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Study of phase stability of β-(BEDT-TTF)2I3 by differential thermal analysis
W. Kang, G. Creuzet, D. Jérome, C. Lenoir
To cite this version:
W. Kang, G. Creuzet, D. Jérome, C. Lenoir. Study of phase stability of β-(BEDT- TTF)2I3 by differential thermal analysis. Journal de Physique, 1987, 48 (6), pp.1035-1040.
�10.1051/jphys:019870048060103500�. �jpa-00210511�
Study of phase stability of 03B2-(BEDT-TTF)2I3 by differential thermal
analysis
W. Kang, G. Creuzet, D. Jérome and C. Lenoir
Laboratoire de Physique des Solides (Associé au CNRS), Bâtiment 510, Université de Paris-Sud, 91405 Orsay,
France
(Reçu le 9 janvier 1987, accepté le 20 f6vrier 1987)
Résumé. 2014 Nous présentons une étude en pression et température du diagramme de phases du composé 03B2- di[bis(éthylènedithio)tétrathiafulvalène ] triiodine, 03B2-(BEDT-TTF)2I3, effectuée par une méthode d’analyse thermique différentielle. Nous montrons que la nature de la phase de ce matériau qui est stabilisée à basse
température dépend du processus de refroidissement. Un point critique (Pc ~ 345 bar, Tc ~ 150 K) joue un
rôle déterminant dans ce processus. On peut obtenir la phase 03B2-H supraconductrice à 8,1 K sous pression atmosphérique en contoumant le point critique dans le sens des aiguilles d’une montre. C’est la phase 03B2-L supraconductrice à plus basse température (probablement la phase présentant une distorsion de réseau
incommensurable) qui est stabilisée par un cyclage P - T effectué dans le sens contraire des aiguilles d’une
montre. Nous avons pu déterminer les limites de stabilité des phases 03B2-L et 03B2-H. Un modèle de Landau utilisant des variations appropriées en pression et température des termes du second et quatrième ordre permet de rendre compte de 03B2-(BEDT-TTF)2I3.
Abstract. 2014 We report a study of the pressure-temperature phase diagram of 03B2-di[bis(ethylenedi- tio)tetrathiafulvalene] triiodide, 03B2-(BEDT-TTF)2I3, by a differential thermal analysis technique. We show that
the nature of the state of the compound at low temperature depends on how cooling is achieved. A critical
point (Pc ~ 345 bar, Tc ~ 150 K) plays a crucial role in this cooling process. The 03B2- H phase displaying superconductivity at 8.1 K at ambient pressure is obtained by a clockwise pressure-temperature cycling around
the critical point. We also show that an anticlockwise P 2014 T cycling with P Pc allows the stabilization of the low Tc superconducting phase (03B2-L) at low temperature, very likely the phase displaying an incommensurate lattice distortion. The limit of stability of both 03B2- Land 03B2-H phases in the P 2014 T diagram has been determined.
The properties of metastability of the various phases of 03B2-(BEDT-TTF)2I3 can be understood in terms of a
Landau expansion of the free-energy with the appropriate pressure and temperature dependence of the second and fourth order terms of the expansion.
Classification
Physics Abstracts
74.10 - 74.30E - 74.70D - 74.90
1. Introduction.
The first ambient pressure sulfur based organic superconductor (BEDT-’I-I’P)213 has attracted much attention after the initial discovery of its supercon-
ducting properties [1, 2] not only because of the
highest SC transition temperature ever found in an
organic material [3] but also because of the many
interesting phases of the compound.
The superconducting transition in /3-(BEDT- TTF)2I3 has been observed originally under ambient
pressure below 1.2 K in the so-called /3-L phase.
