figure>

The latter behaviour is quite unusual, since in all quasi-one-dimensional organic materials conductivity shows a linear rise under pressure, the ratio of which depends on a particular compound. For example, the standard rate of about 30% kbar⁻¹ is observed for (TMTSF)₂X,
(TMTTF)₂X, TTF-TCNQ, Qn(TCNQ)₂ etc. [5]. However, the initial value of about 11% kbar⁻¹ and a tendency to saturation at higher pressures was found in the organic conductors (perylene)₂*M*(M-NT)₂, where *M* = Pt, Au [6].

The obtained temperature-pressure phase diagram for the Cu compound reveals quite unusual features of this material. It can be divided essentially into three pressure domains: I) high pressure (h.p.) for *p* ≥ 6 kbar, II) intermediate (i) for 3.5 kbar ≤ *p* < 6 kbar and III) low pressure (l.p.) for *p* ≤ 3.5 kbar. The highest metal-to-semiconductor transition line, as obtained on cooling, is shown in fig. 2.

<figure><img src="image_4.png" /><figcaption>Fig. 2. – Temperature-pressure phase diagram of (2,5 DM-DCNQI)₂Cu. Open circles for the highest transition temperature (*T*<sub>c1</sub>) obtained on cooling.</figcaption></figure>

I) A typical behaviour of the resistivity *vs.* temperature at 7.4 kbar is shown in fig. 3. One can clearly distinguish two phase transitions towards a semiconducting state: *T*<sub>c1</sub> = 249 K and *T*<sub>c2</sub> = 179 K. Cooling and warming cycles are essentially the same apart from the additional anomaly at about 220 K observed on cooling.

<figure><img src="image_5.png" /><figcaption>Fig. 3. – Logarithm of normalized resistivity of (2,5 DM-DCNQI)₂Cu at 7.4 kbar *vs.* inverse temperature. ○ cooling, ● warming.</figcaption></figure>
II) In the intermediate pressure domain there is a pronounced hysteresis between cooling and warming cycles. Moreover, below about 4.5 kbar measured samples undergo a sharp transition to an insulating state on cooling ($T_{c1}$), while the warming cycle reveals clearly two well-distinguished phase transitions (fig. 4). Note that back to RT, measured samples recover their initial RT resistance.

<figure><img src="image_6.png" /><figcaption>Fig. 4. — Logarithm of normalized resistivity of (2,5 DM-DCNQI)₂Cu at 4.2 kbar vs. inverse temperature. ○ cooling, ● warming.</figcaption></figure>

III) The low-pressure region is the most intriguing one in which measured samples undergo an extremely sharp metal-to-insulator phase transition at 70 K ($T_{c1}$). Note that a resistivity rise of about 6 orders of magnitude occurs in the temperature range of about 5 degrees below the transition. No clear hysteresis is observed. Note also that the low-pressure data (~300 bar) have been obtained after decreasing pressure from some higher value.

*Discussion.* — The (2,5 DM-DCNQI)₂Cu compound shows some marked differences in its chemical and physical properties compared to other one-chain organic systems. The ones we have found in this study concern the variation of the RT conductivity under pressure and the phase diagram under pressure.

The change of the behaviour of the RT conductivity ($dσ/dp$) from positive to negative slope at about (4÷5) kbar is followed by the increase of a metal-to-insulator transition temperature in the phase diagram. Furthermore, the two phase transitions which could be distinguished in the h.p. region are rather smooth, while the only one which appears at in the l.p. region is extremely sharp. Moreover, the i.p. region with its strongly pronounced hysteresis effects reveals that two h.p. transitions collapse into each other giving rise to the one observed at low pressures.

We turn now to the discussion of the most unusual property of the (2,5 DM-DCNQI)₂Cu salt, namely its *p*-T phase diagram which shows that an insulating ground state is stabilized under pressure. This is definitely not the situation which has been encountered so far in the study of low-dimensional organic conductors. On the one hand, for the (TMTSF)₂X series
the insulating ground state at ambient pressure (antiferromagnetic or anion ordered) is suppressed by the application of a pressure of 6 to 10 kbar depending on the nature of the anion (X = PF₆, FSO₃, ReO₄ etc.) [5]. On the other hand, the Peierls insulating ground state of two-chain conductors such as TTF-TCNQ, TSF-TCNQ etc. is only hardly affected by the application of a high pressure (except for commensurability effects) [5]. In this respect, the stabilization of an insulating ground state in (2,5 DM-DCNQI)₂Cu under pressure is quite remarkable and deserves some comments.

First, the pressure-induced insulating ground state of (2,5 DM-DCNQI)₂Cu is very likely due to the onset of a structural distortion and not to some spin density wave mechanism. This statement is based on the observation by Kobayashi *et al.* [7] of superlattice reflections below 210 K associated to a metal-to-insulator transition in the parent compound (MeCl-DCNQI)₂Cu. In addition, the behaviour of the ESR susceptibility and linewidth of the isostructural Ag salt point towards a nonmagnetic ground state [3].

