# Influence of the Anion Order on the Ground State of the Organic Conductor (TMTSF)2 ReO4

S Tomic, Denis Jerome, K Bechgaard

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S Tomic, Denis Jerome, K Bechgaard. Influence of the Anion Order on the Ground State of the Organic Conductor (TMTSF)2 ReO4. Molecular Crystals and Liquid Crystals, 1985, 119, pp.241 - 244. 10.1080/00268948508075164. hal-05010406

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# Influence of the Anion Order on the Ground State of the Organic Conductor (TMTSF)² ReO⁴

S. Tomicª, D. Jéromeª & K. Bechgaardª

ª Laboratoire de Physique des Solides, Bât, 510 91405, Orsay, France

Published online: 17 Oct 2011.

**To cite this article:** S. Tomic, D. Jérome & K. Bechgaard (1985) Influence of the Anion Order on the Ground State of the Organic Conductor (TMTSF)² ReO⁴, Molecular Crystals and Liquid Crystals, 119:1, 241-244

**To link to this article:** http://dx.doi.org/10.1080/00268948508075164

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INFLUENCE OF THE ANION ORDER ON THE GROUND STATE OF
THE ORGANIC CONDUCTOR (TMTSF)₂ReO₄

S. TOMIC*, D. JEROME and K. BECHGAARD⁺

Laboratoire de Physique des Solides, Bât. 510

91405 - Orsay (France)

**Abstract** - We argue that the low temperature behaviour
and the ground state in (TMTSF)₂ReO₄ are sensitive to
the anion ordering which can be controlled by pressure
and cooling rate.

(TMTSF)₂ReO₄ undergoes a metal-insulator transition at 180 K¹
as a consequence of the anion ordering (AO) with a wave vector
(1/2,1/2,1/2)². This transition can be suppressed by pressure and
SC is established at pressures higher than 9 kbar³. A detailed
investigation of the influence of the cooling rate on the ground
state of (TMTSF)₂ReO₄ through carefully controlled thermal treat-
ments at pressures around 10 kbar has been already published⁴.
Here we briefly show these results (Fig. 1 and Fig. 2) and present
new ones obtained at 11 kbar (Fig. 3 and Fig. 4). They can be sum-
marized as follows. (i) Rapid cooling down to the temperature
where the resistivity displays a maximum and slow cooling below
drives a system to an SC ground state at the highest attainable
critical temperature (T<sub>sc</sub> = 1.52 K and 1.46 K at 10.5 and 10 kbar,
respectively). In contrast, the opposite procedure drives the
system into the semiconducting state with very high resistivity
and SC transition is decreased to and below 1.3 K, at 10.5 kbar
and 10 kbar, respectively. (ii) There is an opposite hysteresis
effect in the T-region above and below the resistivity peak, when
the warming rate was much slower than the cooling one. In particu-
lar, in the T region below the peak, the resistivity is lower on
warming, while it is much higher above. (iii) At 10 kbar there is

Permanent address : Institute of Physics of the University
P.O.B. 304, 41001 Zagreb (Yugoslavia).

+Permanent address : H.C. Oersted Institute, Universitetsparken 5,
DK - 2100 Copenhagen (Denmark).
<figure><img src="image_3.png" /><figcaption>FIGURE 1</figcaption></figure>

Normalized resistivity vs T on cooling at 10.5 kbar
Open and solid symbols for slow and rapid cooling, respectively. Note the thermal hysteresis effect in run (c) as indicated by dotted line.

an upturn in the resistivity, above the SC transition, whose
amplitude and the onset temperature depend on the cooling rate.
(iiii) Above 11 kbar when the metallic behaviour is restored ³, an
anomaly in the resistivity is observed around 30 K and the SC
critical temperature reaches 1.66 K.

