A new survey of the physical properties of the
(TMTTF)2 X series. Role of the counterion ordering
C. Coulon, P. Delhaes, S. Flandrois, R. Lagnier, E. Bonjour, J.M. Fabre
To cite this version:
C. Coulon, P. Delhaes, S. Flandrois, R. Lagnier, E. Bonjour, et al.. A new survey of the physical
properties of the (TMTTF)2 X series. Role of the counterion ordering. Journal de Physique,
1982, 43 (7), pp.1059-1067. <10.1051/jphys:019820043070105900>. <jpa-00209482>
HAL Id: jpa-00209482
https://hal.archives-ouvertes.fr/jpa-00209482
Submitted on 1 Jan 1982
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J.
Physique 43 (1982)
1059-1067
JUILLET
1982,
1059
Classification
Abstracts
64.70K - 71.30
Physics
A new survey of the physical properties of the
Role of the counterion ordering
(TMTTF)2 X series.
C. Coulon, P. Delhaes, S. Flandrois
Centre de Recherche Paul Pascal, Domaine Universitaire, 33405 Talence, France
R.
Lagnier,
E.
Bonjour
C.E.N.G., Service Basses Températures, 38041 Grenoble, France
and J. M. Fabre
Laboratoire de Chimie Structurale
(Reçu le 5 octobre 1981, révisé le
Organique (U.S.T.L.),
14
34060
décembre, accepté le 3
mars
Montpellier,
France
1982)
On compare les propriétés physiques des sels (TMTTF)2X (en particulier pour X
Résumé.
BF4, ClO4, PF6
et Br). Ceci nous permet de discuter du rôle de la mise en ordre du contre-ion et de mettre en évidence les propriétés intrinsèques des chaînes de TMTTF. On peut alors établir un parallèle avec les propriétés des analogues
séléniés de ces sels.
=
2014
Abstract.
From the comparison of the physical properties of the (TMTTF)2X salts (mainly X
BF4, ClO4,
PF6 and Br), the role of the counterion ordering is discussed and the intrinsic behaviour of the TMTTF chains is
evidenced. Then, a parallel is drawn with the properties of the selenium analogs of these salts.
=
2014
The recent discovery of high pressuperconductivity in the one chain compound
(TMTSF)2PF6 (bis tetramethyltetraselenafulvalene
hexafluorophosphate) [1] was the precursor of an
intensive study of all the series of the TMTSF salts
1. Introduction.
-
sure
which have revealed some remarkable distinctive
properties of these compounds. The most striking
are : a very high electrical conductivity (more than
105 (1 - ’ cm -1) at low temperature [2] and a competition between a spin density wave (SDW) insulating
ground state [3, 4] and a superconductive state which
can appear at ambient pressure in the CIO 4 (perchlorate) compound [5]. Besides, the usual charge
density wave (CDW) instability is absent in these
materials in which any « 2 kF » or « 4 kF » condensed
superstructure cannot be detected by the diffuse X-ray
technique [6]. The origin of such a distinctive behaviour is still obscure. However, from Barisic’s point of
view, the zig-zag structure of the conducting chain
and the given stoichiometry play an important role
through the occurrence of an external potential
with the wave vector 4 kF coupled with the conduction
electrons [7]. The (TMTTF)2X salts (bis tetramethylte-
trathiofulvalene salts) are known to be isostructural
to their selenium analogs and thus present the same
characteristics. Therefore, the detailed study of their
physical properties can be of prime importance for the
understanding of organic superconductivity. The
recent discovery of superconductivity at about 4 K
and 25 kbar for the bromine salt [8] of this series
clearly supports this point of view.
