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Charge-transfer salts based on BET-TTF and the linear ions AuX2− (X = Br, I)
_ VQTlU,I TIIrr ELSEVIER
Synthetic Metals 70 (1995) 883-886
Charge-Transfer Salts based on BET-TTF and the linear ions AuX2 (X = Br, I) C.Rovira', J.Veciana', J.Tarr6s', E.Molins', M.Mas', D.O.Cowan b and S.Yang b "Imtitut de Ci~ncia de Materials de Barcelona (CSIC), Campus UAB, 08193 Bellaterra (Spain) bDepartment of Chemista3t, The John Hopkins University, Baltimore, MD 21218 (USA)
Abstract The organic ~x-electron donor E-bis(ethylenethio)-tetrathiafulvalene (BET-TI'F) forms several charge-transfer salts with linear AuX 2" (X = Br, I) counterions. Experimental conditions significantly affect the shape and the composition of the charge-transfer crystals. In fact, salts with different stoichiometries are obtained from each counterion. The magnetic behavior of the obtained salts have been studied by EPR speclxoscopy. The EPR parameters (g, and AHa3 of different mixed valence salts with the same counterion are very similar. The crystal structure of (BET-TTF)AuBr 2 has been determined by X-Ray diffraction: triclinic system, P-I, Z = I , a = 5.790(4)/~,, b = 7.786(3)]k, c = 9.716(8)A, ~ = 75.11(5)*, ~ = 89.30(4)*, y = 77.20(5)*, V = 395.5(5)A 3 . Single crystal electrical conductivity measurements of (BET-TI'F)zAuBr2 show that this salt exhibits a semiconducting behavior with a 0(300) = 1.1 Scm t and an energy gap of 0.14 eV.
1. I N T R O D U C T I O N The discovery of superconductivity in [~-(BEDT-TIT)2AuI2 has produced much interest in B E D T - T I T salts with A u X ; (X = halogen) t as a counterion as well as in salts derived from related symmetric and asymmetric multisulfur ~-donors with linear inorganic counterions, z3 We are involved in the synthesis of new mulfisulfur gelectron donors having one sulfur atom placed in different position of outer five member cyclic substituents of the T I T core4 and in the preparation and study of their charge-transfer salts with inorganic counterions of different geometries. 5 Following this protocol, here we report on some magnetic, electronic and structural studies of several charge-transfer salts derived from BET-TI'F and the linear counterions AuX 2" (X = I, Br).
(TCE and CH2C12). The experimental conditions significantly affect the shape and composition of the crystals since one or more salts for each counterion are obtained (Table 1). In some cases two or more salts with different stoichiometries grow in the same electrocrystallization.
2.2. Optical measurements Transmission measurements of finely ground KBr pellet samples with a weight concentration about 1% have already been carried out using a Nicolet interferometer (400-4800 cm t) and a visible-UV Varian (3030-20000 cm -t) spectrometers. The results obtained for mixed valence salts are presented on Table 2. The electronic spectra of all mixed-valence salts indicate the typical charge-transfer bands for conducting salts. 8"9 In all these compounds, two large electronic bands are observed; the lower frequency band is situated between 3300-3600 cm -~ for AuBr 2 salts and between 2990-3000 cm -t for AuI 2- salts (Figure 1).
2. R E S U L T S AND D I S C U S S I O N o
2.1. Growth of charge-transfer salts Charge-transfer salts of BET-'I'FF were obtained both, by electlocrystallization and direct chemical oxidation. Electrocrystallization was carried out in a H cell equipped with platinum electrodes. ~ The concentrations ranged from 0.7 to 1 mM for BET-TTF and from 6 to 7 mM for the tetrabutylammoiun salts [nBu4N]X (X = AuBr2, AuI2) used as supporting electrolytes. Dry organic solvents ( T H E PhCI, TCE) were used and a current density ranging from 0.8 to 1 pA]cm 2 was employed. The tetrabutylammonium salts were synthesized from HAuC14 and several tetrabutylammonium halides. 7 Crystal formation was observed within 24 h, and the fully grown were harvested after about 1 week. Crystals obtained in all three solvents were very thin and plate like. Direct oxidation of the donor with both tetrabutylammonium salts was only observed in some solvents 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved S S D ! 0379-6779(94)02690-Z
'|
.~o.o
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Figure 1: IR spectrum of (BET-TI'F)2AuI v
o
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884
C. Rovira et al. / Synthetic Metals 70 (1995) 883-886
Table 1 Preparative conditions and EPR data of charge-txansfer (BET-TrF).(AuX2)* salts Salts
Method"
Appearance
gb
AI-'I~(G)
(BET-TIT)AuBr2 (BET-TIT)2AuBr2" (BET-'I'FF)3(AuBr2)~ (BET-TTF)4(AuBr2)3~ (BET-TTF)2AuI2~ (BET-TTF)3(AuI2)2¢ (BET-TTF).(Aula).
