HAL Id: jpa-00210331
https://hal.archives-ouvertes.fr/jpa-00210331
Submitted on 1 Jan 1986
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Electron-molecular vibration coupling in 2-D organic conductors : high and low temperature phases of
α-(BEDT-TTF)2I3
M. Meneghetti, R. Bozio, C. Pecile
To cite this version:
M. Meneghetti, R. Bozio, C. Pecile. Electron-molecular vibration coupling in 2-D organic conductors : high and low temperature phases ofα-(BEDT-TTF)2I3. Journal de Physique, 1986, 47 (8), pp.1377- 1387. �10.1051/jphys:019860047080137700�. �jpa-00210331�
Electron-molecular vibration coupling in 2-D organic conductors :
high and low temperature phases of 03B1-(BEDT-TTF)2I3
M. Meneghetti, R. Bozio and C. Pecile
Department of Physical Chemistry, University of Padova, 2, via Loredan, I-35131 Padova, Italy
(Reçu le 24 fevrier 1986, accepté le 11 avril 1986)
Résumé. 2014 On présente les spectres d’absorption dans l’infrarouge en lumière polarisée de monocristaux de
03B1-(BEDT-TTF)2I3 dans la phase conductrice à haute température (T = 300 K) et dans la phase semiconductrice à basse température (T = 20 K). Pour comparer et compléter les données déjà connues, on présente aussi les spectres de conductivité obtenus par transformation de Kramers-Kronig des résultats de réflexion polarisée (80-6 000 cm-1, T = 300 K). On discute d’une manière exhaustive les structures superposées aux larges bandes
causées par les transitions intra- et interbandes et attribuées au couplage des électrons de conduction avec des modes intramoléculaires de BEDT-TTF entièrement symétriques impliquant l’étirement des liaisons C=C et C2014S.
Cette conclusion est basée sur des données de spectroscopie Raman et sur une analyse préliminaire en coordonnées normales de BEDT-TTF neutre. On présente les fondements pour le développement d’un modèle microscopique quantitatif d’oscillations de charge électronique induites par vibration dans les conducteurs organiques 2-D.
Une discussion qualitative du modèle explique le dichroisme observé pour les structures vibroniques de 03B1-(BEDT- TTF)2I3 à température ambiante. La comparaison des spectres à 300 K et à 20 K et l’étude de l’eflet de la tempé-
rature sur les spectres d’absorption de poudres permet l’identification des variations spectrales induites par la transition métal-isolant (M-I) à 135 K. Celles-ci sont discutées par rapport à de possibles variations structurales
impliquées dans la transition M-I. On trouve que les changements spectraux observés doivent nécessairement
impliquer des déplacements de molécules de BEDT-TTF dans les couches organiques.
Abstract. 2014 The polarized infrared (400-4 000 cm-1) absorption spectra of single crystals of the 03B1 form of (BEDT- TTF)2I3 are reported for the high temperature conducting phase at T = 300 K and for the low temperature semi- conducting one at T = 20 K. For comparison and to supplement the already available data, the conductivity spectra obtained by Kramers-Kronig transformation of polarized reflectance data (80-6000 cm-1, T = 300 K)
are also reported. The structures superimposed on the broad absorptions due to electronic intra- and interband transitions are extensively discussed and attributed to the coupling of conduction electrons to totally symmetric
intramolecular modes of BEDT-TTF involving the stretching of C=C and C2014S bonds. Such an assignment is
based on Raman data and a preliminary normal coordinate analysis of neutral BEDT-TTF. The guidelines for the development of a quantitative microscopic model of vibrationally induced electron charge oscillations in 2-D
organic conductors are presented. A qualitative discussion of the model allows us to account for the observed dichroism of the vibronic structures in room temperature 03B1-(BEDT-TIF)2I3. Comparison of the 300 K and 20 K spectra and a study of the temperature dependence of powder absorption spectra allow us to identify the spectral changes induced by the 135 K metal-insulator (M-I) transition. They are discussed in relation to possible structural changes accompanying the M-I transition. It is found that displacements of the BEDT-TTF molecules in the
organic sheets are needed to account for the observed spectral changes.