A first study of the physical properties of the
TMTTF salts has been already published [9]. It was
limited, in particular for magnetic measurements, by
the quality of the samples. We report in this paper a
more complete set of experimental data obtained with
new electrochemical batches allowing a detailed discussion of the low temperature behaviour of these
salts. We will evidence both the role of the counterion
ordering and the intrinsic properties of the TMTTF
chains which can be compared with that of their
selenium analogs. For each physical property we will
present consecutively the phase transition and the
high (and the low) temperature characteristics. This
double aspect will allow us to understand the fundamental role played by the counterions.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019820043070105900
1060
2. Synthesis and characterization of the samples.
Besides the first method already described [9], samples
of better purity have been obtained by the electrochemical technique as described by Bechgaard [2]. The
two routes give samples with the same well-defined
stoichiometry : (TMTTF)2X. Salts with X- BF4,
C104, PF6, N03 , I-, Br-, SCN- have been prepared
and were shown to be isostructural [10, 11]. By electrocrystallization, we have prepared again the Br, PF6
and CI04 salts on which we have particularly focused
our attention in this work. As already noted the structural organization of TMTSF2-X compounds is
also similar to that of this series [12]. Their main
features are schematically reported in figure 1. They
-
try [10, 11]. In particular, for tetrahedral ions such
BF4 or C104 two equivalent statistically occupied
positions are observed at least at ambient temperature [10]. In the following we will distinguish two sets
of anions according as they are centrosymmetric or not.
as
=
3. Review of the
physical properties.
The
-
3.1 ELEC-
temperature
value of a I,, the conductivity along the stacks is reportTRICAL
CONDUCTIVITY.
-
room
are :
the occurrence of a zig-zag stacking of the
TMTTF molecules along the a direction, these chains
being grouped in planes separated by counterions
arrays. Thus, the shortest interchain distances between
sulfur atoms are in the b direction;
the existence of well defined mean positions for
the counterions in the centre of the « cavities » induced
by this structure. However, unusual disorder of the
anions is detected which depends on their symme-
-
Fig. 2. a) Temperature dependence of the electrical conductivity of the TMTTF series. Note the semi-log plot.
b) Behaviour of the logarithm derivative of all for the SCN
(left side) and Br compounds (right side). c) Detailed behaviour of the resistivity of (TMTTF)2C’01 in the vicinity
of the critical temperature. The hysteresis is clearly visible.
The dashed curve gives the temperature dependence for
-
Fig. 1.
reported
-
Crystallographic
[111.
from
structure of
(TMTTF)2PF6
the
PF6
salt.
1061
Table I.
-
Electrical and
crystallographic data for
the series
of TMTTF salts :
the S-S distances
are
reported
from references [10, 11].
ed in table I for the different salts of the series. The
corresponding temperature dependences of pjjI = a - 1
are given in figure 2a (note the semi-log scale).
Cracks » associated with a sudden increase of the
absolute value of the resistivity usually occur during
the thermal cycling of the samples. However, using
several crystals, complete curves can be constructed.
The data plotted in figure 2a were obtained by this
method. In every case, a broad maximum of conducti«
vity
occurs at
relatively high temperature (although
the metallic character of the Br compound is more
pronounced), then the samples become insulating
at lower temperature. According to the symmetry
of the anions different types of anomalies can be
detected on these curves.
A weak anomaly is only detected for the bromine
salt at low temperature (Tc 19 K see Fig. 2 c).
Moreover, with the exception of the SCN compound,
these anomalies (when they exist) occur well below
the temperature of the resistivity minimum independently of the symmetry of the anions. Thus, we can
compare the semiconductor like regime of the different salts.
For the PF6, I, N03, BF4 salts the resistivity is
simply activated in this temperature range, with an
activation energy value of about 600 K ; in comparison
the bromine compound exhibits an activation energy
lower than for these salts.
The paramawith
a
measured
Faraday balance
gnetic susceptibility
was already reported [9] for the BF4, CI04 and Br
salts. For the last one the presence of magnetic impurities prevented any diagnostic about the existence of a
low temperature instability. However, the temperature dependence of the susceptibility of the BF4 and
CI04 compounds evidences clearly a phase transition
respectively at about 40 K and 72 K.
More accurate data obtained with a new batch of
CI04 salt is reported in figure 3. The phase transition
is detected at 75 K and a small bump is also visible
around 10 K : this result is in agreement with the
resistivity measurements. It contrasts with the curves
obtained for the PF6 compound where a phase transition is detected at a lower temperature (Tr -- 15 K).
