J O U R N A L O F C H E M I S T R Y Materials Low temperature crystal structure of the organic metal ([2H8]BEDT–TTF)4Cl2·6D2O [BEDT–TTF= bis(ethylenedithio)tetrathiafulvalene] Philippe Guionneau,*†a Cameron J. Kepert,b Matthew Rosseinsky,b Daniel Chasseau,a Jacques Gaultier,a Mohamedally Kurmoo,c‡ Michael B. Hursthoused and Peter Dayc aL aboratoire de Cristallographie et Physique Cristalline, Universite� Bordeaux I, 351 cours de la L ibe�ration, F-33405 Talence Cedex, France bInorganic Chemistry L aboratory, South Parks Road, Oxford, UK OX1 3QR cT he Royal Institution of Great Britain, 21 Albemarle Street, L ondon, UK W1X 4BS dSchool of Chemistry and Applied Chemistry, University College of Wales, CardiV, UK CF1 3TB ([2H8]BEDT–TTF)4Cl2·6D2O [BEDT–TTF=bis(ethylenedithio)tetrathiafulvalene] exhibits a transition from semimetal to semiconductor at T=160 K (Rosseinsky et al., J.Mater. Chem., 1993, 3, 801). This electronic transition is accompanied by a structural transition that is characterized by the reversible appearance of superstructure reflections corresponding to the doubling of the cell parameter b and a change in space group from Pcca to Pbcn.The crystal structures are very similar above and below T and the calculated intermolecular transfer integrals scarcely change. In contrast, from the diVerence in intramolecular bond lengths, it is clear that the two crystallographically independent BEDT–TTF molecules carry diVerent charges at low temperature, suggesting that a degree of ionicity arising in the BEDT–TTF layer is responsible for the change in electrical behaviour.Progress in understanding the relation between the structures diVerent from those of other ET chloride salts. They belong to the orthorhombic space group Pcca. The structural arrange- and properties of molecular materials has required the development of many new methods. For example, structure refinement ment consists of columns of dimerized ET parallel to c that form layers parallel to the bc plane separated by anion sheets by X-ray diVraction at low temperatures or high pressures is needed to shed light on the crystal structure at the points in (Fig. 1). The main characteristic of the stacks is the alternation of parallel ET with twisted ET, the angle between dimers being the phase diagram where transformation takes place in physical properties.While high pressure X-ray investigations are still about 30° (Fig. 2). The structure is therefore nearer to that of the a¾-ET phase than that of the other ET salts with Cl anions. very rare,1 low temperature crystal structure determination is increasingly accessible as a result of the introduction of area The intermolecular interactions are strong both within the chains and between adjacent stacks, indicating a strong 2D detectors, as evidenced by several recent publications.2 BEDT–TTF (C10H8S8, also ET) salts have attracted great electronic behaviour.The Cl and O atoms of the anion layer create an unusual two-dimensional network formed by hydro- interest because of their diversity in physical properties due to extensive structural polymorphism.The chloride salts form gen bonds. In a previous paper,9 we noted the need for structural information at low temperature to explain the an interesting set, ET3Cl2·2H2O,3 ET4Cl2·4H2O4 and ET3Cl2.5·H5O2 5 are metallic under ambient conditions and mechanism of the electronic transition in this compound. Here, we present the temperature dependence of the crystal struc- undergo a transition towards a less conducting state at low temperature (Tc=100, 50 and 70 K respectively), the 352 salt ture and the intermolecular electronic interactions of ([2H8]ET)4Cl2·6D2O. becoming superconducting at 5 K on applying a pressure of up to 0.5 GPa.The recently reported salt ET3Cl2·5H2O6 behaves as a semiconductor. ([ 2H8]ET)4Cl2·6D2O behaves as Experimental a semimetal7 from room