Furthermore, it was found that pressure has a strong effect on Tc since the pressure coefficient amounts to
dTc/dP - - 0.8 K/kbar [4]. However a sudden
jump of Tc up to 7.5 K was originally noticed at
1.3 kbar followed by subsequent decrease at higher
pressure [5, 6]. The onset of SC at 7.5 K and 8 K was
announced independently by groups in USSR [7, 8]
and Japan [9]. Later, Creuzet et al. [3, 10] reported
the existence of bulk superconductivity in 13 -(BEDT-TTF) 213 at 8.1 K under ambient pressure after an adequate pressure temperature cycling of
the sample (from now on called « Orsay process »).
They also reported that the phase of the compound giving rise to the high SC transition (/3 -H) can be
maintained as long as the sample is not heated above 125 K [10]. It must be noticed that, simultaneously
and independently, the Russian group obtained also the high- Tc state under 1 bar using a very similar
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019870048060103500
1036
process [11]. However, the respective stability of ,8 -H and 8 -L states at different pressures and temperatures remained to be clarified.
Several phase transitions have been reported in this compound. Neutron scattering studies [12] have
detected the onset of an incommensurate lattice modulation below 200 K which remains down to 20 K. The same incommensurate superstructure was observed by X-ray diffuse scattering below
175 K [13]. Furthermore, a diffraction study [14]
performed under pressure has shown that the satel- lite peaks (associated with the incommensurate
structure) do not appear at low temperature pro- vided a pressure in excess of 0.5 kbar is applied at high temperature before cooling down. A similar critical pressure of 0.5 kbar was also reported as the
smallest pressurization which is needed to stabilize the high Tc phase of P-(BEDT-TTF)213 [15].
Recently, Hamzic et al. [16] reported an anomaly
in the c*-resistivity data around 180 K upon cooling
under 1 bar consisting in a divergence in the first
derivative versus temperature. This was assumed to be associated with the transition to the incommensur- ate phase. They also reported the evidence of a well-
defined transition around 132 K which is observed
by heating under 1 bar the B-H state obtained after P - T stabilization. This result was in good agree- ment with the stability limit of the B-H phase that
Creuzet et al. [10] reported before. Around the
same temperature of 132 K Mortensen et al. [17] and Hennig et al. [18] reported a change in the slope of
the thermoelectric power around 125 K. However in these two experiments the compound might have
been in a mixed state because they did not use
pressure and probably performed some temperature cycling under 1 bar (vide infra). Finally Hamzic
et al. [19] also reported in a recent article an another anomaly in the measurement of the thermal expan- sion along the c*-axis at 172 K.
But, in order to understand clearly these transi- tions found independently by different experiments,
more complete determination of the relation be- tween different phases has to be undertaken. In this paper we report the results of thermodynamic study
of the B -(BEDT-TTF) 213 which has been performed
to clarify the necessary conditions for the stability of
the phase displaying superconductivity at 8.1 K. We
have established the P - T diagram showing the
limit of metastability of the {3 - H state at low temperature. We also report the first observation of the /3-L to {3-H conversion at high pressure. A
simple Landau expansion of the free energy provides
a qualitative understanding of most experimental
features.
2. Experimental.
The differential thermal analysis (DTA) technique
was used for detecting the small change of the
sample temperature related to thermal anomalies.
The details of this technique can be found in any textbook on thermal analysis, for instance, in refer-
ence [20]. A relatively large single crystal prepared by the electrochemical method [21] was chosen for
our experiment (2.15 x 1.05 x 0.80 mm 3). The sam-
ple was glued on one of the Cu-constantan ther-
mocouple junctions with GE varnish while the other
junction was located in the sample cell close to the
sample. The absolute temperature of the sample was
measured with an independent pair of thermocouples
which was also put near the sample in the measuring
cell. The sensitivity of the measuring system was better than 1 mK such that an increment of the
sample temperature of several millidegrees could be easily detected. The hydrostatic pressure within the range of 1 to 3 000 bar was achieved by compressing
helium gas in a Be-Ct pressure vessel. The value of the helium pressure was constantly recorded in the
couise of the experiment. Heating rate up to 30 K/min was achieved by the combination of the heater in the cryostat and another additional heating
element installed on the pressure vessel. The cooling
rate was about 7.5 K/min.