Classification
Physics Abstracts
78.30 - 71.38 - 71.30
1. Introduction
The synthesis of a new class of materials based on the sulphur containing organic donor bis(ethylenedithio)
tetrathiafulvalene (BEDT-TTF) has opened new perspectives in the field of organic superconductivity [1, 2]. As a matter of fact such a synthesis represents
the natural development of a tendency towards less
anisotropic electronic interactions compared to those usually found in quasi one-dimensional (1-D) organic charge transfer (CT) conductors. Such a trend had
begun with the"synthesis of the first class of organic superconductors : the Bechgaard salts, (TMTSF)2X.
[3].
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019860047080137700
(BEDT-TTF)2Re04 has been the first material of the new class displaying superconductivity at
T Tr = 2 K under an applied pressure of 4 kbar[l].
Much attention is currently being devoted to the
BEDT-TTF salts containing linear triatomic anions
[4, 9] and exhibiting ambient pressure superconduc- tivity at temperatures up to 8.1 K [9]. A prototype of this subclass is p-(BEDT-TTF)213 (Tr = 1.4 K in the pristine compound [4]; Tc = 8.1 K after thermal and pressure treatment [9]) one of the numerous crystal
forms which are found in the BEDT-TTF : 13 system.
Other forms (a- through s-) [10] are known which are
either superconducting or insulating at low tempe-
ratures. a-(BEDT-TTF)2I3 is known to undergo a
metal-insulator (M-I) transition at 135 K at ambient pressure [11]. The M-I transition can be suppressed by applying a pressure of 15 kbar and a semimetallic state is stabilized down to 0.1 K without achieving superconductivity [6]. It has been reported that doping
the a-form with iodine produces a partial suppression
of the M-I transition and superconducting behaviour at
T 3 K [12].
In this paper we report the results of an investi-
gation of the infrared spectra of Ct-(BEDT - TTF)213
at temperatures both above and below the M-I transition and at ambient pressure. Single crystal polarized absorption and reflection as well as powder absorption spectra have been recorded with the aim either to relate them to known structural and electronic features of this material or to obtain hints on unknown
properties particularly in the low temperature phase.
Our main emphasis has been placed on a discussion
of the spectral features related to the coupling of
conduction electrons to intramolecular vibrations of the BEDT-TTF molecules (e-mv coupling). Besides
the information on the strength of such interactions the analysis can provide knowledge on the electronic structure itself supplementing that obtainable through
the study of interband and intraband electronic transitions.
To this aim, a knowledge of the intramolecular vibrational properties of the BEDT-TTF molecule is a
necessary prerequisite. Limited to the case of the totally symmetric vibrational modes which are most relevant in the e-mv coupling, such a requirement is
fulfilled in the present paper by reporting Raman data
and a normal coordinate analysis of neutral BEDT-.
TTF. Also needed is an ad hoc theoretical model for the electron charge oscillations induced in a 2-D lattice by
intramolecular vibrational modes linearly coupled to
the electrons. Here we report some preliminary
considerations and a qualitative discussion of this model.
2. ExperimentaL
BEDT-TTF has been synthesized and purified accord- ing to previously reported procedures [13, 14]. Crys-
tals of BEDT-TTF triiodide were grown by slow
diffusion at room temperature of 3 x 10-3 mole
litre-1 BEDT-TTF and 10- 2 mole litre-1 tetra-
butylammonium triiodide solutions in 1,1,2-trichloro- ethane in a three chamber apparatus [15] with the
central volume filled with pure solvent. The crystals
collected after several weeks displayed various mor- phologies : thin and thick platelets, needles and block
shaped prisms. Some crystals among the largest platelets (typically a few millimeters wide) were analysed by X-ray diffraction. Both thin (few microns thickness) and thick (tenths of a millimeter thickness) platelets were found to belong to the a-form [16] of (BEDT-TTF)213. For optical measurements the crys- tals were oriented based on their morphology, X-ray
diffraction and optical extinctions observed under a
polarizing microscope.