temperature (s=40 Scm-1) to 160 K where a transition occurs towards a semiconducting state with The quality of each crystal was checked by X-ray diVraction on films before use.The crystals studied were black needles of a maximum activation energy of 30 meV. On applying pressure, the conductivity increases and the transition temperature approximate dimensions 1.4×0.4×0.2 mm,2 all of which had a small fraction of twinning.decreases until the transition is suppressed at pressures greater than 2.4 GPa. The thermopower decreases linearly from room The low temperature structural investigation of this salt was performed in two steps. temperature to 160 K and then increases slightly. The magnetic susceptibility is low at room temperature and scarcely increases from 150 K to 50 K but then rises sharply down to 5 K.The room temperature crystal structure has been solved simultaneously by two groups showing that the deuterated salt, ([2H8]ET)4Cl2·6D2O,7 and the hydrogenated salt, ET4Cl2·6H2O,7 are isostructural but with structures very *E-mail: Philippe.Guionneau@durham.ac.uk † Durham Chemical Crystallography Group, Chemistry Department, Durham, UK DH1 3LE.‡ Institut de Physique et Chimie des Materiaux de Strasbourg, CNRSFig. 1 View of the unit cell of ([2H8]ET)4Cl2·6D2O along b UMR 0046, 23 rue du Loess, 67037 Strasbourg Cedex, France. J. Mater. Chem., 1998, 8(2), 367–371 367At the University of Bordeaux I, evolution of the unit cell parameters was followed from 293 to 18 K with a three circle diVractometer equipped with a closed cycle helium cryostat. The angular positions of sets of eighteen Bragg reflections were determined at five fixed temperatures on cooling and refined to give the cell parameters. Fifteen sets were collected on warming.Data collection of the Bragg reflections was performed at 18 K, but the very low accuracy of the structural resolution factor (R=16%), attributed to the quality of the crystals and to a technical problem leading to temperature fluctuations, does not permit us to present these results.At the University College of Wales, CardiV, we used a four circle diVractometer equipped with a nitrogen vapour jet cooling device operating down to 110 K and with a chargecoupled device (CCD) area detector.Strong superstructure reflections corresponding to the doubling of the b parameter Fig. 3 Temperature dependence of the relative intensities of three were seen at low temperature during a fast preliminary investisuperstructure reflections I=f (T ) and I/Imax=f (T /Tc) gation. Extended studies were performed on two crystals, in each case collecting complete structural data sets at 110 K and We note that the conventional space group corresponding performing scans in w at 5 K temperature increments to obtain to the double cell is Pbcn but, in order to compare the the temperature-dependence of the superstructure intensities. structures at low and high temperatures, it is more convenient Results for the two crystals were identical.The structure at to use the non-standard space group Pnca that arises from 110 K was determined starting from the ambient temperature Pbcn by rotating the axes.coordinates using SHELX76.10 The D atoms of the ET extremities were fixed in theoretical positions and those of Lattice parameter evolution D2O were refined from the room temperature positions. The moderately large magnitudes of the R and Rw factors (Table 1) Fig. 4 shows the temperature dependence of the simple cell and standard deviations of atomic coordinates may be attriparameters of the title compound. There are no observable buted to the small fraction of twinning in the crystal. diVerences between the cell parameter values obtained on cooling or warming. Results and Discussion As shown previously for ET salts,1,9,11 the main characteristic of the temperature dependence of the cell parameters is the Superstructure reflections