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Structure and properties of a new conducting organic charge-transfer salt β-(BEDT-TTF) 2AuBr 2
Solid State Communication,., Vo1.6l,No.8, Printed in Great Britain.
STRUCTURE
AND PROPERTIES
pp.459-464,
OF A NEW CONDUCTING
ORGANIC
1987.
0038-1098187 $3.00 + .OO Pergamon Journals Ltd.
CHARGE-TRANSFER
SALT B_(BEDT-TTF)pAuBrB
M. Kurmoo, D.R. Talham and P. Day Inorganic Chemistry Laboratory, South Parks Road Oxford OX1 3QR, U.K.
Cavendish
I.D. Parker and R.H. Friend Laboratory, Madingley Road, Cambridge
CB3 OHE, U.K.
A.M. Stringer and J.A.K. Howard School of Chemistry, University of Bristol, Cantocks Bristol BS8 lTS, U.K. Received
by R.A. Cowley on 1 October
Close,
1986
A new conducting organic charge-transfer salt, &(BEDT-TTF)2AuBr2 has been synthesised and its crystal structure determined. Single crystal four-probe conductivity has been measured at ambient and applied pressures from 300-1.4K and_EPR spectroscopy from 300-4.2K. S-(BEDT-TTF)~A~B~~ crystallises in PI (Z=l) with a = 9.020(l), b = 5.707(9),Sc = 16.320(1)8; a = 97.60(l), 8 = 92.12(l), y = 102.89(l)'; V = 811.8(2)8 . While the electronic properties are two-dimensional, the molecular packing of the BEDT-TTF molecules is slightly different from that found in other B-phase BEDT-TTF salts with triatomic counterions. 8-(BEDT-TTF)2AuBr2 is metallic to 1.4K at pressures up to 5.6 kbar although at all pressures studied there is appreciable hysteresis in the cooling and warming cycles of the conductivity between 6 and 14K. Abrupt transitions in the temperature dependence of both the peak-to-peak EPR linewidth and integrated signal intensity occur near 20K. The possibility of a low temperature structural modification is discussed.
Introduction
Experimental 8-(BEDT-TTF)2AuBr2 was crystallised by electrochemical oxidation of BEDT-TTF in CH2C12 as the supporting electrowith (n-Bu)kNAuBrp grow as very Crystals small flat lyte. The surface area of the hexagonal plates. crystal faces is generally 0.5mm2 with a thickThey are a minor product ness of O.Ol-0.03mm. of the electrochemical synthesis which yields predominantly a semiconducting 2:l salt." conductivity measurements were Four-probe performed using a pentane pressure medium in a described elsewhere." Electrical system contacts were made to evaporated gold pads at the corners of the plate-like crystals. EPR measurements were performed on single an X-band using reflection crystals Temperature was controlled spectrometer. between 4.2 and 300K in an Oxford Instrument EPR-900 flow cryostat with a DTC-2 temperature controller.
Since the initial report of a conducting salt of BEDT-TTF1, and the subsequent discovery superconductivity under pressure ’ of (BEDT-TTF)2 ReOb2 there has been extensi:: effort into the synthesis and investigation of new charge-transfer salts of BEDT-TTF.3 Among the products of these efforts have been the superconducto;s ambient pressure B8-(BEDT-TTF)zIBrs, and (BEDT-TTF)213,4 BEDT-TTF is bis(ethylene~-(BEDT-TTF)~AuI~~. dithio)tetrathiofulvalene. An unusual feature of BEDT-TTF salts is that they crystallise in morphologies and several crystal stoichiooften under the same metries, synthetic conditions or even in the same electrocrystallisation experiment. For example, in addition to non-superconducting 137, the Bsuperconduc;ors, salts of BEDT-TTF are also AuI2 , and ReO, known. This propensity to crystallise in several different phases is in marked contrast to the TMTCF (C=S,Se) charge transfer salts where a sin le 2:l phase, the Bechgaard salts, predominates 5 . While their structural diversity complicates the study of the properties of the BEDT-TTF salts it provides the opportunity to correlate structures and physical properties in this class of organic conductors. In this communication we report the structure and properties of a new conducting salt of BEDT-TTF, 8-(BEDT-TTF)2AuBr2. Conductivity measurements at ambient and applied pressures together with EPR studies indicate that 8-(BEDT-TTF)2AuBr2 is metallic to 1.4K with evidence of a structural phase transition below 20K.