1 (ICE) 1 (THE TCE) 2 (ICE) 2 (CH2CIz) 1 (PhCI) 2 (ICE) 1 (ICE)
black crystal brown flakes brown powder brown powder brown flakes brown powder black crystal
2.0032 2.0043 2.0037 2.0055 2.0064 2.0053
45 49 48 50 50 49
a. 1. Electrocrystalllzation with V constant; 2. Direct Reaction; THF, tetrahydrofurane; TCE, 1,1,2-trichloroethane; PhCI, chlombenzene. b. Averaged g-values corresponding to randomly oriented samples, c. Correct C,H,S,Br and I elemental analysis.
Table 2 Optical data of mixed valence charge-transfer salts Salts
A Band
B Band
(BET-TTF)2AuBr2 (BET-TTF)s(AuBrz) 2 (BET-TTF)4(AuBr2)3 (B ET-'I'rF)2AuI2 (BET-TTF)3(AuI2)2 (BET-TTF).(AuIz).
3300 3600 3460 3000 2959 2931
10638 10152 10823 11235 10731 11414
could indicate that the different salts with the same linear counterion have a very similar BET-'VFF packing arrangement In fact, BET-TrF salts with the same stoichiometry and different octahedral counterions are isoestructural and have very similar EPR properties, s'n .50-
45-
40-
i 0
~a. 35:22 < iA
30uA
Following the Torrance et al. notation, the lower frequency bands correspond to the charge-transfer "A" peak characteristic of a mixed valence states, ~9 while broad and relatively strong "B" bands, that involves a doubly occupied excited state, appear at similar frequencies for all compounds. Beyond the tail of the electronic "A" band we observe (Figure 1) several vibronic bands (% modes) around 1200 and 1500 cm-l resulting from electronicmolecular vibration (e-my) coupling)° 2.3. EPR properties The EPR speclra (X-band) of polycrystalline samples of all mixed valence salts show a broad, symmetrical EPR signal with lorentzian lineshape. The broad linewidth suggest 2D character for these salts. All mixed valence salts with the same counterion have very similar g and peak-to-peak linewidth (AHrp) values (Table 1) in spite of their different stoichiometries. The temperature dependence of the peak-to-peak linewidth and of the normalized signal intensities (Ifl_300)are also very similar as shown in Figures 2 and 3. Thus linewidths exhibit a rapid decrease with decreasing temperature, and at lower temperatures they change smoothly. The systematic decrease in linewidth can be rationalized in terms of electron-scattering processes in the crystal lattice. H Dynamic spin susceptibilities increase with decreasing temperature over the entire temperature range (300-10OK) whereas the g values do not change significantly. Up to now, different stoichiometry charge transfer salts of BET-'I~I'F with the same counterion had very different g and AH~ values, so that they can be easily differentiated by their EPR parameters: The fact that all (BET-'FI'F)m(AuX2). mixed-valence salts have very similar EPR properties
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(b) Figure 2: Temperature dependence of peak-to-peak EPR linewidth (a) and of the normalized signal intensities (b) of the mixed valence salts (BET-'II'F)2AuBr2 • ,(BET-TYF)3(AuBr2~ ° and (BET-TTF)4(AuBr2)3 •
885
C. Rovira et al. / Synthetic Metals 70 (1995) 883-886
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Figure 4: Molecular structure of (BET-TTF)AuBr 2 showing the atomic labeling. TIK
(a)
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(b) Figure 3: Temperature dependence of peak-to-peak EPR finewidth (a) and of the normalized signal intensities 0a) of the mixed valence salts (BET-TTF)2 AuI 2 • ,(BET-TI'F) 3 (AuI2) 2 • , and (BET-TTF)® (AuI2) * -. 2.4. M o l e c u l a r a n d cry s t a l s t r u c t u r e o f ( B E T - T T F ) A u B r , salt W e have characterized by X-ray analysis the salt ( B E T "I-I'F)AuBr 2. This salt crystallizes in the triclinic space group, P-1 with lattice parameters: a = 5.790(4))[, b = 7.786(3))[, c = 9.716 (8)A, a = 75.11(5)*, I] = 89.30(4)*, 7 = 77-20(5) °, V = 395.5 (5)A 3 , Z = 1. The molecular structure of (BET-TTF)AuBr 2 is shown in Figure 4. Molecules of BET-TI'F in this crystal are centrosymmetric and quasi planar showing only a small distortion towards a chair conformation. The angle between the external rings and the TTF core double bonds mean plane is 1.7(1)* being almost the same found in the neutral donor. '~ Figure 5 shows the crystal packing viewed along b axis. The BET-'ITF molecules are tilted stacked along c-axis and the AuBr 2 anions are between these stacks. Within each stack, adjacent molecules have only three atoms overlapped (Figure 7) with an interplanar separation of 3.728(1) ~. Short intermolecular S---S contacts, lower than the sum of the van der Waals radii, are found in the ab plane between molecules belonging to parallel stacks and axe shown by broken lines in Figure 6. The arrangement adopted by the donor in this salt is very similar to that observed for neutral BET-TTF. 4"
Figure 5: Crystal structure of (BET-TTF)AuBr 2 viewed along baxis.