anisotropy. Here, the relative variation of the parameter c (corresponding to the intrastack separation) is the highest, Analysis of the temperature-dependent intensities of more than about twice as large as that for the interstack direct b.The 100 superlattice reflections shows that the transition displays a parameter, which is perpendicular to the organic layers, no structural hysteresis. The temperature of appearance and remains nearly constant over the temperature range studied.disappearance of the reflections is about 165 K (Fig. 3), corre- The fact that the intrastack parameter decreases more than sponding to the change in electrical behaviour (Tc=160 K).At the interstack parameter mirrors the behaviour of a¾-ET2X9 110 K, the temperature of the crystal structure determination, and ET3CuBr2Y2 (Y=Cl or Br)1 even if in those cases the the superstructure reflection intensities are comparable to those most significant changes in conductivity occur in the interstack of the main reflections. Hence, the crystal structure at 110 K direction.correctly represents the structural evolution in the double cell. The temperature variation of the cell parameters changes The variation of I/Imax of representative reflections with markedly between 160 and 200 K with the onset of the (T /Tc) (Fig. 3) must be interpreted with caution because the superstructure reflections, so the choice of the temperature values of Imax are estimated from the incomplete curve I= intervals is important in calculating the isobaric tensor.Thus f (T ). The variation is quite steep when compared to the spinthe isobaric tensor was calculated over two temperature inter- Peierls system a¾-ET2X [X=Ag(CN)2 and AuBr2].9,11 vals (18–150 and 200–293 K) on either side of the transition zone. The principal components of the isobaric thermal expansion tensor are shown as a function of temperature in Fig. 5. The cell is orthorhombic and so the principal directions of expansion lie along the crystallographic axes. The amplitudes, Table 1 Crystal and experimental data ([2H8]ET)4Cl2·6D2O temperature: 110 K Crystal system orthorhombic reflections: Space group Pnca for the cell 85 ET/cell 16 measured 19841 independent ET 2 independent 4838 a/A ° 32.411(4) observed 3158 b/A ° 13.283(3) parameters 349 c/A ° 14.656(2) I/s(I ) 3 V /A ° 3 6309(4) Rint 0.050 D/Mg m-3 1.808 R(Fo) 0.064 F(000) 3464 vR(Fo) 0.061 Dimensions/mm 1.4×0.4×0.2 l/A ° (Mo-Ka) 0.71069 observation: twinned crystals m/mm-1 1.162 Fig. 2 View of the ET layer showing the ribbons of twisted dimers 368 J. Mater. Chem., 1998, 8(2), 367–371Fig. 4 Temperature dependence of the normalised lattice parameters: (%) a/a0, (#) b/b0, (6) c/c0 and ($) V/V0. Room temperature values are a0=32.497(15), b0=6.722(8), c0=14.826(8) A ° and V0=3238(8) A ° 3. Fig. 5 Temperature dependence of components of the thermal expansion tensors aa, ab and ac vary very little from 293 to 200 K but change abruptly between 150 and 18 K.The three amplitudes become nearly zero at 18 K showing that the crystal is no longer van der Waal distances) and the angles formed by the SMS thermally compressible. direction and the average ET plane. We recall that angles near At room temperature, the bulk modulus (aV-1) is high, zero lead to antibonding interactions.11 The b interactions, indicating a low compressibility. The value for the title com- corresponding to angles between the long axes of the ET pound (5300 K) is close to those of the a¾-ET2X salts (around molecules of around 30°, are quite weak and do not change at 6000 K)9 and much lower than that of ET3CuBr4 (9000 K).1 110 K so the asymmetry introduced by b and b¾ does not appear to be significant.The c interactions correspond to short Crystal structure at low temperature distances that decrease at 110 K, the diVerence between c and c¾ being significant but weak.The f interactions correspond to Below