The paramagnetic susceptibility of the NO 3 salt is also
reported. Unfortunately, the quality of the sample does
not allow any accurate determination of a phase transition.
3.2 MAGNETIC
3.1.1 Anions without inversion symmetry (BF4 ,
C104 , N03 , SCN’). 2013 For the SCN compound a
d Ln (J
in tthee
versus T
shar maximum is detected In
d(1/T)
etecte
at 160 K (cf Fig.
For the CI04 salt, an important jump of resistivity
25 % A strong hysteresis
occurs at 75
curve
K B!AR" ! _
/
is visible (see Fig. 2c). This anomaly is not associated
with a noticeable change of the « regular » slope of the
resistivity which is similar in this temperature range to
that of the PF6 sample (dashed curve in Fig. 2c).
In the case of the BF4 and N03 salts we do not
detect any anomaly but we will see in the following that
at least for BF4, it occurs in a temperature range which
cannot be investigated by resistivity measurements.
3 .1. 2
Centrosymmetric anions (PF6, Br, I).
category of counterions
-
This
contrasts with the first one.
SUSCEPTIBILITY.
-
1062
the narrowest component of the linewidth tensor is
observed, this result however, is significant because
the linewidth anisotropy is constant for the considered
series.
The three components of the g-factor are temperature independent as usually observed in one chain
compounds. The results are presented for one direction in the present study (Fig. 4). The linewidth
decreases with T in the high temperature phase.
Below 100 K the difference between the three salts is
clearly visible :
the linewidth is monotonic above 20 K for
the centrosymmetric anions PF6 and Br with a
minimum around this temperature. For the Br salt
a sharp peak is detected at 16 K. At the same temperature a sudden decrease in the spin-lattice relaxation
time T, has been observed by NMR proton spectroscopy at Orsay which is attributed to the occurrence
of SDW fluctuations at the metal insulator transition [13] ;
on the contrary the linewidth of the CIO 4
compound has a more complicated temperature
dependence, with a break at 75 K and a large bump
around 20 K which might be correlated to the anomaly of the magnetic susceptibility clearly visible in
this temperature domain (see Fig. 3).
-
Fig. 3.
Paramagnetic susceptibility of the PF6, N03,
CIO4 compounds.
-
-
Finally, it must be quoted that the room temperature value of the magnetic susceptibility is nearly
independent of the counterion
(xp -
5-6
x
10-4
emu
and weakly temperature
transition.
CGS/mole at 300 K)
already noted, the room temperature linewidths
(AH300K - 5 G). This will be correlated with the electronic dimensionality of the compounds in the following discussion.
As
dependent
above the
phase
We report briefly in
3.3 EPR SPECTROSCOPY.
4
the
linewidth
and
g-factor EPR data for the
figure
are
very narrow
-
Fig.
4.
-
EPR data for the
CIO4, PF6
and Br
compounds.
PF6, Br and C’04 salts; the temperature dependence
for the BF4 salt has been already reported [9]. These
results have been obtained with a Varian X band
spectrometer, the needle axis of the crystals being
parallel to the static magnetic field. For this position,
3.4 SPECIFIC HEAT. - The temperature dependence
of the specific heat of the BF4 compound was reported
previously [9]. This data is compared with the corresponding curves for the PF6 and CIO 4 salts in
figure 5a.
The behaviour of the BF4 and CI04 compounds
is rather similar : a sharp anomaly is clearly visible
for these two salts respectively at 41 K and 75 K.
However, for the last one the anomaly is quite large
and is spread from 50 K to 80 K as shown in figure 5b
(for comparison the specific heat of the BF4 salt is
also plotted at the same scale). As is usually done for
the study of structural phase transition we have
calculated the excess entropy AS(T) defined by :
where LBCp(T) is the anomalous specific heat obtained
after subtraction of the normal specific heat determined by smooth interpolation of the low and the
high temperature dependences of Cp (light full line
in figure 5b).
AS(T) for the CI04 compound is given in figure 5c.