Although thermodynamic anomalies which can be detected by this equipment are essentially anomalies accompanied by a latent heat, we could also detect
changes in the specific heat.
3. Results.
Figure 1 displays the diagram of /3-(BEDT-TTF)213
which can be drawn from the results of present study. In the rest of this section we shall comment
about these data. The line (a) marks the reversible
phase transition between the /3-H phase at high temperature and the incommensurate modulated
Fig. 1. - P - T diagram of 13-(BEDT-TfF)2I3. Second
order transition between B -Hand 13 - L phases (+). Open
circles and closed circles mean the limits of stability of 13 - Hand 13 - L states respectively upon increasing tempera-
ture (see text).
I3-L phase at low temperature. The (b) line stands for the limit of stability of the 13 - H phase observed
when raising the temperature after the 13 -H phase
has been obtained at low temperature by the appro-
priate P - T cycling (« Orsay process »). The line (c) shows the limit of stability of the 13-L phase
obtained at low temperature after a regular cooling
of the sample under ambient pressure. (From now
on, we shall call « anti-Orsay process » the P - T
path leading the observation of the phase line (c)).
Strictly speaking, (b) and (c) lines do not represent phase transition lines as they are not related to the
reversible transformations from one phase into the
other. We shall call them phase conversion lines.
The diagram in figure 1 shows that the (a) transi-
tion line and the (b) and (c) conversion lines meet at
a critical point (possibly a tricritical point) with
coordinates Pc = 345 bar and Tc = 150 K.
The transition (a) can be detected by a change of
the slope of the DTA signal versus temperature observed on heating and cooling curves. A typical signal is shown in figure 3a. According to the small change of the slope it is clear that the change of specific heat related to that transition cannot be
large. This transition is presumably related to the
transition detected by X-ray and neutron scattering [12, 14] thermal expansion [19] and c*-resistivity [16]
in the temperature domain 175-200 K. Under atmos-
pheric pressure we detected the DTA anomaly at
181 ± 2 K. The pressure coefficient at this transition line is negative (- - 25 K/kbar).
A typical signature of the conversion line (b) is
shown in figure 3b. As compared to the signal
obtained from the (c) line (vide infra) the latent heat
associated with the (b) line is rather small. In
addition, the change in the specific heat is rather small. The small downward temperature peak is not fully understood but is removed by either increasing
pressure or the warming speed. (However in the
later case, the removal may be due to the reduction
Fig. 2. - Schematic diagrams of the various P - T cycles
used in this experiment. (P1 Pc P2) (see text).
Fig. 3. - Typical DTA signals. /3-H to (3-L conversion signal at P =1 bar (a) {3 - H to (3 - L conversion signal at
P = 63 bar (b) {3 - L to (3 - H conversion signal at
P = 831 bar (c).
of resolution). Under ambient pressure the 8’-H to j8-L conversion line (b) occurs at 131.5 K and
increases with pressure. The conversion was easily
observed only up to 300 bar, and no DTA signal
could be detected above 350 bar. The deduced (a)
and (b) lines as shown in figure 1 somewhat con-
tradicts data of neutron scattering experiment [14]
where satellite reflections (i.e. f3-L phase) could be
observed when the sample was cooled from room temperature under a pressure of 0.5 kbar. In the
present study we have performed an « Orsay pro-
cess » under controlled pressures of 380 bar and 356 bar and we have checked that the f3 - H to j8-L conversion signal could be observed at 131.5 K
on warming under ambient pressure in both cases
similarily to the « Orsay process » performed with a
pressure exceeding 0.5 kbar. An another subtle
1038
P - T cycling (Fig. 2c) has also been used to locate
more accurately the position of the tricritical point.