C
Figure 6: Intermolecular S---S contacts in the BET-TI'F layers. $2--- S 3 = 3.601(2)/~
C. Rovira et aL / Synthetic Metals 70 (1995) 883-886
886
ACKNOWLEDGEMENTS This work was supported by a grants from the Programa Nacional de Quh'nica Fina, (CIRIT-CICyT) Spain (QFN93-4510-CO1)and the Human Capital and Mobility Program of the UE (ERBCHRX CT93-0271). Work at Hopkins was suppomcd by a grant from the National Science Foundation (DMR-9Z23481) REFERENCES
Figure 7: Molecular overlap pattern for BET-TTF molecules in the ~ of (BET-TTF)AuBr2. 2.5. Electrical conductivity measurements The fotw probe dc electrical conductivity meastwemcots on crystals of (BET-TTF)~AuBr2 give a zoom temperature conductivity of 1.1 Scm "t. Variable temperature conductivity measurements was c.azrieclout from 300 to 80 K (Figure 8). The resistivity curve shows that (BET-TFF)zAuBr2 is semiconducting with an activation energy of 0.138 eV.
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TEMPERATURE(K} Figure 8: Resistance versus temperature plot for (BET-TI'F)= AuBr:. CONCLUDING REMARKS We have synthesized a new family of charge-transfer salts derived from bis(ethylenedithio)tetrathiafulvalene(BET-TTF) and linear AuX2- (X = I, Br) counterions that display interesting optical and magnetic properties. Further efforts are being made in order to obtain suitable crystals of the mixed valence salts for structure determination and conductivity measurements.
1. H.H. Wang, M.A. Beno, U. Geiser, M.A. Firestone, K.S. Webb, L. Nunez, G.W. Grabtree, ICD. Carlson ,J.M. WiUiams, L.J. Azcvedo, J.F. Kwak and LE. S c ~ , Inorg. Chetm ~__, 2465 (1985). 2. IC Kikuchi, IC Murata, Y. Honda, T. Tamiki, IC Saito, H. Anzai, IC Kobayashi, T. Ishiguro and L lkemoto, J. Phys. Society of Japarg 56, 4241 (1987). 3. A.M. Kini, M.A. Beno, D. Son, H.H. Wang, K.D. Carlson, L.C. Porter, U. Welp, B.A. Vogt, JaM. ~/illlam~:, D. Jtmg, M. Evain, M.H. Whangbo, D.L. Overmyer and J.E. $chirber, Solid State Communication, ~ 503 (1989). 4. a) C. Rovira, N. Santal6 and J. Vcciana, Tetrahedron l.,ctt, 30, 7249 (1989). b) C. Royira, N. Santal6, J. Vcciana, J. Claret, E./Violins and C. Miravitlles, Synth. Met~ 41-43,2205 (1991). c) C. Rovira, J. Veciana, N. Santal6, J. Tarrds, J. Cirujeda, E. Molins, J. Llorca and E. Espinosa, J. Org. Chem. 59, 3307 (1994). 5. a) C. Rovira, N. Santal6, J. Vcciana E. Molins and C. MiravitHe.s, Synth. Met. 41-43 2199 (1991). b) C. Rovira, J. Vcciana, J. Tarrds, N. Santal6 in "New Organic Materials" C. S¢oane, N. Martin Eds. Universidad Complutcnsc Madrid, 1994, p. 58 6. M.M. Lee et aL, Mol. Cryst. Liq. Cryst. 79 145 (1982). 7. P. Braunstein, R-J.H. Clark, W. Ramsay, R. Foster and C. Ingold, J. C. S. Dalton 1845 (1982). 8. C. Garrigou-Lagrange, A. Graja, C. Coulon and P. Delhaes, J. of Physics, C17, 5437 (1984). 9. J.B. Torrance, B.A. Scott, B. Walter, F.B. Kanfman, P.E. Seiden, Phys. Rev, BI9, 730 (1979). 10. a) C. Garrigou-Lagrange, E. Dupart, J.P. Morand and P. Delhaes, Synth. Met., 27, B537 (1988); b) R. Bozio, I. Zanon, A. Girlando, C. Pecile, J. Chem. Phys., 71, 2282 (1979). 11. E. L. Venturini, J. E. Azevedo, J. E. Schirber, J. M. Williams, H. H. Wang, Phys. Rev. B. Condens. Matter 32, 2819 (1985). 12. C. Rovira et al. manuscript in preparation.