Results 8-(BEDT-TTF)2AuBr212 crystallises in the space group pi with unit cell triclinic parameters a = 9.020(l), b - 5.707(g), c = OL = 97.60(l), 8 = 92.12(l), y = 16.32O(l)A; 102.89(1p; 2 = 1; V = 811.8(2)A3. We label this salt a "B-phase" due to the similarity of the crystal data, including unit cell volume, to other b-BEDT-TTF salts of triatomic anions. 14 The organic molecules form stacks of dimerised crystallographically and equivalent BEDT-TTF molecules which along with adjacent stacks form 459
A NEW CONDUCTING
460
ORGANIC
CHARGE-TRANSFER
a 2-dimensional layer of donor molecules separated by a columnar layer of AuBrq counterions In contrast to other @-BEDT-TTF (Figure 1).
(ET>
2
AuBr
SALT B-(BEDT-TTF)~AUB~~
Vol. 61, No. 8
to 1.4K. Figure 4 shows the temperature dependent resistance of a typical sample below 200K at pressures between 2 kbar and 5.6 kbar.
2
Fip.1. Structure of k(BEDT-TTF)2AuBr2 showing the alternating organic and inorganic layers.
rialts of trlatomlc anionsI BEDT-TTF molecules within a stack are slightly displaced relative to one another such that the stacking axis forms an angle (4) of about 66 with the plane of the BEDT-TTF molecules (Figure 2). Also in contrast to other p-phase salts, molecules in adjacent side-by-side situated such are that stacks interstack nearest neighbours are nearly coplanThis is similar to the interstack ar (@=P). arrangement found in (BEDT-TTF)2C104.TCEO_ 51. The side-by-side arrangement of BEDT-TTF units is pictured in Figure 3 with short interstack sulphur-sulphur distances labelled. The anion is constrained by symmetry to be linear (the Au-Br bond length = exactly 2,380(1)A and lies within the cavities formed by At room temperathe BEDT-TTF stacks along b. ture, the anion network is ordered. Conductivity was found to be nearly isotropic within the plane of the plates, and highly it.The perpendicular to roomanisotropic conductivity is of the order of temperature 10 Q-'cm-l although absolute conductivity is difficult to determine because the samples used are small so that the contact pad dimensins are comparable to their separation (-60vm). All samples remained metallic to temperatures down
Below pressures of 3.8 kbar there is a knee in the conductivity near 1lOK. This anomaly is removed with increasing pressure such that at it is completely suppressed. At 5.6 kbar ambient pressure there is a slight upturn in below SK which is progressively resistance removed as pressure is increased. Figure 5 resistance vs temperature for several plots At all pressures in the low temperature region. the cooling and warming cycles show pressures, hysteretic behaviour centred near 8K (between 6 and 14K). At a given pressure successive warming cycles yield slightly and cooling possibly due to different different curves, cooling and heating rates. studied show an appreciable All samples The resistance magnetoresistance at 4K. increases up to 50% when a field of 1.3T is applied perpendicular to the sample plane, while a lower increase in R (-15%) is observed with the field parallel to the sample plane. were Lorentizian at signals all EPR the for crystal orientation temperatures here. The maximum (orientation reported peak-to-peak linewidth dependent) (AH) at room-temperature is 58G. Figure 6 shows the temperature dependence of the linewidth and the
Vol. 61, No. 8
A NEW CONDUCTING
ORGANIC
CHARGE-TRANSFER
SALT 8-(BEDT-TTF)2AuBr2
TEMPERATURE View down the long molecular axis of the Fig.2 BEDT-TTF molecules illustrating the inter- and intrastack interactions between donor molecules.
Fig.3 Side-by-side arrangements of BEDT-TTF molecules. The atom numbering scheme as well as some sulphur-sulphur contact distances are labelled.
461
(K I
Fig.4
Temperature dependence of the resistance for B_(BEDT-TTF)2AuBr2 at pressures of 2.0, 3.8 The anomaly near 1OOK is removed and 5.6 kbar. at higher pressures.