In such a case the pressure was applied during the
145 to 137 K stage of the cooling. Then, the sample
was cooled down to 70 K and the pressure was released. Thus doing, the (b) conversion line was
observed if the pressure exceeds 350 bar. These data show that the tricritical point is located below
350 bar (Pc = 345 ± 5 bar). This study shows conclu- sively that a pressure less than 0.5 kbar (say 345 bar)
is sufficient in the « Orsay process » for stabilization of the 8 -H phase at low temperature.
The conversion line (c) is obtained with the so-
called « anti-Orsay process » where pressure is ap-
plied at low temperature and the sample temperature is increased under pressure (P > 360 bar). A typical shape of the DTA signal is shown in figure 3c. There
occurs a very distinct peak related to the heat
released in the conversion from 3 - L to f3 - H state (latent heat). The conversion temperature is only weakly pressure dependent (104-105 K) above
600 bar and begins to increase rapidly in the vicinity
of the tricritical point. The area under the DTA peak decreases along the (c) line while approaching
the tricritical point and the conversion becomes difficult to detect below 400 bar. As shown in
figure 3c, a small step was observed around 96 K
independently of the conversion peak. This is prob- ably concerned with the dynamic effect in the zone
(IV) in figure 4 which will be discussed later.
So far, our equipment allows only a qualitative
determination of the latent heat and no quantitative
studies have been performed.
Fig. 4. - The phase diagram established after the Landau
phenomenological model. The inset shows the assumed behaviour of a (p, T) and c (P, T) at two different pressu-
res.
4. Phenomenological approach and discussion.
As we suggested before [3, 10, 16], the present
thermodynamic data have confirmed the very pro- nounced metastability of the {3 -H state at low temperature. This means that when the system is
prepared following the « Orsay process » it does not sit in the absolute minimum of its free energy. We will show that the salient features of phase metastab- ility of {3-(BEDT-TIF)2I3 can be understood in terms of a simple Landau phenomenological model
with two adjustable parameters [22]. Accordingly,
we postulate the existence of a polynomial expansion
of the free energy versus the order parameter
q (the amplitude of the lattice modulation).
G = a(P, T) q 2 + c(P, T) n 4 + g(P, T) n 6 .
The existence of transition or conversion lines in the
(P, T) diagram arises from the (P, T) dependence
of the coefficients a, c and g. At this stage, we have
no idea about the microscopic process which governs such dependences. However, the large amount of experimental data rapidly tends to focus on the
evolution of G (P, T, n ) as shown in figure 5. In- deed, the evolution between the high temperature
region I (q = 0 stable) and the low temperature on region III :0 0 thermodynamically more stable than q = 0) can be made via two different manners :
if the G curve is deformed through region II (left
part of the diagram) the q = 0 state is always
stabilized at low temperature. On the contrary, if the
Fig. 5. - The phase diagram with the G ( n ) configurations
in each regions. The useful cycling are schematically represented for lines (a) (----) (b) ( ) and (c) (-.-).
G curve is deformed through regions V and IV, the
n = 0 state can be stabilized at low temperature under certain conditions as we will discuss later in
more details.
We can suggest a very simple (P, T ) dependence
of the coefficients a, c and g which leads to such a diagram. First of all, the (P, T) dependence of the
coefficient of the sixth-order term can be neglected
in our discussion. Only its sign matters and must be positive in order to warrant the stability of the
system. Furthermore, we assume that a (P, T ) has a parabolic shape versus temperature at constant pressure with two zeros at T, and T2 (P P c ) and no
root when*? > P C. For c (P, T ) we assume a linear temperature dependence. It crosses the T-axis at a temperature slightly above Tl. With the above
mentioned behaviour for coefficients a and c (shown
in the inset of Fig. 4) we could reproduce the diagram as shown in figure 4. Each configuration of
G as shown in figure 5 can then be easily deduced in
each region of the « phase diagram ». From this
model one can understand the (P, T) cycles leading
to the observation of lines (a), (b) and (c).