Synthetic Metals 70 (1995) 883-886
Charge-Transfer Salts based on BET-TTF and the linear ions AuX2 (X = Br, I) C.Rovira', J.Veciana', J.Tarr6s', E.Molins', M.Mas', D.O.Cowan b and S.Yang b "Imtitut de Ci~ncia de Materials de Barcelona (CSIC), Campus UAB, 08193 Bellaterra (Spain) bDepartment of Chemista3t, The John Hopkins University, Baltimore, MD 21218 (USA)
Abstract The organic ~x-electron donor E-bis(ethylenethio)-tetrathiafulvalene (BET-TI'F) forms several charge-transfer salts with linear AuX 2" (X = Br, I) counterions. Experimental conditions significantly affect the shape and the composition of the charge-transfer crystals. In fact, salts with different stoichiometries are obtained from each counterion. The magnetic behavior of the obtained salts have been studied by EPR speclxoscopy. The EPR parameters (g, and AHa3 of different mixed valence salts with the same counterion are very similar. The crystal structure of (BET-TTF)AuBr 2 has been determined by X-Ray diffraction: triclinic system, P-I, Z = I , a = 5.790(4)/~,, b = 7.786(3)]k, c = 9.716(8)A, ~ = 75.11(5)*, ~ = 89.30(4)*, y = 77.20(5)*, V = 395.5(5)A 3 . Single crystal electrical conductivity measurements of (BET-TI'F)zAuBr2 show that this salt exhibits a semiconducting behavior with a 0(300) = 1.1 Scm t and an energy gap of 0.14 eV.
1. I N T R O D U C T I O N The discovery of superconductivity in [~-(BEDT-TIT)2AuI2 has produced much interest in B E D T - T I T salts with A u X ; (X = halogen) t as a counterion as well as in salts derived from related symmetric and asymmetric multisulfur ~-donors with linear inorganic counterions, z3 We are involved in the synthesis of new mulfisulfur gelectron donors having one sulfur atom placed in different position of outer five member cyclic substituents of the T I T core4 and in the preparation and study of their charge-transfer salts with inorganic counterions of different geometries. 5 Following this protocol, here we report on some magnetic, electronic and structural studies of several charge-transfer salts derived from BET-TI'F and the linear counterions AuX 2" (X = I, Br).
(TCE and CH2C12). The experimental conditions significantly affect the shape and composition of the crystals since one or more salts for each counterion are obtained (Table 1). In some cases two or more salts with different stoichiometries grow in the same electrocrystallization.
2.2. Optical measurements Transmission measurements of finely ground KBr pellet samples with a weight concentration about 1% have already been carried out using a Nicolet interferometer (400-4800 cm t) and a visible-UV Varian (3030-20000 cm -t) spectrometers. The results obtained for mixed valence salts are presented on Table 2. The electronic spectra of all mixed-valence salts indicate the typical charge-transfer bands for conducting salts. 8"9 In all these compounds, two large electronic bands are observed; the lower frequency band is situated between 3300-3600 cm -~ for AuBr 2 salts and between 2990-3000 cm -t for AuI 2- salts (Figure 1).