spin susceptibility (xs) measured in arbitrary units from the integrated signal intensity and normalised to 300K. The data in Fig. 6 were recorded with the static field parallel, and the microwave field perpendicular, to the plane of plate-like crystal. the orientation This signal maximized the intensity at roomtemperature relative to a 96 rotation. Compared to other p-phase BEDT-TTF salts, the EPR linewidth in the AuBrp- salt is very For example, AH at 300K is less than 20G broad. in the superconductor 8-(BEDT-TTF)2Au1215. In BEDT-TTF salts have large linewidths general, relative to other sulphur x-donors 16 a result that has been attributed to the increased dimensionality15 ,17 brought interaction about by the BEDT-TTF stacks. between The linewidth narrows with decreasing temperatures to 1OG at 30K, below which a sharp decrease occurs near 24K resulting in a peak-to-peak linewidth of less than 1G at temperatures below 16K. At 4.2K, AH = 0.66. Such a narrow linewidth is exceptional in reported BEDT-TTF salts, although a 1G linewidth at low temperature was recently reported for (BEDT-TTF),(C~OI,)~~*. The spin also susceptibility shows a temperature
A NEW CONDUCTING
ORGANIC
CHARGE-TRANSFER
SALT B-(BEDT-TTF),AuBr, ‘
,
Discussion
I
I
I
p-IBE0T-TTF)2A~Br2
I
I
a
L
TEMPERATURE
I
I
12
16
i !O
lK1
pig.5
Resistance vs. temperature as a function of pressure illustrating the hysteretic behaviour observed upon temperature cycling below 2OK.
50
+F+
t 0
AuBr2
++ *
L
0
P-IEEDT-TTF12
z
* 1
I
100
2lnl TEMPERATURE
vol. 61, No. 8 L
IKI
!
300
Temperature dependence of the EPR Fig.6 peak-peak linewidth and intergrated signal intensity (inset) for &-(BEDT-TTF)2AuBr2. The static field, Ho, is aligned perpendicular to the face of the plate-like crystals.
dependence, decreasing to about 0.3 of the room below which xs temperature value at 50K. levels off until a sharp decrease occurs near 20K (inset Figure 6). So far, we have been unable to detect any change in the EPR behaviour associated with the anomaly seen at 1lOK in the transport measurements. This may be due to the limits of precision associated with the broad and weak signals obtained at these temperatures from the small single crystals.
While the crystal data is similar to other 8- and 8'- phase conducting BEDT-TTF salts of triatomic anions, the molecular packing is somewhat different in this AuBr2 salt. This may result from the relatively short AuBr2- anion (anion length = 8.78 calculated from the Au-Br bond length plus the Van der Waal radii of the terminal bromine atoms14). The direct stacking seen in the other p-phases (g-96)" is perhaps less favourable in salts containing shorter anions due to increased core repulsio;sof the out of plane terminal ethylene groups . The mean vertical separation within the stack of the molecules is BEDT-TTF and sideways 3.28 displacement of the BEDT-TTF molecules within a stack reduces the core repulsion. While this also reduces the degree of HOMO overlap between molecules relative to a stacking angle of $=96, a1.2o have calculated that ,$=6Cl' Mori et to a local maximum in a plot of corresponds stacking angle vs HOMO overlap. The mode of stacking with a nearly BEDT-TTF coupled arrangement of side-by-side molecules in adjacent stacks is similar to that found in the first reported BEDT-TTF salt (BEDT-TTF)2C10,+.TCE condYing calculations2" 5on.Mo~~eCu'Stt~~bi~~~po~~~ band predict a two-dimensional band structure. This was refle;;;;E;red abny con,duci;;;:;tez;d polari;;: with character of the two-dimensional properties presented here for @-(BEDT-TTF)2AuBr2. measurements The conductivity reveal at least two possible phase transitions near 1lOK The low temperature region has been and 8K. studied on several crystals extensively from different batches. One sample studied had a sudden loss of resistance at 11K (pressure of 1 bar) which could be recovered by the application of a magnetic field of 1 Tesla. Under a pressure of 2 kbar the same sample showed a partial (50%) drop in resistance at 6K which was reversed by applying a field of 1.3T. While reminiscent of of this is the behaviour b -(BEDT-TTF)~I~ at ambient pressure22, attribufilamentary ted to superconductivity, the hysteretic behaviour observed in most of our AuBrp crystals !s inconsistent with this and is more probably a signature of a structural phase Similar behaviour has been observed transition. in (BEDT-TTF),ReO,' and some TMTSF23 and TMTTF24 salts. For example, (BEDT-TTF),ReOb undergoes an abrupt metal-insulator transition near 77K under pressure which is suppressed until at pressures above 4 kbar superconductivity is observed near 2K2. However, there is a region Oil the pressure-temperature phase diagram of (BEDT-TTF)zR~O,+~ where hysteresis is observed in the conductivity on cooling and warming which is associated with a displacive structural modulation25. The sharp transition seen in both the EPR linewidth and xs above 20K is consistent with the latter assessment although the magnetic transition appears 5-10K higher than the anomaly in the conductivity. Again, such behaviour is similar to that of (BEDT-TTF)2Re0,,17 in which a transition order first ambient occurs at in both AH and xs several degrees pressure higher than the metal insulator transition as observed in the conductivity2 and the super-
Vol. 61, No. 8
A NEW CONDUCTING
ORGANIC
CHARGE -TRANSFER
lattice formation observed by diffraction25. To confirm that the low temperature transition is indeed structural, further investigation including low temperature X-ray is necessary In some salts of the TMTCF series diffraction. have been similar observations (C=S,Se), However, in associated with anion ordering. as in most of the reported 8-(BEDT-TTF)2AuBr2, is already BEDT-TTF salts, the anion network Si le crystal ordered at room temperature. n&j below 77~ X-ray diffraction on (BEDT-TTF)ZReOk suggests that the phase transition in that salt of both the donor and involves displacement although the driving sublattices, counterion It seems force behind it is still unknown. possible that interactions between the terminal counterions could the groups and ethylene A low temperature ordering of the beimportant. been observed in has grou 6 ethylene the room However, in g-(BEDT-TTF)zIg & . reported here for structure temperature the terminal ethylene groups 8-(BEDT-TTF)2AuBr2,
463
SALT B-(BEDT_TTF)2AuBr2
ordered are features.
