Synthetic Metals 156 (2006) 75–80

Evaluation of charge transfer degree in thebis(ethylenethio)tetrathiafulvalene salts

by Raman spectroscopy

Aneta Kowalskaa,∗, Roman Wojciechowskia, Jacek Ulanskia,Marta Mas-Torrentb, Elena Laukhinab,Concepcio Rovirab, Kyuya Yakushic

a Department of Molecular Physics, Technical University of Lodz, 90-924 Lodz, Polandb Institut de Ciencia de Materials de Barcelona, Campus UAB, E-08193 Bellaterra, Spain

c Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan

Received 25 May 2005; received in revised form 5 October 2005; accepted 25 October 2005Available online 6 December 2005

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bstract

Raman spectroscopy studies for a series of charge-transfer salts based on the bis(ethylenethio)tetrathiafulvalene (BET-TTF) were crder to analyse the charge distribution on the donor molecules in the unit cell of crystals. With the help of the density functional theory cns

or BET-TTF0 and BET-TTF+1 molecules it was shown that the Raman spectroscopy can be applied to determine the stoichiometry in thealts. For salts exhibiting increase of the resistivity below ca. 100 K, the Raman spectra at variable temperature indicate that this behaelated to the charge disproportionation phenomenon.

2005 Elsevier B.V. All rights reserved.

eywords: Raman spectroscopy; Charge transfer salts; Charge disproportionation

. Introduction

Conducting low dimensional organic crystals are usuallyormed by planar aromatic molecules that exhibit interfacialtaking allowing overlap of the� electron orbitals. This stackingode leads to one-dimensional systems, which are vulnera-le at low temperature to Peierls lattice distortions that result

n an insulating state[1,2]. Therefore, to avoid this Peierlsistortion is important to promote good contacts between theolecules in the adjacent stacks giving rise to a system with

onductivity in two-dimensions (2-D). So far the most suc-essful results have been obtained using halogenated salts ofis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) in which 2-electronic structures are formed through S· · ·S contacts[1,2].ne of the ways to develop new conducting organic crystals

s the synthesis of the new donor molecules, which, similar to

∗ Corresponding author. Tel.: +48 42 6313205; fax: +48 42 6313218.E-mail address: chwiejcz@p.lodz.pl (A. Kowalska).

BEDT-TTF, can also promote S· · ·S interactions. The donbis(ethylenethio)tetrathiafulvalene (BET-TTF)[3] is hence agood candidate material. This molecule does not show theical conformational isomerism of the ethylene groups founBEDT-TTF due to the higher rigidity of the five memberings, but it was found that BET-TTF exists in the two imeric forms shown inFig. 1a and b. There exist a numberthe charge transfer BET-TTF-based salts that below 100 Ka metal–insulator transition of unknown nature[3]. Structuraphase transitions in molecular crystals can be investigateX-ray diffraction, which is a standard technique used for demination of the crystal structures. In addition, the analysthe X-ray data allows one to calculate the electronic structumolecular crystals. However, there is a need for an experimtechnique useful for rapid characterization of newly obtacrystals, since they are often obtained as a mixture of mucrystalline phases, which have also often different comptions[1,2]. Even an approximate stoichiometry evaluation gsome indication if the prepared crystals may potentially hthe desired electrical properties. For this purpose, the m

379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2005.10.012

76 A. Kowalska et al. / Synthetic Metals 156 (2006) 75–80

Fig. 1. Chemical structure oftrans (a) andcis (b) isomers of BET-TTF andschematically shownνa, νb andνc vibrations in the BET-TTF molecule (c).

Raman spectroscopy has been successfully applied, because ofthe frequencies of some vibrational modes that are sensitive tothe charge population, i.e. oxidation state of the donor molecules[4–7]. In general, the TTF-like core has three charge sensitivevibrations connected with the CC bonds stretching, two ofthem are symmetrical and one is unsymmetrical (seeFig. 1c)[8,9].