It is clear from this figure that the excess entropy
is discontinuous at 75 K. The bump visible between
45 and 75 K in the Cp temperature (Fig. 5b) is revealed
as a continuous increase ofAS(T). The excess entropy
1063
any temperature. Indeed the obtained Debye temperatures are respectively 51 K, 59 K and 55 K for the
BF4, C104 and PF6 compounds.
properties. 4 .1 THE
STATE.
Organic conductors are
usually considered as synthetic metals when their conductivity is large enough (0’11(300 K) &#x3E; 100 0 - I cm -1
for example) and increases rapidly with decreasing
4. Discussion of the physical
« METALLIC »
(0’11 (T) -...;
1 to 2). The
of
this
class of salts.
examples
(TMTSF)2X’
However a large number of compounds behave
differently. Although there is no gap at their Fermi
level from a band structure point of view (one electron
approximation), their conductivity is smaller
(III (300 K) - 20-100 fl-’ cm-’) and shows a broad
maximum at high temperature. This behaviour is
particularly revealed for the (TMITF)2X’ salts because
the phase transition occurs only at relatively low
temperature, except for the SCN compound (the
transition temperatures are collected in table I.
They are discussed in the following). Thus, even
in the high temperature phases they sometimes appear
to be semiconductor like. This paradox can be explained by the low dimensionality of these materials.
It is well known that a real metal is not truly metallic
in one dimension. The smallest disorder for example
gives a localization of the electrons [14], but this
effect can also be induced by electron-electron [15]
or electron phonon interaction [16]. This localization
is weakened but also appears in two dimensions.
We expect this effect to be relevant for organic conductors of low dimensionality when t 1. the interchain
transfer overlap is very weak. We think that it is the
explanation for the apparent semiconducting regime
of the TMTTF salts. To support this point of view
one can note that the magnetic susceptibility in the
high temperature phase is metal-like (high and nearly
temperature independent value of Xp). This point of
view is also in agreement with the behaviour of
irradiated metallic samples : even if the metal insulator
transition temperature decreases, one observes a
broadening of the conductivity maximum which
appears at a higher temperature than in the pure
sample [17, 18]. This is also in agreement with the
behaviour of the TMTTF salts under pressure :
they become quickly metallic even if the temperature
of the metal insulator transitions are not significantly
temperature
salts
Fig. 5. a) Specific heat of BF 4’ CIO, and PF6 compounds.
b) Detailed behaviour of the specific heat of the BF, and
CIO4 compounds plotted at the same scale. The light full
line is considered as the regular part of Cp and used to calculate the excess entropy. c) Temperature dependence of the
excess entropy of the CIO4 compound. The discontinuity at
75 K is approximately R Ln 2.
-
for the BF4 compound is much smaller as expected
from figure 5b. In this case, the value of AS/R above
the temperature of the phase transition is about 0.2.
The specific heat of the PF6 salt is regular without
any anomaly in the whole temperature range (5 K
to 300 K) within the accuracy of our experiment.
Note, to conclude, that the absolute value of the
normal specific heat of the three salts is similar at
-
-
T-a
ot -
are
changed [19].
A semi-quantitative comparison of the different
salts of the series is possible using the data collected
in table I [20]. From the figure 1, the only appreciable
interchain transfer overlap appears to be in the b
direction. Thus the shortest sulfur-sulfur distance
between chains in this direction gives an estimation
for t 1.. This distance can be correlated to the conducti-
vity data
:
61I
ITa§ 300K,
oo, Tmax,
TO’ allmaxamax (conductivity at ambient
IT 300K
temperature, temperature of the maximum of aII
1064
and ratio of the corresponding value of the conductivity
and of aMOOK). The shorter the S-S distance, the
more pronounced is the metallic character (high
value for
a§oo
and
Imax
max,
a 300K
low value for
B
/
T max .
For example, the bromine compound corresponding
to the shortest S-S distances has also the most metallic
behaviour. More systematically, as far as the dimensionality is concerned, the data of the table I allows
classification of the salts within the series :
means of higher dimensionality than).