with
no
unusual
geometrical
salt conducting new, summary, a In 8-(BEDT-TTF)ZAuBrp has been synthesised and The relatively short AuBr2characterised. slightly packing molecular forces a anion found in other p-phase different from that counterions, triatomic with salts BEDT-TTF still network is BEDT-TTF the although remains two-dimensional. l 4K 8;iBEDT-TTF)2AuBr2 and ambient applied metallic to . Hysteretic behaviour in the pressures. conductivity at pressures up to 5.6 kbar and sharp transitions seen in the EPR linewidth and susceptibility near 20K suggest a low spin temperature structural phase transition. This work has been supported by the U.K. Science and Engineering Research Council.
References
1.
H. Kobayashi, A. Kobayashi, Y. Sasaki, G. H. Inokuchi, J. Am. T. Enoki, Saito, Chem. Sot. 105 , 297 (1983).
2.
S.S.P. Parkin, E.M. Engler, R.R. Schumaker, V.Y. Lee, J.C. Scott, R.L. R. Lagier, Greene, Phys. Rev. Lett. 50 , 270 (1983).
A3.
For a recent review of the structure and see: properties of organic conductors Proceedings of the International Conference on the Physics and Chemistry of Low Dimensional Synthetic Metals Mol. Cryst. Liq. Cryst. 119 , (1985).
4.
E.B. Yagubskii, I.F. Shchegolev, V.N. Kononovich, M.V. Laukhin, P.A. Kartsovnick, A.V. Zuapykina, L.I. Buravov, JETP Lett. 39 , 12 (1984).
5.
J.M.
6.
Crabtree, L. Nunez, Carlson, G.W. K.D. Beno, U. Geiser, M.A. H.H. Wang, M.A. Firestone, K.S. Webb, J.M. Williams, Solid State Commun. 57 , 89 (1986).
7.
Shibaeva, R.P. Yagubskii, Mol. 361 (1985).
V.F. Cryst.
Kaminskii, Liq. Cryst.
E.B. 119 ,
P. Day, J.A.K. Howard, M. Kurmoo, A. Stringer, D.R. Talham, unpublished results.
9.
S.S.P. Parkin, E.M. Engler, V.Y. Lee, R.R. Schumaker, Mol. Cryst. Liq. Cryst. 119 , 375 (1985).
p.
by,
Stringer,
Howard, M. J.A.K. Kurmoo, A. D.R. Talham, unpublished results.
P.C.W. M.A. Beno, M.A. Firestone, L.M. Sowa, H.H. Wang, J.M. Williams, State Commun. 57 , 735 (1986). J. Phys. E.
Leung, Solid
19
,
11.
D.R.P. Guy, R.H. Friend, 430 (1986).
12.
Crystal data for C2nHlsSIsBr2Au M 1126.2; space group pi (no.2). a,, = Triclinic c = 16.320Cl)A. a 9.020(l), b = 5.707(g), = 97.60(l), f3 = 92.12(l), y = 102.89(1P. 295K. Z=1;D,2.30gcm-3;U= F(000) 542; n(MoK,) J9.6cma ; 811.8(2)A3; A (graphite monochromator, Mo-X-Radiation) Final R(I$,,) 0.044 (0.044) for 0.71069A. absorption corrected independent 2556 intensities [28>5rP, 1>3o(I)] recorded on a diffractometer (w -28 P3m in Nicolet Structure solved by Patterson and scans). Fourier techniques and refined by blocked cascade least squares-l3 All non-hydrogen refined were with anisotropic atoms displacement parameters and all hydrogen were located and refined freely atoms Atomic coordinates, bond lengths (Uiso). angles, and atomic displacement and parameters, together with structure factor have been listings deposited at the Cambridge Crystallographic Data Centre. observed and The calculated structure amplitudes are retained by the Authors.