In this paper, we present results of micro-Raman spec-troscopy studies of the BET-TTT donor and its charge transfersalts: (BET-TTF)9(ReO4)4·2THF [10], (BET-TTF)3(ReO4)2,[10], (BET-TTF)(ReO4) [10], (BET-TTF)2PF6 [11], (BET-TTF)2SCN, [12] and (BET-TTF)SCN[12] as well as calcula-tions of the frequencies of the CC bond stretching modes inthe neutral BET-TTF0 molecules and monovalent BET-TTF+1

ions using the density functional theory (DFT)[13]. We willshow a real potential for application of micro-Raman spec-troscopy for determination of stoichiometry of newly preparedBET-TTT-based salts as made before for charge transfer salbased on BEDO-TTF or BEDT-TTF[4,8]. The second part ofthe paper is devoted to the study of the temperature dependence of the Raman spectra measured down to 15 K in ordeto investigate if the charge disproportionation is the origin ofthe conductivity decrease occurring below 100 K in metallicBET-TTF-based salts. It is known that in such a case a characteristic splitting of the Raman bands steaming from the chargesensitive vibrations should be observed[5,6]. For example, the�2 tot

2

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B odu is se( them itht eo rrieo

2.2. Micro-Raman spectroscopy

Raman spectra at 80 K were recorded using a Micro-RamanSpectrometer (Yobin-Yvon 64000) and 514.5 nm laser light(with 1 cm−1 spectral resolution). The laser beam was defocusedin the microscope to avoid light induced sample degradation.Single crystals were mounted by grease on a cold stage in ahomemade helium cryostat, and cooled down. First, the spectrafor all the crystals were collected for three different orientationsof the laser light polarization plane and the orientation yieldingthe strongest signal was chosen for further studies. Additionally,Raman spectra were collected down to 15 K using a RenishowRamanscope System 1000 and 780 nm diode laser excitation(with 2 cm−1 spectral resolution).

2.3. Materials

Single crystals of the neutral BET-TTF donor were obtainedfrom CS2. Single crystals of its related charge transfer saltswith four different stoichiometries (BET-TTF)9(ReO4)4·2THF,(BET-TTF)2PF6, (BET-TTF)2SCN, (BET-TTF)SCN, (BET-TTF)(ReO4) and (BET-TTF)3(ReO4)2 were synthesized byelectrocrystallisation according to the previously described pro-cedures[3,11,12]. The neutral BET-TTF molecules in thecrystals present a positional disorder of the external sulphuratoms (85%trans majority orientation). For some salts, thee TFd -tTc rep[( irr sis-t ls of( r ofi ft

3

3

o ec ,t tioni TTFm g asi inglec oat”o

s dv

′′(BEDT-TTF)3(HSO4)2 salt shows one band at 1488 cm−1 at00 K (corresponding toν2 mode) that splits below 100 K in

wo at 1473 and at 1527 cm−1 [5,6].

. Experimental

.1. DFT calculations

The molecular geometry of the BET-TTF0 (neutral) andET-TTF+1 (cation) were first optimised by the DFT methsing the scheme B1-LYP employing the Gaussian bas6–31 G(d)) for each atom. There were no restrictions onolecular symmetry. All the calculations were performed w

he GAUSSIAN 98 program package[13]. Based on thesptimised structures the vibrational analyses were then caut.

ts

-r

-

t

d

lectrocrystallization results in isomerization of the BET-Tonor. The (BET-TTF)2PF6, and (BET-TTF)3(ReO4)2 salts con

ain mainly trans-isomer, while the (BET-TTF)2SCN, (BET-TF)SCN and (BET-TTF)ReO4 salts containcis-isomer. In thease of the (BET-TTF)9(ReO4)4·2THF salt both isomers aresent. Four of selected salts, i.e. (BET-TTF)9(ReO4)4·2THF

10], (BET-TTF)2PF6 [10,11], (BET-TTF)2SCN [3,12] andBET-TTF)3(ReO4)2 [10] show metal-like behaviour of theesistivities from 300 K down to 100 K; below 100 K the reivity start to increase. On the other hand, single crystaBET-TTF)ReO4 salt demonstrate semiconductor behaviouts resistance in the range 300–80 K[10] and single crystals ohe (BET-TTF)SCN salt are insulators[3,12].