the structural transition temperature the unit cell is short distances and favourable angles for strong interactions. doubled without any change in the degree of symmetry. The The transfer integrals corresponding to f and f ¾ are similar at transition therefore leads to the emergence of two crystallolow temperature.Hence, the intermolecular interactions appear graphically independent ET molecules, labelled A and B. At a little stronger at low temperature, the salt retains its layer low temperature the ET stacks formed an …ABABA… character and any irregularity caused by the structural diVersequence.In the simple cell there are five significant near entiation between the two ET is minor. neighbour interactions within each layer (Fig. 6) (a1, a2, b, c, The anionic layer consists of a two-dimensional network: Cl f ). It should be noted that the interactions of the type (x, y, and O atoms linked by H-bonds form chains connected to z)-(1-x, -0.5+y, 0.5-z) are not considered significant each other by short OMCl contacts.At low temperature, the owing to the very large intermolecular distances involved. The distances within these chains scarcely decrease as the interchain number of major interaction modes increases to eight in the OMCl distances increase (Fig. 7). There are no close contacts doubled cell due to the existence of the two independent ET between anions and cations either at room or low temperature, (a1, a2, b, c, f, b¾, c¾, f¾).The stacks remain based on alternation of a1 and a2 in a manner that hardly changes from room temperature to 110 K: the averaged interplanar distances Table 3 Shortest intermolecular SMS distances (A ° ) with the contact between the ET decrease from 3.69(1) to 3.63(1) A ° (a1) and angles [°] between ET of adjacent stacks.Standard deviations are less than 0.005 A ° and 1.0° from 3.68(1) to 3.60(1) A ° (a2) and the torsion angles from 0° to 1(1)° (a1) and 32(1)° to 30(1)° (a2). Table 3 lists the short Interactions 293 K 110 K interstack SMS distances ( less than 1.2 times the sum of the b 4.046 [54.9] 3.938 [53.2] b¾ as for b 3.969 [53.2] Table 2 Averaged intramolecular bond lengths (A ° ) and charge for the 4.026 [51.1] two independent ET, A and B, at 110 K c 3.951 [5.8] 3.892 [7.9] 3.466 [4.0] 3.492 [6.1] 3.468 [4.6] 3.346 [6.4] 3.892 [7.3] 3.839 [8.1] 3.578 [11.1] 3.582 [12.1] 3.476 [9.9] 3.434 [9.8] c¾ as for c 3.932 [8.1] 3.384 [6.2] 3.533 [6.0] 3.880 [7.8] 3.528 [11.9] 3.503 [9.7] f 4.054 [52.3] 4.034 [52.3] 3.709 [57.1] 3.675 [58.1] 3.858 [58.0] 3.909 [58.4] a b c d d charge 3.729 [64.5] 3.618 [65.3] f ¾ as for f 4.024 [51.0] 293 K: 1.363 1.740 1.744 1.341 0.780 0.52 110 K: 3.686 [57.1] 3.773 [57.4] A 1.358 1.734 1.744 1.346 0.774 0.60 B 1.348 1.748 1.753 1.346 0.807 0.38 3.655 [63.2] J.Mater. Chem., 1998, 8(2), 367–371 369Fig. 6 Labelling scheme adopted for the ET layer in the double cell and the symmetry operations Fig. 7 Detail of the anion network and interatomic distances (A ° ) at contrary to the situation found in the a¾-ET2X series or in 293 (italic) and 110 K ET3CuBr4. Electronic interactions and the bond lengths are closer to the theoretical values. Such an ordering has been also observed in the a¾-ET2X series9 and The transfer integrals between neighbouring ET molecules in ET3Cl2·3H2O.13 have been calculated from the Hu� ckel method using a double- The central SMC and CNC bond lengths are quite diVerent f basis set12 for both the room temperature and the low in A and B (Fig. 9). Recently we established a correlation temperature structures (Table 4). between these intramolecular bond lengths and the charge of Results for the room temperature structure indicate that the the ET.14 This method, based on the parameter d= intrachain interactions are quite strong but the degree of (b+c)-(a+d) where b, c are the SMC and a, d the CNC dimerization is high (a1/a2=2.1) and of the