13.
"Shelxtl", Nicolet XED (Madison) and G. Sheldrick, University of Gottingen, 1981.
14.
P.C.W. Leung, T.J. Emge, A.J. Schultz, M.A. Beno, K.D. Carlson, H. Wang, M.A. Firestone, J.M. Williams, Sol id State Commun. 57 , 93 (1986).
15.
D.R. Talham, M. Kurmoo, P. Day, S.D. Obertelli, I.D. Parker, R.H. Friend, J. Phys. C: Solid State Phys. 19 , L383 (1986).
Williams, H.H. Wang, M.A. Beno, T.J. P.T. Copps, F. Behroozi, Emge, L.M. Sowa, G.W. Crabtree, L.N. Hall, K.D. Carlson, Inorg. Chem. 23 , 3839 (1984).
8.
10a.
b.
464
A NEW CONDUCTING
ORGANIC
CHAKGE-TRANSFER
16.
A.N. Bloch, Organic Conductors and Semiconductors, Lecture Notes in Physics (Berlin: Springer) 65 , 317 (1977).
17.
K. Carneiro, J.C. Scott, E.M. Engler, State Commun. 50 , 477 (1984).
SALT B-(BEDT-TTFj2AuBr2
Vol. 61, No. 8
22.
K.D. Carlson, G.W. Crabtree, M. Choi, L.N. Hall, P.T. Copps, H.H. Wang, T.J. Emge, M.A. Beno, J.M. Williams, Mol. Cryst Liq. Cryst. 125 , 145 (1985).
23.
S.S.P. Parkin, D. Jerome, Mol. Cryst Liq. Cryst., 79
Solid K.
Bechgaard,
, 213 (1982).
18.
T. Enoki, K. Imaeda, M. Inokuchi, G. Salto, Phys. 1553 (1986).
Kobayashi, H. Rev. B. 33 ,
24.
S.S.P. Parkin, C. Coulon, D. Phys. C: Solid State Phys. (1983).
19.
M.-H. Whangbo, J.M. Williams, M.A. Beno, T.J. Emge, H.H. Chem. 24 , 3502 (1985).
P.C.W. Leung, Wang, Inorg.
25.
s. Ravy, R. Moret, J.P. Pouget, R. Comes, 33 S.S.P. Parkin, Phys. Rev. B. 2049 (1986).
20.
T. Mori, A. Kobayashi, Y. Sasaki, H. Kobayashi, G. Saito, H. Inokuchi, Bull. Chem. Sot. Jpn. 57 , 627 (1984).
26a.
P.C.W. Leung, T.J. Emge, M.A. Beno, H.H. Williams, V. Wang. J.M. Petricek, P. Coppens, J. Am. Chem. Sot. 107 , 6184 (1985).
21.
H. Tajima, K. Yakushi, H. Kuroda, H. Inokuchi, Solid State Commun. (1984).
G. Saito, 49 , 769
b.
Y. Maniwa, T. Takahashi, G. Phys. Sot. Jpn. 55 , 47 (1986).
Jerome, J. 16 L209
Saito,
-J.
STRUCTURE
AND PROPERTIES
pp.459-464,
OF A NEW CONDUCTING
ORGANIC
1987.
0038-1098187 $3.00 + .OO Pergamon Journals Ltd.
CHARGE-TRANSFER
SALT B_(BEDT-TTF)pAuBrB
M. Kurmoo, D.R. Talham and P. Day Inorganic Chemistry Laboratory, South Parks Road Oxford OX1 3QR, U.K.
Cavendish
I.D. Parker and R.H. Friend Laboratory, Madingley Road, Cambridge
CB3 OHE, U.K.