<figure><img src="image_4.png" /><figcaption>FIGURE 2 - Normalized resistivity vs T at 10 kbar on cooling. Open and solid symbols for low and rapid cooling T-regions, respectively.</figcaption></figure>

(TMTSF)₂ReO₄ is a quasi-1D metal at ambient pressure and
high temperatures (HT) as its parent
compound (TMTSF)₂ClO₄. Anions are
distributed statistically in two
symmetrically equivalent positions
and so create a pseudouniform
Coulomb potential along organic
chains which does not disturb the
electron conduction. On cooling this
structure changes either in (i) non-
ordered structure obtained by fast
quenching which freezes in HT phase
(ex: the Q-state in (TMTSF)₂ClO₄²⁻)
(ii) new ordered structure establi-
shed below a phase transition driven
by the anion-electron coupling. If
a wave vector is such that the unit
cell is doubled only along the b
(or c) direction, the system enters
<figure><img src="image_5.png" /><figcaption>FIGURE 3 - R vs T curve at 11 kbar. The dotted line is a guide for the eye.</figcaption></figure>

a SC ground state (ex: a structure $q_2$ = (0,1/2,0) at 1 bar in ($TMTSF_2ClO_4$, $T_{AO}$ = 24 K)². The other case can be that a wave vector is such that a gap opens in the electronic spectrum at the FS and the system enters in an insulating ground state (ex: a structure $q_1$ = (1/2,1/2,1/2) at p = 1 bar in ($TMTSF_2ReO_4$, $T_{AO}$ = 180 K). Such a transition can be suppressed either by pressure, or by fast quenching, or again combining the both effects. Once the metallic behaviour is restored and the SC ground state established, the question arises about the structure of a LT phase.

This question already can be asked at lower p when the system at higher T displays semiconducting behaviour and a metallic one at LT, before entering in the SC ground state. First possibility is that the pressure restores HT phase and the other is that it transforms a structure $q_1$ into a new one compatible not only with a single-particle conduction but also with SC. As pointed out by

<figure><img src="image_6.png" /><figcaption>FIGURE 4</figcaption></figure>

R vs T curve at 11 kbar below 4 K. The SC transition width is about 0.11 K, $T_{SC}$ = 1.66 K and it is suppressed by the field of 400 gauss.
Emery¹, there are only two "good" anion structures for SC due to the existence of a short Se-O contact between each TMTSF molecule and one nearly non-centrosymmetric anion. These are (0,1/2,0) and (0,0,1/2) which create an uniform potential along conduction chains. Regarding HT phase we expect that the thermic motion of the anions ceases at LT leading so to a certain non-ordered structure. Therefore, it is tempting to interpret a behaviour of (TMTSF)₂ReO₄ under pressure as a consequence of a transformation from a (1/2,1/2,1/2) AO to a (0,1/2,0) or (0,0,1/2) AO, as described in a theoretical model by Bruinsma and Emery ¹. Our experiments favour the latter interpretation for the following reasons (i) for pressures 9 kbar<p<11 kbar slow cooling below a peak in the resistivity gives lower resistance than a rapid one ; (ii) even fast cooling throughout all T-region down to 4.2 K drives the system into the SC ground state and we know from (TMTSF)₂ClO₄ in the Q-state that a frozen disorder suppresses SC ; (iii) for p > 11 kbar there is a kink in R vs T curve below which the resistivity decreases more rapidly, similarly to the ClO₄ case in the R-state at p = 1 bar.

We wish to acknowledge fruitful discussions with W.J. Emery.

REFERENCES

1. C.S. Jacobsen, H.J. Pedersen, K. Mortensen, G. Rindorf, N. Thorup, J.B. Torrance and K. Bechgaard, _J. Phys. C_ : _Solid State Phys._, **15**, 2657 (1982).

2. Proc. Int. Conf., Les Arcs 1982, J. Physique, C3 (1983).

3. S.S.P. Parkin, D. Jérôme and K. Bechgaard, Molec. Cryst. Liq. Cryst., **79**, 213 (1981).

4. S. Tomic, D. Jérôme and K. Bechgaard, _J. Phys. C_ : Solid State Phys., **17**, L11 (1984).