same order as that averaged bond lengths of the central TTF (Table 2), enables in the a¾-ET2X series.The interstack transfer integrals between the charge to be estimated with an accuracy of about 0.1 e.coplanar ET are negative and comparable to the smaller The structure refinement is good enough to use this method intrachain one. The high value of the interstack transfer integral in the present case (lt;8% and standard deviations on bond f confirms the two dimensional nature of this salt. It is lengths less than 0.01 A ° ). At room temperature the existence interesting to note the coexistence between an intrachain of a single crystallographically independent ET requires that dimerization tendency and a 2D character.At low temperature, the transfer integrals increase slightly. The intrastack dimerization does not increase below the transition, in contrast to ET3Cl2·2H2O where the dimerization is more pronounced at low temperature.The emergence of two independent ET has little eVect: the diVerences between f, f ¾ and c, c¾ are quite small (20 meV) but agree with the change in electrical behaviour. The diVerence in the site energy of the two ET is an intermediate result in the dimer splitting calculations and arises from the diVerence in charge carried by the two ET. At room temperature this diVerence is zero because all the cations are identical, but at low temperature it increases to 60 meV as a result of the diVerence between A and B molecules.A similar variation has been found in the a¾-ET2X series and interpreted as the appearance of a degree of ionicity in the ET layers. ET conformations and bond lengths At room temperature, the two ethylene extremities of the ET are strongly disordered, as evidenced by the small bond lengths (1.381 and 1.389 A ° instead of 1.53 A ° ) and large atomic thermal coeYcients (Beq>10 A ° 2) (Fig. 8). At 110 K, the ethylene groups Fig. 8 Thermal ellipsoids (90% probability) of the ET molecules at (a) of A and B are ordered indicating that the disorder observed 293 and (b) 110 K at high temperature is dynamic: the values of these coeYcients correspond to those expected at this temperature (Beq<2.8 A ° 2) Table 4 Values of the intermolecular transfer integrals (meV) interactions 293 K 110 K intrastack a1 148 148 a2 70 76 interstack b 30 36 b¾ 30 40 c -80 -100 c¾ -80 -80 Fig. 9 Intramolecular bond lengths of the two independent ET (A and f 207 223 B) in ([2H8]ET)4Cl2·6D2O at 110 K. All standard deviations are less f ¾ 207 205 than 0.008 A ° . 370 J. Mater. Chem., 1998, 8(2), 367–371all the molecules carry a charge of +1/2, in agreement with in the ethylenic extremities and a change in the charge distribution. the charge calculated by the d-method [+0.52(10)]. We also note that the sum of the calculated charge for A and B at We are grateful to Simon Coles for his help in CardiV and to 110 K corresponds to that expected (A+B=+0.98 instead of Laurent Ducasse for the transfer integrals calculation program.+1). At 110 K, the SMC bonds in molecule A become shorter Financial support was received from the EPSRC (UK), the and the CNC longer than in B, corresponding to a diVerence European Community (HCM network) and the Re�gion in charge. From the d-method we estimate that the charges in Aquitaine (France).A and B at 110 K are respectively +0.60(10) and +0.38(10), a diVerence that is quite suYcient to open a gap at the Fermi energy and cause the transition from semimetal to semicon- References ductor. An analogous phenomenon is observed in the a¾-ET2X 1 P. Guionneau, J. Gaultier, D. Chasseau, G. Bravic, Y. Barrans, series where a partial localization of the charges leads to L.Ducasse, D. Kanazawa, P. Day and M. Kurmoo, J. Phys. I Fr., similar values of the cation charges (in this case: 0.66 for one 1996, 6, 1581. ET and 0.33 for the other at 120 K) below the transition 2 See for example: T. Burgin, T. Miebach, J. C. HuVman, towards a less conducting state. L. K. Montgomery, J. A. Paradis, C. Rovira, M.