A.M. Stringer and J.A.K. Howard School of Chemistry, University of Bristol, Cantocks Bristol BS8 lTS, U.K. Received
by R.A. Cowley on 1 October
Close,
1986
A new conducting organic charge-transfer salt, &(BEDT-TTF)2AuBr2 has been synthesised and its crystal structure determined. Single crystal four-probe conductivity has been measured at ambient and applied pressures from 300-1.4K and_EPR spectroscopy from 300-4.2K. S-(BEDT-TTF)~A~B~~ crystallises in PI (Z=l) with a = 9.020(l), b = 5.707(9),Sc = 16.320(1)8; a = 97.60(l), 8 = 92.12(l), y = 102.89(l)'; V = 811.8(2)8 . While the electronic properties are two-dimensional, the molecular packing of the BEDT-TTF molecules is slightly different from that found in other B-phase BEDT-TTF salts with triatomic counterions. 8-(BEDT-TTF)2AuBr2 is metallic to 1.4K at pressures up to 5.6 kbar although at all pressures studied there is appreciable hysteresis in the cooling and warming cycles of the conductivity between 6 and 14K. Abrupt transitions in the temperature dependence of both the peak-to-peak EPR linewidth and integrated signal intensity occur near 20K. The possibility of a low temperature structural modification is discussed.
Introduction
Experimental 8-(BEDT-TTF)2AuBr2 was crystallised by electrochemical oxidation of BEDT-TTF in CH2C12 as the supporting electrowith (n-Bu)kNAuBrp grow as very Crystals small flat lyte. The surface area of the hexagonal plates. crystal faces is generally 0.5mm2 with a thickThey are a minor product ness of O.Ol-0.03mm. of the electrochemical synthesis which yields predominantly a semiconducting 2:l salt." conductivity measurements were Four-probe performed using a pentane pressure medium in a described elsewhere." Electrical system contacts were made to evaporated gold pads at the corners of the plate-like crystals. EPR measurements were performed on single an X-band using reflection crystals Temperature was controlled spectrometer. between 4.2 and 300K in an Oxford Instrument EPR-900 flow cryostat with a DTC-2 temperature controller.
Since the initial report of a conducting salt of BEDT-TTF1, and the subsequent discovery superconductivity under pressure ’ of (BEDT-TTF)2 ReOb2 there has been extensi:: effort into the synthesis and investigation of new charge-transfer salts of BEDT-TTF.3 Among the products of these efforts have been the superconducto;s ambient pressure B8-(BEDT-TTF)zIBrs, and (BEDT-TTF)213,4 BEDT-TTF is bis(ethylene~-(BEDT-TTF)~AuI~~. dithio)tetrathiofulvalene. An unusual feature of BEDT-TTF salts is that they crystallise in morphologies and several crystal stoichiooften under the same metries, synthetic conditions or even in the same electrocrystallisation experiment. For example, in addition to non-superconducting 137, the Bsuperconduc;ors, salts of BEDT-TTF are also AuI2 , and ReO, known. This propensity to crystallise in several different phases is in marked contrast to the TMTCF (C=S,Se) charge transfer salts where a sin le 2:l phase, the Bechgaard salts, predominates 5 . While their structural diversity complicates the study of the properties of the BEDT-TTF salts it provides the opportunity to correlate structures and physical properties in this class of organic conductors. In this communication we report the structure and properties of a new conducting salt of BEDT-TTF, 8-(BEDT-TTF)2AuBr2. Conductivity measurements at ambient and applied pressures together with EPR studies indicate that 8-(BEDT-TTF)2AuBr2 is metallic to 1.4K with evidence of a structural phase transition below 20K.
Results 8-(BEDT-TTF)2AuBr212 crystallises in the space group pi with unit cell triclinic parameters a = 9.020(l), b - 5.707(g), c = OL = 97.60(l), 8 = 92.12(l), y = 16.32O(l)A; 102.89(1p; 2 = 1; V = 811.8(2)A3. We label this salt a "B-phase" due to the similarity of the crystal data, including unit cell volume, to other b-BEDT-TTF salts of triatomic anions. 14 The organic molecules form stacks of dimerised crystallographically and equivalent BEDT-TTF molecules which along with adjacent stacks form 459
A NEW CONDUCTING
460
ORGANIC
CHARGE-TRANSFER
a 2-dimensional layer of donor molecules separated by a columnar layer of AuBrq counterions In contrast to other @-BEDT-TTF (Figure 1).
(ET>
2
AuBr
SALT B-(BEDT-TTF)~AUB~~
Vol. 61, No. 8
to 1.4K. Figure 4 shows the temperature dependent resistance of a typical sample below 200K at pressures between 2 kbar and 5.6 kbar.