. Results and discussion

.1. DFT calculations

The DFT calculations were made for thetrans andcis isomersf the BET-TTF molecule (Fig. 1a and b). According to thalculations for the TTF derivatives[14] with the charge of 0he BET-TTF molecule adopts a distorted “boat” conforman the neutral ground state, but it is known that neutral BET-

olecules crystallize with a “planar” conformation, havinlightly higher energy than the “boat” conformation[3]. Taking

nto account that Raman experiments were prepared for srystals, we employ the “planar” conformer instead of the “bne.

The planar BET-TTFtrans andcis isomers haveC2h andC2vymmetries, respectively. Therefore, three CC stretching baseibrations are divided into two ag and one bu modes for thetrans-

A. Kowalska et al. / Synthetic Metals 156 (2006) 75–80 77

Table 1Frequencies of symmetrical and unsymmetrical CC stretching vibration forcis and trans isomers of neutral BET-TTF and its monocations calculated byDFT and obtained experimentally

Calculated (cm−1) Experimental (cm−1)

νa νc νb νa νc νb

cis-BET-TTF0 1602 1589 1550 1594 – 1527, 1535trans-BET-TTF0 1602 1589 1550cis-BET-TTF+1 1510 1463 1405 1505 1463 1415trans-BET-TTF+1 1509 1463 1405

BET-TTF, while there are two a1 and one b1 modes for thecis-BET-TTF. The molecular symmetries are lowered in the tricliniccrystal structures, therefore in our calculationsC1 point groupsymmetry was used for both BET-TTF isomers. To distinguishbetween the symmetrical and unsymmetrical vibrations we haveanalysed the calculated relative velocities of the atoms relatedto the selected vibrational mode.

Table 1shows the DFT calculated frequencies for the threeC C bonds stretching modes of the neutral and BET-TTF+1

molecules. The calculations show that there are no differencesin the frequencies of corresponding modes between thecis andtrans isomers. For the neutral molecules two of these modes aresymmetrical, composed of mixed central and in-phase ring CCstretching, while the third mode is found to be unsymmetricalstretching of the ring CC bonds. We are labelling symmetricalmodes asνa, νb and unsymmetrical asνc (seeFig. 1c). Calcula-tions for the cation BET-TTF+1 show, that among the symmetri-cal modes, one (νa) is mainly ring C C bonds stretching, whilethe other one (νb) is only central C C bond stretching. The thirdmode is found to be unsymmetrical stretching of the rings CCbonds, similarly as for the neutral molecule. The calculationsshow also some other modes composed of the low amplitudeC C bonds stretching mixed with CH bending mode.

3.2. Raman spectra

ml vecT d( calCs ctionw esemi s ora

l inFTsS latedbν

Fig. 2. Raman spectra of neutral BET-TTF (a) and its salts: (BET-TTF)9(ReO4)4·2THF (b), (BET-TTF)2SCN (c), (BET-TTF)2PF6 (d), (BET-TTF)SCN (e), (BET-TTF)ReO4 (f), taken for 514.5 nm laser line at 80 K.

Raman spectra of the salts containing BET-TTF+ (salt sto-ichiometry 1:1) are shown inFig. 2e and f. In each of thesespectra, the two most intensive bands are observed at 1505,1415 cm−1 and 1511, 1409 cm−1 labelled as e1, e2, and f1, f2bands, respectively. The experimental frequencies are again ingood agreement with the symmetrical CC stretching modescalculated at 1510 and 1405 cm−1 for BET-TTF+ (Table 1.).Therefore, we have assigned the bands e1 and f1 to theνa modeand e2, and f2 to theνb mode (Table 2.).