The single thin platelet crystal used for recording polarized absorption spectra was mounted on a caesium iodide window. Reflectance data were taken from a single thick platelet crystal and normalized to the reflectance of an aluminium mirror. Powder
absorption spectra were obtained from Nujol mulls
on caesium iodide windows. Absorption and reflec- tance measurements have been carried out using an
infrared Fourier transform spectrometer (Bruker
Model 113 v). A closed cycle refrigerator (Cryodyne,
Model 21) has been used for low temperature measu-
rements. The mechanical vibrations produced by the refrigerator caused the appearance of some spectral
noise in the Fourier transformed spectra. Such a noise is however localized in the 530-500 cm-1 region and
does not interfere with our identification of narrow
vibrational structures (see below). Raman spectra of neutral BEDT-TTF have been recorded from
powder samples at 20 K using a SPEX 1403 double monochromator and the exciting line at 488.0 nm
from an argon laser (Spectra Physics Model 165).
3. Results.
Figure 1 shows the absorption spectra (400-
4 000 cm-’) of a thin platelet of ex-(BEDT-TTF)2I3 at
room temperature (upper panel) obtained with the electric field vector of the incident radiation parallel
to the a and b crystal axes which coincide with direc- tions of maximum optical anisotropy [17]. The ordi-
nate scale is expressed as optical density since the difficulty in measuring accurately the sample thick-
ness prevented us from giving absolute absorbance
values.
The structure of ex-(BEDT-TTF)2I3 [16] contains
sheets of organic donors parallel to the (001) plane separated along the c axis by sheets of I3 anions.
Stacking of the BEDT-TTF molecules occurs along
the a axis. The spectra in figure 1 exhibit a moderate anisotropy in the order of 1.5 : 1 with the higher absorption measured for E 11 b. The broad continuous
absorption extending over the whole spectral region investigated is attributable to intra- or interband electronic transitions or, most likely, to their overlap.
Fig. 1. - Polarized absorption spectra (400-4000 cm-1)
of a thin platelet crystal of a-(BEDT-TTF)2I3 at 300 K (upper panel) and 20 K (lower panel). The E II a and E b
curves have been obtain6d with the electric field of the incident radiation parallel to the a and b crystal axis respec-
tively.
Therefore the data imply that, as is usually found with
other 2 : 1 BEDT-TTF salts, interstack electron transfer interactions are at least comparable to the
intrastack ones [17-22]. This conclusion is also in agreement with polarized reflectivity data of the
a-form previously reported by Vlasova et al. [23]
and by Kaplunov et al. [24].
A broad structure is observed for both polarizations
between 1 000 and 1 400 cm-1. Other noteworthy
features include the asymmetric band at 877 cm-1
in the E II b spectrum and the absorption upturn observed for both polarizations on approaching the
lowest frequency limit. Weak narrow bands (see Fig. 3
below for a clearer appreciation of the details) are
found at 1 402, 1 132 and 880 cm-1 for E 11 a and
652 cm-1 for E b. In addition, the latter spectrum shows narrow dips at 1 298 and 1 182 cm -1.
For the sake of comparison with the absorption spectra and with previously published data [23, 24],
we have recorded polarized reflectance spectra from thick plateletes at room temperature between 80 and 10 000 cm-1. Our data, shown in figure 2, upper panel,
Fig. 2. - Room temperature polarized reflectance (upper panel) and conductivity (lower panel) spectra of a-(BEDT- TTF)2I3 single crystal for E II a and E II b in the 80-6 000 cm-1 region.
are quite similar to those of Vlasova et al. [23] and Kaplunov et al. [24]. The reflectance values agree within 10 % in all cases and the general shape is quite reproducible except for the fact that the data of reference [24] do not show any structure around 400 cm-1. A Kramers-Kronig transformation has been performed by extrapolating our reflectance data to zero frequency with a constant value equal to that
measured at 80 cm-1 for both polarizations. In extrapolating the E a data to high frequencies we
have simulated the presence of a previously observed
band at 20 000 cm-1 [17, 21, 24]. At higher frequencies,
as well as for the E II b polarization for m > 10 000
cm-1, the reflectance has been extrapolated accord- ing to standard procedures [25]. The conductivity spec- tra (80-6 000 cm-’) obtained in this way are shown in
figure 2 lower panel. They look similar to those of
Kaplunov et al. but for the higher resolution of our
data and for the presence of a remarkable band at 370 cm-1. The latter band was already implied in some
structure observed in the reflectance spectra obtained
by us as well as by Vlasova et al.