2
Fip.1. Structure of k(BEDT-TTF)2AuBr2 showing the alternating organic and inorganic layers.
rialts of trlatomlc anionsI BEDT-TTF molecules within a stack are slightly displaced relative to one another such that the stacking axis forms an angle (4) of about 66 with the plane of the BEDT-TTF molecules (Figure 2). Also in contrast to other p-phase salts, molecules in adjacent side-by-side situated such are that stacks interstack nearest neighbours are nearly coplanThis is similar to the interstack ar (@=P). arrangement found in (BEDT-TTF)2C104.TCEO_ 51. The side-by-side arrangement of BEDT-TTF units is pictured in Figure 3 with short interstack sulphur-sulphur distances labelled. The anion is constrained by symmetry to be linear (the Au-Br bond length = exactly 2,380(1)A and lies within the cavities formed by At room temperathe BEDT-TTF stacks along b. ture, the anion network is ordered. Conductivity was found to be nearly isotropic within the plane of the plates, and highly it.The perpendicular to roomanisotropic conductivity is of the order of temperature 10 Q-'cm-l although absolute conductivity is difficult to determine because the samples used are small so that the contact pad dimensins are comparable to their separation (-60vm). All samples remained metallic to temperatures down
Below pressures of 3.8 kbar there is a knee in the conductivity near 1lOK. This anomaly is removed with increasing pressure such that at it is completely suppressed. At 5.6 kbar ambient pressure there is a slight upturn in below SK which is progressively resistance removed as pressure is increased. Figure 5 resistance vs temperature for several plots At all pressures in the low temperature region. the cooling and warming cycles show pressures, hysteretic behaviour centred near 8K (between 6 and 14K). At a given pressure successive warming cycles yield slightly and cooling possibly due to different different curves, cooling and heating rates. studied show an appreciable All samples The resistance magnetoresistance at 4K. increases up to 50% when a field of 1.3T is applied perpendicular to the sample plane, while a lower increase in R (-15%) is observed with the field parallel to the sample plane. were Lorentizian at signals all EPR the for crystal orientation temperatures here. The maximum (orientation reported peak-to-peak linewidth dependent) (AH) at room-temperature is 58G. Figure 6 shows the temperature dependence of the linewidth and the
Vol. 61, No. 8
A NEW CONDUCTING
ORGANIC
CHARGE-TRANSFER
SALT 8-(BEDT-TTF)2AuBr2
TEMPERATURE View down the long molecular axis of the Fig.2 BEDT-TTF molecules illustrating the inter- and intrastack interactions between donor molecules.
Fig.3 Side-by-side arrangements of BEDT-TTF molecules. The atom numbering scheme as well as some sulphur-sulphur contact distances are labelled.
461
(K I
Fig.4
Temperature dependence of the resistance for B_(BEDT-TTF)2AuBr2 at pressures of 2.0, 3.8 The anomaly near 1OOK is removed and 5.6 kbar. at higher pressures.
spin susceptibility (xs) measured in arbitrary units from the integrated signal intensity and normalised to 300K. The data in Fig. 6 were recorded with the static field parallel, and the microwave field perpendicular, to the plane of plate-like crystal. the orientation This signal maximized the intensity at roomtemperature relative to a 96 rotation. Compared to other p-phase BEDT-TTF salts, the EPR linewidth in the AuBrp- salt is very For example, AH at 300K is less than 20G broad. in the superconductor 8-(BEDT-TTF)2Au1215. In BEDT-TTF salts have large linewidths general, relative to other sulphur x-donors 16 a result that has been attributed to the increased dimensionality15 ,17 brought interaction about by the BEDT-TTF stacks. between The linewidth narrows with decreasing temperatures to 1OG at 30K, below which a sharp decrease occurs near 24K resulting in a peak-to-peak linewidth of less than 1G at temperatures below 16K. At 4.2K, AH = 0.66. Such a narrow linewidth is exceptional in reported BEDT-TTF salts, although a 1G linewidth at low temperature was recently reported for (BEDT-TTF),(C~OI,)~~*. The spin also susceptibility shows a temperature
A NEW CONDUCTING
ORGANIC
CHARGE-TRANSFER
SALT B-(BEDT-TTF),AuBr, ‘
,
Discussion
I
I
I
p-IBE0T-TTF)2A~Br2
I
I
a
L
TEMPERATURE
I
I
12
16
i !O
lK1
pig.5
Resistance vs. temperature as a function of pressure illustrating the hysteretic behaviour observed upon temperature cycling below 2OK.
50
+F+
t 0
AuBr2
++ *
L
0
P-IEEDT-TTF12
z
* 1
I
100
2lnl TEMPERATURE
vol. 61, No. 8 L
IKI
!