Table 2Raman bands frequencies of the symmetric CC vibrationsνa andνb observed at80 K and the charge on the donor molecule calculated from known stoichiometryof the salts (δstoich)

Compound δstoich νb (cm−1) νa (cm−1)

BET-TTF 0.00 1535 1594(BET-TTF)9(ReO4)4·2THF 0.44 1483 1554(BET-TTF)2SCN 0.50 1474 1551(BET-TTF)2PF6 0.50 1484 1555(BET-TTF)3(ReO4)2 1.00 1420 1509(BET-TTF)SCN 1.00 1415 1505(BET-TTF)ReO4 1.00 1409 1511

The Raman spectra shown inFig. 2were taken with 514.5 naser excitation at 80 K for neutral BET-TTF (a), and fiharge transfer salts: (BET-TTF)9(ReO4)4·2THF (b), (BET-TF)2SCN (c), (BET-TTF)2PF6 (d), (BET-TTF)SCN (e) anBET-TTF)ReO4 (f). For all these salts, the two symmetri

C stretching-based vibrational modes labelledνa and νbhould be observed in the Raman spectra and in this see will limit our discussion only to the bands assigned to thodes. We assume that other bands are related to CH bend-

ng modes or result from splitting of the Raman active bandctivation of some IR modes.

The Raman spectrum of the neutral BET-TTF crystaig. 2a exhibits two main bands, a1 at 1594 and a2 at 1535 cm−1.he calculated frequencies of the symmetrical CC bondstretching vibrations are at 1602 and 1550 cm−1 (seeTable 1.).atisfactory agreement of experimental and DFT calcuands enable us to assign the a1 and a2 bands as theνa andb normal modes, respectively.

78 A. Kowalska et al. / Synthetic Metals 156 (2006) 75–80

Fig. 3. View of the crystal packing of (BET-TTF)3(ReO4)2 alonga-axis.

For the salts with the BET-TTF donor with oxidation statebetween 0 and +1 we expect the frequencies of theνa andνbmodes to be found between the frequencies observed for thecrystals composed of the BET-TTF0 and BET-TTF+1 molecules.Fig. 2c and d show the Raman spectra taken for the 2:1 sto-ichiometry salts, i.e. with BET-TTF+0.5. The two most inten-sive bands for salt (BET-TTF)2SCN and salt (BET-TTF)2PF6are found, at 1551, 1474 cm−1 (c1, c2) (Fig. 2c) and at 1555,1484 cm−1 (d1, d2) (Fig. 2d), respectively. We have assignedthese bands asνa andνb, consequently. The Raman spectrumof the (BET-TTF)9(ReO4)4·2THF salt (Fig. 2b), with an aver-age of +0.44 charge on the BET-TTF molecule, exhibits twomain bands, b1 and b2, at 1554 and 1483 cm−1. These bandsare at almost the same frequencies as the d1 and d2 bands forthe (BET-TTF)2PF6 salt (cf.Fig. 2d). Because the stoichiometrydifference is small we assign these bands also to theνa andνbmodes, respectively.

The crystal structure of the (BET-TTF)3(ReO4)2 salt containstwo types of donor layers[10]. According to the X-ray diffrac-tion data one layer is composed of BET-TTF+1 having very shortcontacts with the anions, while the second layer consists of twoequivalent BET-TTF+0.5molecules having a�-type crystal pack-ing (seeFig. 3). The metallic properties of this salt have beenattributed to the�-type mixed valence layer. Molecules withdifferent oxidation states are stacked along different crystal-lographic directions. We have, therefore, recorded the Ramans rdert tlyc alw itionf m ‘ai r toe tensba s-t nedf ationy st r-i ,w F( edf enseb F

Fig. 4. The Raman spectra of the (BET-TTF)3(ReO4)2 salt taken for 514.5 nmlaser line with three mutually perpendicular polarisations of the incident light;spectrum (a) corresponds to incident light polarisation parallel to the long axisof BET-TTF+1, (b) and (c) were taken in the plane perpendicular to the long axisof BET-TTF+1 (see text).