We must note however the anomalous value of
Tmax for the BF4 compound which contrasts with its
(&#x3E;
II
max
and 11
value of oril
300K d
’
*
300K
to give a definitive answer to this
role
the
of the impurities or other extrinsic
problem,
defects on this localization process must be clarified.
For example, the results on TMTTF 2Br under
pressure are sensitive to the chemical preparation [8],
and further investigations are necessary to specify
the differences between the samples.
To conclude this discussion, we can remark that
the S-S distances are in every case larger than the
Van der Waals distance for sulfur atoms ( = 3.60 A).
That means that the TMTTF salts have a very low
dimensionality. This is in agreement with the observation of narrow EPR linewidths [21]. As expected
the linewidth of the Br compound is the largest one.
Furthermore,
The main part of
4.2 THE PHASE TRANSITIONS.
this paper is devoted to the study of phase transitions.
In every case, they are metal insulator transitions
(the nature of the metallic state being clarified previously), but they present very different characteristics. As already noted, the samples can be ranged
in two groups according to the symmetry of the anions.
-
low symmetry.
The BF4 and
C104 belong to this group. For these two salts the
phase transition is associated with a detectable specific heat anomaly. In these cases, Pouget et al. [6, 22]
have found that the phase transition is not of Peierls
type but is due to the ordering of the counterion
sublattice. In fact, the data of figure 5b, and 5c for
the C104 are very similar to those observed for usual
order-disorder transitions in inorganic crystals [23,
24, 25] : the specific heat anomaly is extended over
a rather large temperature range, the AS(T) exhibits
a jump at the phase transition. Note that it corresponds
within the accuracy to R Ln 2 per mole of
(TMTTF)2CIO4. This is in agreement with the analysis of the counterion disorder in the metallic phase :
the phase transition seems to be associated with the
loss of the two fold degeneracy in the tetrahedron
positions. This induces the doubling of the longitudinal
periodicity of the counterion sublattice and opens a
4.2.1 Anions
of
-
gap at the Fermi surface of the conduction electrons
in the band structure picture. The magnitude of this
gap can be estimated from the magnetic susceptibility
data assuming that xp(T) below T c obeys an activated
law :
The extrapolated value of the gap at zero kelvin
appears to be much lower than 600 K. This result
could explain that there is no change in activation
energy of the electrical conductivity at T c’ because
at this temperature the gap induced by the localization
process in the metallic phase appears to be larger than
the gap open at the phase transition. The conductivity
gain at Tr could be induced by the loss of disorder.
Note that the ordering of the counterion sublattice
also leads to a more conducting low temperature
phase in (TMTSF)2N03 although the temperature
dependence of all(T) is different [2]. Below Tr ,, the
ordering is not complete and the loss of entropy
when decreasing T is still important (Fig. 5c). This
is in agreement with the EPR linewidth temperature
dependence which presents anomalies around 20 K.
These effects might involve ordering of CH3 groups
of the organic chains.
The BF4 salt also presents a specific heat anomaly
and the transition has been recognized as induced by
the anions [22]. However, the associated jump of
AS(T) is much smaller than the expected value
R Ln 2. To reconcile these two results one can remember that the order-disorder transition is only a limiting
case of the instability. More generally one can describe
a continuous change from this kind of instability to a
purely second order displacive transition [26]. Thus,
a more pronounced displacive character of the phase
transition of the BF4 compound could explain our
specific heat data. This assumption is actually supported by the room temperature crystal structure
of BF4 salt [10] : two statistical positions for the
pyramid with a different centre of gravity have been
distinguished.
To pursue this discussion we must note that the
SCN salt shows a metal insulator transition at high
temperature (Tc 160 K). The study of its electrical
properties has also revealed the extrinsic nature of
this instability [19]. The anion dipolar character could
be important to explain the order of magnitude of Tc.
A complete crystal structure of the low temperature
phase might allow the clarification of this point and
to evidence a possible displacive character in the
phase transition as just proposed for the tetrafluoroborate analog.