First, cooling under normal pressure or weak pressures leads to the transition from region I (q = 0, J3-H state) to region II (q = q , , 3 -L state)
which are thus separated by the (a) transition line.
Secondly, dealing with the « Orsay process », the J3 - H state is absolutely stable for energy con-
figurations (I) or (V) and even in (IV) if the secondary minimum of the free energy is not oc-
cupied. The system remains in the J3 - H phase at low temperature (zone (III)) since there is a potential
barrier higher than the thermal energy separating
the two potential wells. When the sample is heated
in zone (III) after pressure release the height of the
barrier becomes smaller. Finally a conversion into a
J3 - L state (q = n + ) occurs in zone (II). So the (b)
conversion line appears to be the separation line
between regions III and II. But since at this stage there is not much energy difference between the two different states no strong latent heat is related to this conversion process. At higher temperature a regular
second order phase transition occurs when a = 0 at T = T2.
Similarly, the same model provides a natural understanding of the « anti-Orsay process ». Starting
from a /3-H configuration (zone (I)), the system undergoes a second order phase transition at T = T2 towards the configuration of zone (II) which
becomes the configuration of zone (III) upon further
cooling. Warming the system at P > P c leads to a
situation where 13-H phase should be more stable
than 13-L (zone (IV)). However, because of the high potential barrier existing between the two minima of the free energy the conversion into the absolutely
stable J3 - H phase will not occur until zone (V) is reached, leading to the identification of the separ-
ation line between regions IV and V as the (c)
conversion line.
Additional experiments (see Figs. 2d and 2e) have
also been performed in order to determine the influence of the dynamical effects on the phase
conversion process. After achievement of an « Orsay
process » the sample was kept at a temperature 125- 127 K for 15 min (P = - 1 bar). After this waiting
time it was further cooled down. The I3-L to {3 - H conversion (line(c)) was thus obtained upon
subsequent heating under pressure P > P c but the sample did not reveal any thermal anomaly at 131 K
when heated under P Pc. This effect was not
observed when the sample was kept below 120 K ’
during the waiting period. Inversely, the I3-L phase
obtained after an « anti-Orsay process » was main- tained between 95-98 K for 15 min under a pressure of 850 bar. After this waiting period the sample
showed all characteristic features of the I3-H.state.
Both above described experiments indicate that
dynamical effects can play a role only in the neigh-
bourhood of the conversion lines. These results are
in agreement with previous studies of the system by
ac susceptibility techniques [10].
Despite the fact that no microscopic explanation is given for the transition and conversion lines, this phenomenological model gives a rather useful framework in complete agreement with the present DTA experimental data and also with all previous
related experiments discussed in the first section.
Finally, one can easily retain that the simple recipe
to stabilize the {3 - H state at low temperature is to avoid the triangle between (a) and (b) lines when the
sample is cooled down. We must notice that we
always used an hydrostatic helium-gas pressure and results were perfectly reproducible : as a conse-
quence, it seems that the discussion about the relative importance of non-hydrostatic pressure in the stabilization of the P-H state [4, 15] is probably meaningless.
Acknowledgments.
We thank H. J. Schulz and F. Creuzet for very fruitful discussions. We also thank J. C. Ameline for his skillful technique in the high pressure exper- iment.
Note added in proof. - After submission of this article we have received the article of GINODMAN,
V. B. et al., JETP-Lett. 44 (1986) 523 which reports
a study of (3-(BEDT-TTF) 213 performed by resistive techniques versus pressure and temperature. In addition, in a recent publication ENDRES, H. et al.,
Z. Naturforschung 41a (1986) 1319 claimed that the lattice distortion is commensurate at 100 K. This
question is still puzzling as the latter authors have not found any transition around 200 K where an
American (LEUNG, P. C. et al., J. Amer. Chem.