-H.Whangbo, S. N. Magonov. S. I. Khan, C. E. Strouse, D. L. Overmyer and J. E. Schirber, J. Mater. Chem., 1995, 5(10), 1659; H. Kobayashi, K. Kawano, T. Naito and A. Kobayashi, J. Mater. Chem., 1995, Conclusions 5(10), 1681. 3 M. J. Rosseinsky, M. Kurmoo, D. R. Talham, P. Day and The semimetal to semiconductor transition in ([2H8]- D. Watkin, J. Chem. Soc. Chem. Commun., 1988, 88.BEDT–TTF)4Cl2·6D2O is associated with a structural trans- 4 R. P. Shibaeva, R. M. Lobkovskaya, L. P. Rozenberg, L. I. Burarov, A. A. Ignatiev, N. D. Kushch, E. E. Laukhina, ition [(Pcca, a, b, c)<(Pnca, a, 2b, c)] that gives rise to an M. K. Makova, E. B. Yagubskii and A. V. Zvarykina, Synth. Met., overall increase in the strength of the intermolecular inter- 1988, 27, 189. actions; the two dimensional character remains at low tempera- 5 T.Mori and H. Inokuchi, Bull. Chem. Soc. Jpn., 1988, 61, 591. ture. The change in physical properties is not caused by a 6 G. Ono, A. Izuoka, T. Sugawana and Y. Sugawana, Mol. Cryst. molecular rearrangement but by the onset of a diVerence in L iq. Cryst., 1996, 285, 63. 7 M. J. Rosseinsky, M. Kurmoo, P. Day, I. R. Marsden, R.H.Friend, ionicity between the two independent ET. Indeed, the main D. Chasseau, J. Gaultier, G. Bravic and L. Ducasse, J. Mater. structural change resulting from the electronically driven trans- Chem., 1993, 3, 801. ition is the distribution of bond lengths within the ET unit, 8 M. B. Inoue, M. A. Bruck, M. Carducci and Q. Fernando, Synth. which splits into two crystallographically independent units at Met., 1990, 38, 353.low temperature. We believe that it is the separation in energy 9 P. Guionneau, J. Gaultier, M. Rahal, G. Bravic, J. M. Mellado, D. Chasseau, L. Ducasse, M. Kurmoo and P. Day, J.Mater. Chem., between the two independent ET, rather than any major 1995, 5, 1639 changes in the intermolecular interactions, that is important 10 G.M. Sheldrick, SHELX 76, University of Cambridge, 1976. in the development of a semiconducting band structure. The 11 D. Chasseau, J. Gaultier, G. Bravic, L. Ducasse, M. Kurmoo and proposed mechanism for the semimetallic to semiconducting P. Day, Proc. R. Soc. L ond. A, 1993, 442, 207. transition is a lowering of the free electron energy by band 12 A. Fritsch and L. Ducasse, J. Phys. I, 1991, 1, 855.narrowing, which accompanies the separation of charge in the 13 D. Chasseau, S. He�brard, G. Bravic, J. Gaultier, L. Ducasse, M. Kurmoo and P. Day, Synth.Met., 1995, 70, 947. ET layer. Such a mechanism is analogous to the charge- 14 P. Guionneau, C. J. Kepert, D. Chasseau, M. R. Truter and P. Day, density-wave induced metal to semiconductor transition. Synth.Met., 1997, 86, 1973. The analogy between the behaviour of the chloride and the 15 See e.g. ref. 2, 3, 9, 11, also V. E. Korotkov, V. N. Molchanov and a¾-ET2X series [X=Ag(CN)2 or AuBr2] is evident. R. P. Shibaeva, Kristallografiya, 1992, 37, 1437. M. Fettouhi, As the number of complete crystallographic studies at low L. Ouahab, D. Grandjean and L. Toupet, Acta Crystallogr., Sect. B., 1993, 49, 685. M. Fettouhi, L. Ouahab, D. Grandjean and temperature on this class of molecular conductors increases, it L. Toupet, Acta Crystallogr. Sect. B, 1992, 48, 275. M. Kurmoo, will be possible to correlate the transition in physical properties A. W. Graham, P. Day, S. J. Coles, M. B. Hursthouse, with the structural behaviour to extract general trends. From J. M. Caulfield, J. Singleton, L. Ducasse and P. Guionneau, J. Am. the results published up to now,15 it is clear that cooling Chem. Soc., 1995, 117(49), 12 209. strongly influences the intramolecular conformation of the ET, the general outcome being the disappearance of the disorder Paper 7/04818A; Received 7th July, 1997 J. Mater. Chem., 1998, 8(2), 367–371