Comparison with the’ absorption spectra from thin
platelets shows a remarkable agreement particularly
for what concerns the vibrational structures including
both the broad features and the narrow bands and
dips. Two differences are worth mentioning. (i) The intensity of the electronic band displays a more
distinct maximum in the E II b conductivity than in the
corresponding absorption spectrum. Such a difference is however expected, at least qualitatively, when comparing conductivity and absorption [25]. (ii) The anisotropy is significantly higher in the conductivity
data and this might be due to some inaccuracy in the
absolute reflectance values. The most important
information provided by the conductivity spectra
concerns the presence of the 370 cm-1 band which is certainly related with the above noted upturn in absorption at 400 cm-1.
The lower panel of figure 1 shows the polarized absorption spectra of a-(BEDT-TTF)2I3 at 20 K.
While the optical density and anisotropy are not much affected, some changes in the overall shape and
structure of these spectra compared to those at room temperature are clearly evident. For both polarizations
the maximum of the absorption intensity due to
electronic transitions becomes more apparent. Parti-
cularly noteworthy is the rather sharp decrease of
absorption intensity on approaching the low frequency
limit observed for both polarizations.
The changes in the fine structures can be better
appreciated with reference to figure 3 where enlarged spectra (400-1600 cm-1) at the two temperatures are directly compared The E 11 b and E a spectra are shown in the upper and lower panel respectively. With E II b the broad (about 200 cm-1 wide) feature which
was centred at 1 275 cm-1 in the 300 K spectrum shifts to 1 350 cm-1 but does not change appreciably
in intensity though its shape becomes less dispersive.
A number of narrow bands and dips observed at room temperature become sharper and some remarkable
new features appear. The latter include the bands at
1 483, 1 462 and 1 430 cm-1 already noted in the 100 K reflectance spectrum reported by Kaplunov
et al. [24]. As a matter of fact it is rather hard to decide whether these features, growing on the wing of the
broad 1350 cm-1 absorption, are really bands or dips.
An additional sharp band is also observed at 480 cm-1.
The E 11 a spectrum shows more dramatic changes
which parallel the sharpening of the broad electronic band noted above. The rather smeared room tempe-
rature absorption at about 1 160 cm-1 (which actually
appears as a shallow dispersive feature extending over
more than 200 cm -1 in Fig. 3) shifts to 1 270 cm -1
while its oscillator strength strongly increases. A
markedly asymmetric band grows at 875 cm-’
corresponding to the 877 cm-1 absorption observed
at both temperatures in the E b spectra. Two pro- minent sharp peaks not observed at room temperature appear at 466 and 441 cm-1. In addition to the shar-
pening and splitting of other weak bands, two narrow
Fig. 3. - Polarized absorption spectra of a thin platelet crystal of a-(BEDT-TTF)2I3 for E b (upper panel) and for
E 11 a (lower panel) at 300 K and 20 K in the 400-1600 cm-1 region.
dips appear at 1 294 and 1 184 cm -1, nearly at the
same frequency as the analogous features observed in
the E 11 b spectra but with larger amplitude.
We have investigated on the relation between the temperature induced spectral changes and the M-I
phase transition at 135 K. Such a relation is in fact
already suggested by the presence in the 100 K reflectance spectrum of some of tie additional features shown by our 20 K absorption spectrum. Our
approach has been to study the temperature depen-
dence of powder absorption spectra of a-(BEDT- TTF)2I3. Despite their unpolarized nature and some smearing effect due to small particle diffusion, such spectra compare rather well with the thin platelet
ones particularly for what concerns the narrow vibra-
tional structures. The most suitable spectral region for studying the temperature dependence is in the range 300-500 cm-1 because of the presence of comparatively
narrow intense bands coincident with bands observed in the polarized absorption spectra at 20 K. Spectra
recorded at various temperatures above and below the M-I transition are shown in the left panel of figure 4. The detailed temperature dependence of the
Fig. 4. - Left panel : temperature evolution of the powder absorption spectra of a-(BEDT-TTF)2I3 in the 500-300 cm-1 region. Right panel : temperature dependence of the inte-
grated absorption intensity of the bands at 480 cm-’ 1 (0), 467 cm-1 (m), 441 em -1 (8) and 394 cm-1 (A) nor-
malized to its value at 20 K.