In any case, these three salts are examples for which
the phase transition does not reveal any intrinsic
property of the TMTTF chains. This behaviour
contrasts with the salts belonging to the second group.
1065
4.2.2 Anions of high symmetry.
Examples of
of
this
are
compounds
given by the PF6 and
group
the Br salts. In the first one no counterion ordering
has been observed by X-ray analysis. On the contrary,
a weak one dimensional 2 kF instability is detected
around 15 K [6]. Because of the shape of the Br ions
we do not expect any ordering for the second salt.
Thus we think that the physical properties of these
two salts reflect the intrinsic behaviour of the TMTTF
chains. The magnetic and electrical data (Fig. 2b)
give similar values of the gap at 0 K : A(0) = 20 K.
This is in agreement with the low value of Tc ( rr 15 K
for the PF6 salt and rr 16 K for the Br salt as obtained
from EPR data) [27]. In these two salts the low temperature ground state appears to be non magnetic
even if, for the Br salt, magnetic fluctuations are
detected around the phase transition [13]. At least
for the PF6 compound a lattice distortion is detected
at the transition (2 kF superstructure) which can be
considered as the signature of a CDW instability.
The remaining remark is that the transition occurs
for the two compounds at a very low temperature
similar to that of the metal antiferromagnetic transition in the TMTSF salts. This gives a distinctive
character to the TMTTF series and could be related
to the occurrence of superconductivity under high
pressure. Thus a more complete comparison with
the TMTSF series is noteworthy. This will be the
aim of the end of this paper.
-
5. Concluding remarks : comparison with the
At first sight, the behaviour of
TMTSF series.
the TMTTF and TMTSF salts seems to be very
different. An illustration of this remark is given by
the shape of the temperature dependence of the electrical conductivity. But this difference in behaviour can
be explained by the dimensionality of the two series :
the TMTSF salts are known to be much less onedimensional (tl is much larger). This is easily proved
when it is noted that the Se-Se shortest distances
between
neighbouring chains (3.78 A for
(TMTSF)2C’04 for example [12]) are smaller than
the corresponding Van der Waals distance which
is about 4.0 A and the localization process is not any
more effective for the selenium compounds. In that
frame (TMTTF)2Br appears to be intermediate between the other TMTTF salts and the TMTSF series.
However, the study of the phase transitions allows
certain analogies to be evidenced for which the
symmetry of the anions is important.
-
5.1 NON
CENTROSYMAETRIC ANIONS.
-
For the
ordering of anions of low symmetry
can take place at low temperature. The induced
effect on the properties of the conducting chains
has been widely studied for the TMTSF salts. It
depends strongly on the nature of the anion :
For (TMTSF)2N03 a 2 a x b x c superstructure appears at about 41 K, indicating an ordering
two
-
series,
an
of the anions at this temperature [28]. At the same
time, the induced effect on the conducting chains is
rather weak : an anomaly is detected in a(T) which
changes only slightly with increasing pressure [29],
but no effect is seen on magnetic susceptibility [30].
On the opposite for the ReO 4 compound, the
appearance of a 2 a x 2 b x 2 c superstructure [28]
is associated with a metal insulator transition which
disappears by applying pressure [31]. The occurrence
of strong hysteresis at intermediate pressure suggests
that the ordering is not achieved at high pressure
because of kinetic effects. The behaviour of the BF4
compound seems to be similar [31].
-
These two examples suggest a general framework
to discuss the role of the ordering of non centro-
anions. Because of the periodicity 2 a
of the superstructure along the directions of the
chains a gap is opened at the Fermi surface by the
ordering process. However, its effect on the electrical
properties depends strongly on the transverse periodicity of the superstructure.
A comparison with the properties of the TMTTF
salts is straightforward. As already noted an ordering
of the anions has been detected by X-rays technique
for the BF4 and CI04 salts and is suspected by conductivity measurements for the SCN compound.
Because of the localization process affecting a(T)
the induced gap is only visible for the two first salts
from magnetic susceptibility but can be detected
for the SCN salt with conductivity data. Moreover,
the high temperature phase transition is also suppressed under pressure for the SCN salt which behaves
like (TMTSF)2Re04 [19].