with oxidation state lower than +1, however they do not corre-spond to BET-TTF+0.5 and do not show higher intensities forthe laser light polarization parallel to the molecular long axis ofthe nominally +0.5 charged BET-TTF. Moreover, the two bandsfound in the frequency range 1600–1650 cm−1 are due to theReO4

− anions, as we have verified by taking the Raman spectrafor KReO4 powder samples. From the chemical composition,every BET-TTF molecule cannot have +1 charge. Therefore, weconsider that the bands corresponding to the BET-TTF+0.5 arenot visible probably due to the weak resonance enhancement inBET-TTF+0.5, which has a different resonance condition fromthat of BET-TTF+1.

Fig. 5shows the existent relation between charge (δ) on theBET-TTF molecules (0≤ δ ≤ 1) calculated from the nominalstoichiometry (δstoich) of the salts and the experimental fre-quencies of the charge sensitive CC bonds stretching modes.We have found a good linear relationship between theδstoichand the frequencies of theνa and νb modes as shown inFig. 5. The straight lines can be described by two equations:νa =−87δexp+ 1594 andνb =−122δexp+ 1535, whereδexpis thecharge on the BET-TTF molecules based on experimental data.Thus, by applying these equations to the frequencies of Ramanspectra bands of newly synthesized BET-TTF salts, the degreeof charge transfer and the stoichiometry of these salts can beestimate.

F eda latedf

pectra for three mutually perpendicular polarisations in oo distinguish theνa andνb modes stemming from the differenharged molecules (seeFig. 4). The first orientation of the crystas established by rotating the crystal in order to find the pos

or which the spectrum was the most intensive—see spectrun Fig. 4. The two following orientations were perpendiculaach other and to the first one. In all spectra the two most inands, g1 at 1509 cm−1 and g2 at 1420 cm−1, correspond toνandνb modes of the BET-TTF+1. Taking into account the cry

al structure[10] and relative intensities of the bands obtaior different polarisations, we have deduced that the polarisielding the most intensive g1 and g2 bands (Fig. 4a) correspondo the direction along the BET-TTF+1 long axis. For any polasation in the plane perpendicular to the BET-TTF+1 long axise were not able to detect the bands related to the BET-TT+0.5

see, for example,Fig. 4b and c). Similar results were obtainor two other laser lines, 488 and 633 nm. The two less intands found at 1535 and 1452 cm−1 could be due to BET-TT

’

e

ig. 5. Plots of theνa andνb symmetric C C vibration frequencies determint 80 K vs. oxidation states of the BET-TTF molecules in various salts calcu

rom their stoichiometry (cf.Table 2).

A. Kowalska et al. / Synthetic Metals 156 (2006) 75–80 79

Fig. 6. Raman spectra of the (BET-TTF)2PF6 salt taken for 785 nm laser line atdifferent temperatures.

The (BET-TTF)9(ReO4)4·2THF, (BET-TTF)2 SCN, (BET-TTF)2PF6 and (BET-TTF)3(ReO4)2 salts show phase transitionsfrom the metallic to the semiconducting state at 75, 70, 100 and125 K, respectively[10,11,12]. Since all of these compoundsshow no discontinuous change in electrical resistivity, these insulating states are regarded not as band insulating states butlocalized states[10,11]. The low-temperature phases of (BET-TTF)9(ReO4)4·2THF and (BET-TTF)3(ReO4)2 are interpretedas a weakly localized state which are driven by the positionadisorder of the external sulphur atoms[10]. The low-temperaturephase of (BET-TTF)2PF6 is interpreted as a Mott–Hubbard insu-lator[11]. These interpretations should be examined by an additional experiment, because the Wigner crystal-like localization(charge disproportionation) should be also taken into accoun[5,6]. In order to examine the nature of the phase transitions, wehave measured the temperature dependence of the Raman sptra of these salts. The Raman spectra of the three compoundshowed no temperature dependence except for the narrowinof linewidth and a small frequency shift.Fig. 6 shows, as anexample, the Raman spectra measured for the (BET-TTF)2PF6salt with 785 nm laser line, for polarization perpendicular to theconducting plane, i.e. almost parallel to the long axis of BET-TTF. Except the usual thermal narrowing and small shift of the