integrated absorption intensity of the bands at 480 cm-’, 467 cm-1, 441 cm-1 and 394 cm-1 is plotted in the left panel of the same figure. In all cases
the curves display a knee around 135 K and a steep increase of intensity at lower temperatures. The non vanishing intensity above the M-I transition tempe-
rature indicated by the curves for the bands at 467 cm-1
and at 394 cm-1 is likely due to an approximately temperature independent component related to the
presence of defects in the powder sample. The above
results firmly establish that the main additional features of the low temperature spectra are induced by
the M-I phase transition.
4. Discussion,
4.1 HIGH TEMPERATURE CONDUCTING PHASE. - Crys-
tals of (x-(BEDT-TTF)2I3 are triclinic (Pl) and
contain two formula units per cell [16]. The BEDT-
TTF molecules belong to three crystallographically independent sets. Molecules A (and the symmetry related A’) occupy general positions and pile along
the a axis to form type I stacks. Molecules B and C are
independent, lie on inversion centres and form type II stacks along a. Type I and type II stacks
alternate in the b direction to form sheets parallel to
the (001) plane. The molecular planes of the BEDT-
TTF units in stack I make a large dihedral angle with
those of stack II. The bond lengths of B and C molecules
are significantly different from those of the A (and A’)
ones suggesting the possibility that the average charge
on the former is smaller than on the latter.
Considering the HOMO’s alone and their inter- actions in the (ab) plane, the four molecules per unit cell give rise to four 2-D tight binding bands. Neglect- ing electron-electron interactions, the six electrons
occupying the HOMO’s hll up the lowest three bands.
Depending on the relative magnitude of the gap between the third and fourth band and of their
dispersion in the first Brillouin zone, the material may be a semiconductor or semimetal. The results of
preliminary band structure calculations [26] are not fully conclusive but the temperature dependence
of the dc. conductivity and thermopower [11] favour
a semimetallic structure.
According to the latter hypothesis, the infrared spectrum should display an overlap of inter- and intraband transitions. The clear maximum at about 3 000 cm-1 exhibited by the E II b conductivity spectrum at 300 K (Fig. 2, lower panel) indicates that,
with this polarization, the conductivity is dominated
by interband transitions. On the other hand in the
E 11 a spectrum the conductivity maximum is less
pronounced and shifted to lower frequencies which
indicates a smaller contribution from interband transitions. The conductivity and the polarized absorp-
tion spectra (Fig. 1, upper panel) consistently show
that the interstack electronic interactions are stronger than the intrastack ones. This also agrees with the indication coming from the observed ratio of d.c.
conductivities, abl a. >, 2 [16]. As a matter of fact
the observed optical anisotropy could be accounted for mainly in terms of the interstack interactions.
This is suggested both by the calculated values of the intermolecular overlap [26] and by the observation that all the S... S intermolecular contacts within the stacks are greater than the sum of Van der Waals radii whereas the interstack ones are markedly
smaller [16].
4.1.1. Vibronic structures and their assignment. - We turn now to a discussion of the broad structures observed at room temperature on top of the back- ground of electronic transitions below 1600 cm-1.
There are a number of evidences showing that these
structures are due to the coupling of the conducting
electrons to intramolecular vibrations of the BEDT- TTF units.
Features of similar frequency and shape are obser-
vable in the spectra of other 2 : 1 BEDT-TTF salts whose electronic structure is markedly different from that of the a-form [18-20, 22, 24]. This excludes a
purely electronic origin for the above features. On the other hand their strong intensity, asymmetric shape
and unusually large width rules out the possibility
that they originate from purely vibrational transi- tions.
It is well known from the infrared spectroscopy [27]
of quasi 1-D organic conductors that, for molecular constituents whose frontier orbitals are non-degene-
rate, the only intramolecular modes which couple
with conduction electrons are the totally symmetric
ones [28]. We have therefore carried out a preliminary
vibrational analysis of BEDT-TTF with the aim of
relating the observed vibronic structure in the spectra of conducting salts to specific intramolecular modes.