Finally, the paramagnetic susceptibility of the N03
salt does not show any anomaly above 20 K and if
an ordering does exist above this temperature its
effect must be comparable with that observed for
the selenium analog.
In conclusion, this comparison demonstrates the
similarities between the sulfur and the selenium
series as far as the problems of anions ordering are
concerned. We point out moreover that the physical
problem is a tridimensional one; in any case, it must
not be considered as a classical 1 d situation.
symmetric
Other analogies
5.2 CENTROSYMMETRIC ANIONS.
observed in the study of the intrinsic properties
of the conducting chains. In this case, three different
instabilities are concerned namely the CDW, SDW
and superconducting instabilities. As already noted,
only the two last ones have been observed in the
TMTSF series.
At ambient pressure, only metal insulator transitions are observed. The low temperature phase is
clearly non magnetic even if magnetic fluctuations
are involved for the Br salt [13]. Moreover, a « 2 kF »
lattice distortion has been detected for the PF6
salt [6].
Thus the ambient pressure ground state seems to
-
are
1066
be a CDW state. However, we must note that it
appears at exactly the same mode (2 kF
I71a)
as the SDW instability and a coupling between the
CDW and the SDW order parameters must be introduced to describe the phase diagram. According to
the value of this coupling a ground state with both
CDW and SDW character is not excluded [32].
Furthermore, the recent discovery of superconductivity at high pressure for TMTTF2Br could be the
sign of the simultaneous occurrence of CDW, SDW
and superconductive ground states in the (T, P) phase
diagram of the TMTTF series. To get a more complete
=
experimental data the study of solid solutions
of TMTTF and TMTSF salts is in progress.
To conclude, we can simply notice that the TMTTF
series could be still more fascinating than its selenium
analog. It reactivates the interest for the sulfur compounds which are not so difficult to prepare as the
selenium ones.
set of
Acknowledgments.
S. S. P. Parkin, J.
We
grateful to Drs
T. Takahashi and
F. Creuzet for fruitful discussions and communications of their results prior to publication.
-
P.
are
Pouget,
References
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[2] BECHGAARD, K., JACOBSEN, C. S., MORTENSEN, K.,
[15]
PEDERSEN, H. J. and TORUP, N., Solid State
Commun. 33 (1980) 1119.
[3] ANDRIEUX, A., JEROME, D. and BECHGAARD, K., J.
[16]
Physique-Lett. 42 (1981) L-87.
[4] MORTENSEN, K., TOMKIEWICZ, Y., SCHULZ, T. D. and
ENGLER, E. M., Phys. Rev. Lett. 46 (1981) 1234.
[5] BECHGAARD, K., CARNEIRO, K., OLSEN, M., RASMUSSEN, F. B., JACOBSEN, C., Phys. Rev. Lett. 46 (1981)
852.
[6] POUGET, J. P., COMES, R., BECHGAARD, K., FABRE, J. M.,
GIRAL, L., Physica 108B (1981) 1187.
POUGET, J. P., Congrès de la « Société Française de Physique » Clermont-Ferrand (1981).
[7] BARISIC, S., Congrès de la « Société Française de Physique » Clermont-Ferrand (1981).
S. S. P., CREUZET, F., RIBAULT, M., JEROME,
PARKIN,
[8]
D., BECHGAARD, K., FABRE, J. M., International
conference on low dimensional conductors (Boulder Colorado Aug. 1981) Mol. Cryst. Liq. Cryst.
79 (1982) 605.
[9] DELHAES, P., COULON, C., AMIELL, J., FLANDROIS, S.,
TORREILLES, E., FABRE, J. M. and GIRAL, L.,
Mol. Cryst. Liq. Cryst. 50 (1979) 43.
[10] GALIGNE, J. L., LIAUTARD, B., PEYTAVIN, S., BRUN, G.,
MAURIN, M., FABRE, J. M., TORREILLES, E.,
GIRAL, L., Acta Crystallogr. B. 35 (1979).
