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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�
A new survey of the physical properties of the (TMTTF)2 X series.
Role of the counterion ordering
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 Organique (U.S.T.L.), 34060 Montpellier, France (Reçu le 5 octobre 1981, révisé le 14 décembre, accepté le 3 mars 1982)
Résumé. 2014 On compare les propriétés physiques des sels (TMTTF)2X (en particulier pour X = 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 pro-
priétés intrinsèques des chaînes de TMTTF. On peut alors établir un parallèle avec les propriétés des analogues
séléniés de ces sels.
Abstract. 2014 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.
Classification Physics Abstracts
64.70K - 71.30
1. Introduction. - The recent discovery of high pres-
sure superconductivity 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 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 compe- tition 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 (per- chlorate) 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 beha- viour 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 dis- cussion 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 fun- damental 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 electroche- mical 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 elec- trocrystallization, 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 struc-
tural organization of TMTSF2-X compounds is
also similar to that of this series [12]. Their main
features are schematically reported in figure 1. They
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. 1. - Crystallographic structure of (TMTTF)2PF6 reported from [111.
try [10, 11]. In particular, for tetrahedral ions such
as BF4 or C104 two equivalent statistically occupied positions are observed at least at ambient tempera-
ture [10]. In the following we will distinguish two sets
of anions according as they are centrosymmetric or not.
3. Review of the physical properties. - 3. 1 ELEC-
TRICAL CONDUCTIVITY. - The room temperature value of a I,, the conductivity along the stacks is report-
Fig. 2. - a) Temperature dependence of the electrical con-
ductivity 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 beha- viour 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
the PF6 salt.
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 p jj I = 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.
3.1.1 Anions without inversion symmetry (BF4 , C104 , N03 , SCN’). 2013 For the SCN compound a shar maximum is detected in the d Ln (J
versus T
curve at 160 K (cf Fig. etecte
In t e
d(1/T) versus
For the CI04 salt, an important jump of resistivity
occurs at 75 K AR _ B! " ! 25 % / A strong hysteresis
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). - This category of counterions contrasts with the first one.
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 indepen- dently of the symmetry of the anions. Thus, we can
compare the semiconductor like regime of the diffe- rent 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.
3.2 MAGNETIC SUSCEPTIBILITY. - The parama-
gnetic susceptibility measured with a Faraday balance
was already reported [9] for the BF4, CI04 and Br
salts. For the last one the presence of magnetic impuri-
ties prevented any diagnostic about the existence of a
low temperature instability. However, the tempera-
ture 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 transi-
tion 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 transi-
tion.
1062
Fig. 3. - Paramagnetic susceptibility of the PF6, N03, CIO4 compounds.
Finally, it must be quoted that the room tempera-
ture value of the magnetic susceptibility is nearly independent of the counterion
(xp - 5-6 x 10-4 emu CGS/mole at 300 K)
and weakly temperature dependent above the phase
transition.
3.3 EPR SPECTROSCOPY. - We report briefly in figure 4 the linewidth and g-factor EPR data for the
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,
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 tempera-
ture independent as usually observed in one chain
compounds. The results are presented for one direc-
tion 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 tempe-
rature a sudden decrease in the spin-lattice relaxation
time T, has been observed by NMR proton spectro- scopy at Orsay which is attributed to the occurrence
of SDW fluctuations at the metal insulator transi- tion [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 ano-
maly of the magnetic susceptibility clearly visible in
this temperature domain (see Fig. 3).
As already noted, the room temperature linewidths
are very narrow (AH300K - 5 G). This will be corre-
lated with the electronic dimensionality of the com- pounds in the following discussion.
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 cor- responding 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 deter- mined 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
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 calcu-
late 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
any temperature. Indeed the obtained Debye tempe-
ratures are respectively 51 K, 59 K and 55 K for the
BF4, C104 and PF6 compounds.
4. Discussion of the physical properties. - 4 .1 THE
« METALLIC » STATE. - Organic conductors are
usually considered as synthetic metals when their con-
ductivity is large enough (0’11(300 K) > 100 0 - I cm -1 for example) and increases rapidly with decreasing temperature (0’11 (T) -...; T-a ot - 1 to 2). The
(TMTSF)2X’ salts are examples of this class of salts.
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 explain-
ed 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 con-
ductors 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 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 : a§ oo, TO’ max 61I (conductivity at ambient vity data : IT 300K, Tmax, all amax (conductivity at ambient
IT 300K
temperature, temperature of the maximum of a II
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
Imax B
value for a§oo and max, low value for T max .
a 300K /
For example, the bromine compound corresponding
to the shortest S-S distances has also the most metallic behaviour. More systematically, as far as the dimen- sionality 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 II
value of oril d max
value of 300K and
11 ’ *
300K
Furthermore, to give a definitive answer to this
problem, the role of the impurities or other extrinsic 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 obser- vation of narrow EPR linewidths [21]. As expected
the linewidth of the Br compound is the largest one.
4.2 THE PHASE TRANSITIONS. - The main part of 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 pre-
viously), but they present very different characte- ristics. As already noted, the samples can be ranged
in two groups according to the symmetry of the anions.
4.2.1 Anions of low symmetry. - The BF4 and C104 belong to this group. For these two salts the
phase transition is associated with a detectable speci-
fic 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 ana- lysis 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
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 remem-
ber 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 sup-
ported 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 tetrafluoro- borate 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.