2. R E S U L T S AND D I S C U S S I O N o
2.1. Growth of charge-transfer salts Charge-transfer salts of BET-'I'FF were obtained both, by electlocrystallization and direct chemical oxidation. Electrocrystallization was carried out in a H cell equipped with platinum electrodes. ~ The concentrations ranged from 0.7 to 1 mM for BET-TTF and from 6 to 7 mM for the tetrabutylammoiun salts [nBu4N]X (X = AuBr2, AuI2) used as supporting electrolytes. Dry organic solvents ( T H E PhCI, TCE) were used and a current density ranging from 0.8 to 1 pA]cm 2 was employed. The tetrabutylammonium salts were synthesized from HAuC14 and several tetrabutylammonium halides. 7 Crystal formation was observed within 24 h, and the fully grown were harvested after about 1 week. Crystals obtained in all three solvents were very thin and plate like. Direct oxidation of the donor with both tetrabutylammonium salts was only observed in some solvents 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved S S D ! 0379-6779(94)02690-Z
'|
.~o.o
.tsa.t
1.67.m
17~.u
itH.o
tTaT.m cc~a~
tlH.
Figure 1: IR spectrum of (BET-TI'F)2AuI v
o
loem.
B
7am .oe
.on
.eo
884
C. Rovira et al. / Synthetic Metals 70 (1995) 883-886
Table 1 Preparative conditions and EPR data of charge-txansfer (BET-TrF).(AuX2)* salts Salts
Method"
Appearance
gb
AI-'I~(G)
(BET-TIT)AuBr2 (BET-TIT)2AuBr2" (BET-'I'FF)3(AuBr2)~ (BET-TTF)4(AuBr2)3~ (BET-TTF)2AuI2~ (BET-TTF)3(AuI2)2¢ (BET-TTF).(Aula).
1 (ICE) 1 (THE TCE) 2 (ICE) 2 (CH2CIz) 1 (PhCI) 2 (ICE) 1 (ICE)
black crystal brown flakes brown powder brown powder brown flakes brown powder black crystal
2.0032 2.0043 2.0037 2.0055 2.0064 2.0053
45 49 48 50 50 49
a. 1. Electrocrystalllzation with V constant; 2. Direct Reaction; THF, tetrahydrofurane; TCE, 1,1,2-trichloroethane; PhCI, chlombenzene. b. Averaged g-values corresponding to randomly oriented samples, c. Correct C,H,S,Br and I elemental analysis.
Table 2 Optical data of mixed valence charge-transfer salts Salts
A Band
B Band
(BET-TTF)2AuBr2 (BET-TTF)s(AuBrz) 2 (BET-TTF)4(AuBr2)3 (B ET-'I'rF)2AuI2 (BET-TTF)3(AuI2)2 (BET-TTF).(AuIz).
3300 3600 3460 3000 2959 2931
10638 10152 10823 11235 10731 11414
could indicate that the different salts with the same linear counterion have a very similar BET-'VFF packing arrangement In fact, BET-TrF salts with the same stoichiometry and different octahedral counterions are isoestructural and have very similar EPR properties, s'n .50-
45-
40-
i 0
~a. 35:22 < iA
30uA
Following the Torrance et al. notation, the lower frequency bands correspond to the charge-transfer "A" peak characteristic of a mixed valence states, ~9 while broad and relatively strong "B" bands, that involves a doubly occupied excited state, appear at similar frequencies for all compounds. Beyond the tail of the electronic "A" band we observe (Figure 1) several vibronic bands (% modes) around 1200 and 1500 cm-l resulting from electronicmolecular vibration (e-my) coupling)° 2.3. EPR properties The EPR speclra (X-band) of polycrystalline samples of all mixed valence salts show a broad, symmetrical EPR signal with lorentzian lineshape. The broad linewidth suggest 2D character for these salts. All mixed valence salts with the same counterion have very similar g and peak-to-peak linewidth (AHrp) values (Table 1) in spite of their different stoichiometries. The temperature dependence of the peak-to-peak linewidth and of the normalized signal intensities (Ifl_300)are also very similar as shown in Figures 2 and 3. Thus linewidths exhibit a rapid decrease with decreasing temperature, and at lower temperatures they change smoothly. The systematic decrease in linewidth can be rationalized in terms of electron-scattering processes in the crystal lattice. H Dynamic spin susceptibilities increase with decreasing temperature over the entire temperature range (300-10OK) whereas the g values do not change significantly. Up to now, different stoichiometry charge transfer salts of BET-'I~I'F with the same counterion had very different g and AH~ values, so that they can be easily differentiated by their EPR parameters: The fact that all (BET-'FI'F)m(AuX2). mixed-valence salts have very similar EPR properties
•
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150
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i
200
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250
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(b) Figure 2: Temperature dependence of peak-to-peak EPR linewidth (a) and of the normalized signal intensities (b) of the mixed valence salts (BET-'II'F)2AuBr2 • ,(BET-TYF)3(AuBr2~ ° and (BET-TTF)4(AuBr2)3 •
885
C. Rovira et al. / Synthetic Metals 70 (1995) 883-886
.50-
.,m
A"
,sl
21 A
Ae
m
Ao|a AA8 u o
(-~ 4o cL .° 35,
AA| Aeg
Ae Ae•
30
251 I I"
2oi
Figure 4: Molecular structure of (BET-TTF)AuBr 2 showing the atomic labeling. TIK
(a)
0
2.0 ¸ o o
ee ee
1.5
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•[it'-'.'.