[11] LIAUTARD, B., PEYTAVIN, S. and BRUN, G., private
communication;
CHASSEAU, D., GAULTIER, J., HAUW, C. and LIAU-
(to be published).
example the data collected for (TMTSF)2ClO4
by : BECHGAARD, B., CARNEIRO, K., RASMUS-
TARD, B.
[12]
See for
SEN, F.
SEN, C.
B., OLSEN, M., RINDORF, G., JACOBS., PEDERSEN, H. J. and SCOTT, J. C.,
JACS 103 (1981) 2440.
[13] TAKAHASHI, T., ANDRIEUX, A., CREUZET, F., JEROME,
D. and FABRE, J. M. (to be published).
[14] MOTT, N. F., TWOSE, W. D., Adv. Phys. 10 (1961) 107.
[17]
A review of localization effects including the role of
e-e interaction is given for example by : ZELLER,
H. R., Adv. Solid State Phys. 13 (1973) 31.
RASHBA, E. I., GOGOLIN, A. A., MELNIKOW, W. I., in
the proceedings of Int. Conference on organic
conductors and semiconductors (Siofok), Lect.
Notes in Phys. (Springer Verlag) 1977, p. 265.
CHIANG, C. K., COHEN, M. J., NEWMAN, P. R. and
HEEGER, H. J., Phys. Rev. B 16 (1977) 5163.
[18] ZUPPIROLI, L., BOUFFARD, S., BECHGAARD, K., HILTI,
B., MAYER, C. W., Phys. Rev. B 22 (1980) 6035.
[19] This is for example the case for TMTTF2SCN for
which the conductivity increases by a factor of 9
when the decrease of Tc is only 20% : PARKIN,
S. S. P., COULON, C., to be published.
[20] In this table the structural data (S-S distances) are collected from reterences [10] and [11].
[21] For a more complete discussion of the EPR data see for
example : FLANDROIS, S., COULON, C., DELHAES,
P., CHASSEAU, D., HAUW, C., GAULTIER, J., FABRE,
J. M. and GIRAL, L., International conference
on low dimensional conductors (Boulder Colorado Aug. 1981) Mol. Cryst. Liq. Cryst. 79 (1982)
663.
[22] POUGET, J, P., Private communication.
[23] A review about orientational disorder in solids. See in
particular the specific heat data of NaClO4 is
given in : PARSONAGE, N. G. and STAVELEY,
N. A. K., Disorder in crystals (Clarendon Press,
Oxford) 1978.
[24] STAVELEY, L. A., GREY, N. R., LAYZELL, M. J., Zelt.
Natur. Tel. A 18 (1963) 148 and references therein.
[25] MORIYA, K., MATSUO, T., SUGA, H., SEKI, S., Bull.
Chem. Soc. Japan 52 (1979) 3152 and references
therein.
[26] AUBRY, S., J. Chem. Phys. 60 (1974) 2446.
[27] As already noted, the determination of Tc for the
Br salt from conductivity measurement gives a
slightly different value Tc ~ 19 K. This situation
often occurs when the transitions at very low T, in
this case the magnetic data appears to be more
accurate.
1067
[28] POUGET, J. P., MORET, R., COMES, R., BECHGAARD, K.,
J. Physique-Lett. 42 (1981) L-543 (see also ref. [6]).
[29] MAZAUD, A., Thèse de 3e cycle, Orsay (1981).
[30] PEDERSEN, H. J., SCOTT, J. C. and BECHGAARD, K.,
Phys. Rev. B 24 (1981) 5014.
[31] PARKIN, S. S. P., JEROME, D., BECHGAARD, K., Proceedings of the international Conference on low-
[32]
dimensional conductors (Boulder Colorado August
1981) Mol. Cryst. Liq. Cryst. 79 (1982) 569.
These problems of coupled order parameters are
extensively studied in the field of phase transition
theory.- See for example : IMRY, Y., SCALAPINO,
D. J., GUNTHER, L., Phys. Rev. B 10 (1974) 2900
and references therein.