Secondly, a particular feature of (2,5 DM-DCNQI) salts is the role of ligand played by the organic anion between metallic cations: transition metals Cu, Ag or alkali metals Li, Na and K through strong nitrogen-metal-nitrogen coordinate bonding. For the silver salt, the 3d levels of Ag⁺(3d¹⁰) do not take part in the density of states at the Fermi level, since all d-orbitals are complete and they are located below the Fermi energy of the organic 1/4 filled band [2, 3].

As far as the copper salt is concerned, there exists the possibility for the metallic cation to take an intermediate valence state between +1 and +2 in the tetrahedral crystal field. Some experimental features (X-ray, XPS and UPS data [7, 8]) do point towards this direction, namely Cu⁺¹³⁄₂, and suggest the existence of a strong mixing between Pπ orbitals of the organic molecules and d-levels of the transition metal cations. Hence, partially filled d-levels contribute to the density of states and to the Fermi surface, as well.

Keeping that in mind, it would appear that it is mainly a Cu oxidation state which is at the origin of a departure from a simple one-dimensional band structure and therefore, influences the stabilization of a particular ground state. It is also worth noticing that the Cu-mixed valent state at 1 bar could be understood as a consequence of a slightly distorted tetrahedral coordination of Cu atoms to the N atoms of four N-cyano groups. Namely, the perfect tetrahedral geometry is compatible with a +1 oxidation state, while a distorted one is expected to favour a +2 oxidation state [9]. The effect of applied pressure and of lowering the temperature could be to change the existing Cu coordination geometry. Even if such changes would not influence the initial valency of copper ions, one might expect changes in the band structure and, therefore, a different low-temperature behaviour [7].

Consequently, it is tempting to propose that the internal structure of the *p*-T phase diagram may be a consequence of a change in the environment of the Cu cations and/or their oxidation state. Moreover, the low- and high-pressure phase at 300 K might correspond to two, essentially different, Cu states. In that view, in the intermediate-pressure region (near critical *p*<sub>c</sub> ~ (4 ÷ 5) kbar) these two phases would coexist and the competition between their corresponding ground states would reflect itself in the low-temperature behaviour (hysteresis effects).

Indeed the structural data of Kobayashi *et al.* [7] on the parent compound (MeCl-DCNQI)₂Cu show that lowering the temperature makes copper tetrahedral environment more distorted and the distance Cu-N shorter. Assuming that the effect of applied pressure corresponds to that of decreasing the temperature one would expect that pressure makes Fermi surface resemble more and more the surface of a one-dimensional conductor with consequently enhanced possibilities for electronic instabilities driven by Fermi surface nesting.

Finally, we are aware that the high-pressure technique used in the present work does not
allow a very accurate determination of the actual pressure value at low temperatures in the low-pressure region ($p \le 5$ kbar). This pressure domain should deserve in the future an investigation by the helium gas pressure technique [10].

***

We acknowledge the skillful technical assistance of J. C. AMELINE and useful discussions with P. BATAIL. One of us (ST) would like to thank CNRS for a temporary associate research position at Orsay in 1986-87.

## REFERENCES

[1] AUMÜLLER A., ERK P., KLEBE G., HÜNIG S., VON SCHÜTZ J. U. and WERNER H. P., *Angew. Chem. Int. Ed. Engl.*, **25** (1986) 740.

[2] VON SCHÜTZ J. U., WERNER H. P., WOLF H. C., AUMÜLLER A., ERK P. and HÜNIG S., *Ampere Colloquium*, Rome (1986).

[3] A preliminary report of this work has been given by J. U. VON SCHÜTZ in the paper by VON SCHÜTZ J. U., BAIR M., WERNER H. P., WOLF H. C., AUMÜLLER A., ERK P. and HÜNIG S., *Nato ASI series volume: Organic and inorganic low-dimensional crystalline materials*, edited by P. DELHAES and M. DRILLON (1987).

[4] TOMIĆ S., JÉROME D. *et al.*, to be published.

[5] JÉROME D. and SCHULZ H. J., *Adv. Phys.*, **31** (1982) 299.

[6] HENRIQUES R. T., ALCACER L., JÉROME D., BOURBONNAIS C. and WEYL C., *J. Phys. C*, **19** (1986) 4663.

[7] KOBAYASHI A., KATO R., KOBAYASHI H., MORI T. and INOKUCHI H., *Solid State Commun.*, **64** (1987) 45.

[8] SCHMEIßER E. *et al.*, to be published.

[9] COTTON F. A. and WILKINSON G. (eds.), *Advanced Inorganic Chemistry* (J. Wiley and Sons, New York, N. Y.).

[10] TOMIĆ S., JÉROME D. *et al.*, to be published.