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(b) Figure 3: Temperature dependence of peak-to-peak EPR finewidth (a) and of the normalized signal intensities 0a) of the mixed valence salts (BET-TTF)2 AuI 2 • ,(BET-TI'F) 3 (AuI2) 2 • , and (BET-TTF)® (AuI2) * -. 2.4. M o l e c u l a r a n d cry s t a l s t r u c t u r e o f ( B E T - T T F ) A u B r , salt W e have characterized by X-ray analysis the salt ( B E T "I-I'F)AuBr 2. This salt crystallizes in the triclinic space group, P-1 with lattice parameters: a = 5.790(4))[, b = 7.786(3))[, c = 9.716 (8)A, a = 75.11(5)*, I] = 89.30(4)*, 7 = 77-20(5) °, V = 395.5 (5)A 3 , Z = 1. The molecular structure of (BET-TTF)AuBr 2 is shown in Figure 4. Molecules of BET-TI'F in this crystal are centrosymmetric and quasi planar showing only a small distortion towards a chair conformation. The angle between the external rings and the TTF core double bonds mean plane is 1.7(1)* being almost the same found in the neutral donor. '~ Figure 5 shows the crystal packing viewed along b axis. The BET-'ITF molecules are tilted stacked along c-axis and the AuBr 2 anions are between these stacks. Within each stack, adjacent molecules have only three atoms overlapped (Figure 7) with an interplanar separation of 3.728(1) ~. Short intermolecular S---S contacts, lower than the sum of the van der Waals radii, are found in the ab plane between molecules belonging to parallel stacks and axe shown by broken lines in Figure 6. The arrangement adopted by the donor in this salt is very similar to that observed for neutral BET-TTF. 4"
Figure 5: Crystal structure of (BET-TTF)AuBr 2 viewed along baxis.
C
Figure 6: Intermolecular S---S contacts in the BET-TI'F layers. $2--- S 3 = 3.601(2)/~
C. Rovira et aL / Synthetic Metals 70 (1995) 883-886
886
ACKNOWLEDGEMENTS This work was supported by a grants from the Programa Nacional de Quh'nica Fina, (CIRIT-CICyT) Spain (QFN93-4510-CO1)and the Human Capital and Mobility Program of the UE (ERBCHRX CT93-0271). Work at Hopkins was suppomcd by a grant from the National Science Foundation (DMR-9Z23481) REFERENCES
Figure 7: Molecular overlap pattern for BET-TTF molecules in the ~ of (BET-TTF)AuBr2. 2.5. Electrical conductivity measurements The fotw probe dc electrical conductivity meastwemcots on crystals of (BET-TTF)~AuBr2 give a zoom temperature conductivity of 1.1 Scm "t. Variable temperature conductivity measurements was c.azrieclout from 300 to 80 K (Figure 8). The resistivity curve shows that (BET-TFF)zAuBr2 is semiconducting with an activation energy of 0.138 eV.
+1.00
.
.
.
.
.
.
.
.
.
.
.
t...-J
x
+8.80
÷0.60 =: +0 40 m
~ +0.20 +0.00
i - - i '
i
loo
)
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i
360
TEMPERATURE(K} Figure 8: Resistance versus temperature plot for (BET-TI'F)= AuBr:. CONCLUDING REMARKS We have synthesized a new family of charge-transfer salts derived from bis(ethylenedithio)tetrathiafulvalene(BET-TTF) and linear AuX2- (X = I, Br) counterions that display interesting optical and magnetic properties. Further efforts are being made in order to obtain suitable crystals of the mixed valence salts for structure determination and conductivity measurements.
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