h1 (1555 cm−1) and h2 (1484 cm−1) bands (νa andνb, respec-tively) we have found no splitting of the Raman bands in thespectra from room temperature down to 15 K. If the charge-disportionation occurs like the�-type and�-type BEDT-TTFsalts, for example, the charge distribution changes from BET-TTF+0.5BET-TTF+0.5to BET-TTF+0.2BET-TTF+0.8[5,6]. In thiscase,νa mode should show splitting of∼52 cm−1, because ofthe linear relation between the charge and frequency obtained inthe preceding paragraph. The experimental results indicate thatthe low temperature increasing of resistivity in the salts (BET-TTF)9(ReO4)4·2THF, (BET-TTF)2 SCN and (BET-TTF)2PF6 isnot ascribed to the charge disproportionation. The experimentalresults support the interpretation by Khasanov et al. for (BET-TTF)9(ReO4)4·2THF and Tarres et al. for (BET-TTF)2PF6. Aswe could not observe the Raman bands of BET-TTF+0.5of (BET-TTF)3(ReO4)2, we were able to add no information about thelow-temperature insulating state of this compound.

Finally, we give a brief comment on the additional bands otherthan h1 and h2 of (BET-TTF)2PF6. We can notice in the spectrain Fig. 6also some other bands marked as h3 (1526 cm−1) andh4 (1515 cm−1). These two bands are seen in the entire mea-sured temperature range and, therefore, they can not be relatedto the C C bonds symmetrical stretching responsible for thephase transition. Eventual splitting of theνa andνb would takeplace if the crystal symmetry was lowered at the phase tran-s ees calv ionso sis-t ET-T dw ET-T4 -t

4

trond n thisd cy oft hec for-m e oft andt pec-t norsw

low-t ET-Ts nom-e thec re byt rs.

-as

l

-

t

ec-sg

ition. The presence of at least one of h3 and h4 bands can bxplained by Raman activation of the unsymmetrical CC bondstretching mode (νc). In certain conditions the unsymmetriibrations of one molecule is changing to symmetrical vibratf two molecules forming a dimer. This observation is con

ent with the fact that molecules in the structure of the (BTF)2PF6 salts are slightly dimerized[12]. This additional banas observed for three salts with different stoichiometry: (BTF)ReO4 (BET-TTF)3(ReO4)2 and (BET-TTF)2PF6 (Figs.2f,and 6, at frequency of 1463, 1470 and 1523 cm−1, respec

ively). Therefore, we assigned them asνc.

. Conclusions

By studying the Raman spectra of the BET-TTF eleconor as well as six different charge transfer salts based oonor, we have found a linear dependence of the frequen

he charge sensitive symmetric CC stretching modes, upon tharge on the BET-TTF molecule. The application of thisula should allow for the estimation of the oxidation stat

he donor BET-TTF molecules in newly synthesized salts,hus, the determination of their stoichiometries by Raman sroscopy (the situation can be more complicated when doith different charges are present in the same crystal).Low-temperature Raman studies show that the

emperature increase of the resistivity, observed in the (BTF)9(ReO4)4·2THF, (BET-TTF)2PF6 and (BET-TTF)2 SCNalts, is not associated with charge disproportionation phena; it results most probably from the weak localization ofharge carriers, which can be provoked at low temperatuhe orientational disorder of BET-TTF in the BET-TTF laye

80 A. Kowalska et al. / Synthetic Metals 156 (2006) 75–80

Acknowledgments

We wish to thank to Prof. P. Paneth from Technical Universityof Lodz for help in performing the DFT calculations using theGaussian 98. This work was financed partially by grant KBN Nr7 T08A 013 20, BQU2003-00760, and 6FP NAIMO IntegratedProject No NMP4-CT-2004-500355.

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