300
Temperature dependence of the EPR Fig.6 peak-peak linewidth and intergrated signal intensity (inset) for &-(BEDT-TTF)2AuBr2. The static field, Ho, is aligned perpendicular to the face of the plate-like crystals.
dependence, decreasing to about 0.3 of the room below which xs temperature value at 50K. levels off until a sharp decrease occurs near 20K (inset Figure 6). So far, we have been unable to detect any change in the EPR behaviour associated with the anomaly seen at 1lOK in the transport measurements. This may be due to the limits of precision associated with the broad and weak signals obtained at these temperatures from the small single crystals.
While the crystal data is similar to other 8- and 8'- phase conducting BEDT-TTF salts of triatomic anions, the molecular packing is somewhat different in this AuBr2 salt. This may result from the relatively short AuBr2- anion (anion length = 8.78 calculated from the Au-Br bond length plus the Van der Waal radii of the terminal bromine atoms14). The direct stacking seen in the other p-phases (g-96)" is perhaps less favourable in salts containing shorter anions due to increased core repulsio;sof the out of plane terminal ethylene groups . The mean vertical separation within the stack of the molecules is BEDT-TTF and sideways 3.28 displacement of the BEDT-TTF molecules within a stack reduces the core repulsion. While this also reduces the degree of HOMO overlap between molecules relative to a stacking angle of $=96, a1.2o have calculated that ,$=6Cl' Mori et to a local maximum in a plot of corresponds stacking angle vs HOMO overlap. The mode of stacking with a nearly BEDT-TTF coupled arrangement of side-by-side molecules in adjacent stacks is similar to that found in the first reported BEDT-TTF salt (BEDT-TTF)2C10,+.TCE condYing calculations2" 5on.Mo~~eCu'Stt~~bi~~~po~~~ band predict a two-dimensional band structure. This was refle;;;;E;red abny con,duci;;;:;tez;d polari;;: with character of the two-dimensional properties presented here for @-(BEDT-TTF)2AuBr2. measurements The conductivity reveal at least two possible phase transitions near 1lOK The low temperature region has been and 8K. studied on several crystals extensively from different batches. One sample studied had a sudden loss of resistance at 11K (pressure of 1 bar) which could be recovered by the application of a magnetic field of 1 Tesla. Under a pressure of 2 kbar the same sample showed a partial (50%) drop in resistance at 6K which was reversed by applying a field of 1.3T. While reminiscent of of this is the behaviour b -(BEDT-TTF)~I~ at ambient pressure22, attribufilamentary ted to superconductivity, the hysteretic behaviour observed in most of our AuBrp crystals !s inconsistent with this and is more probably a signature of a structural phase Similar behaviour has been observed transition. in (BEDT-TTF),ReO,' and some TMTSF23 and TMTTF24 salts. For example, (BEDT-TTF),ReOb undergoes an abrupt metal-insulator transition near 77K under pressure which is suppressed until at pressures above 4 kbar superconductivity is observed near 2K2. However, there is a region Oil the pressure-temperature phase diagram of (BEDT-TTF)zR~O,+~ where hysteresis is observed in the conductivity on cooling and warming which is associated with a displacive structural modulation25. The sharp transition seen in both the EPR linewidth and xs above 20K is consistent with the latter assessment although the magnetic transition appears 5-10K higher than the anomaly in the conductivity. Again, such behaviour is similar to that of (BEDT-TTF)2Re0,,17 in which a transition order first ambient occurs at in both AH and xs several degrees pressure higher than the metal insulator transition as observed in the conductivity2 and the super-
Vol. 61, No. 8
A NEW CONDUCTING
ORGANIC
CHARGE -TRANSFER
lattice formation observed by diffraction25. To confirm that the low temperature transition is indeed structural, further investigation including low temperature X-ray is necessary In some salts of the TMTCF series diffraction. have been similar observations (C=S,Se), However, in associated with anion ordering. as in most of the reported 8-(BEDT-TTF)2AuBr2, is already BEDT-TTF salts, the anion network Si le crystal ordered at room temperature. n&j below 77~ X-ray diffraction on (BEDT-TTF)ZReOk suggests that the phase transition in that salt of both the donor and involves displacement although the driving sublattices, counterion It seems force behind it is still unknown. possible that interactions between the terminal counterions could the groups and ethylene A low temperature ordering of the beimportant. been observed in has grou 6 ethylene the room However, in g-(BEDT-TTF)zIg & . reported here for structure temperature the terminal ethylene groups 8-(BEDT-TTF)2AuBr2,
463
SALT B-(BEDT_TTF)2AuBr2
ordered are features.
with
no
unusual
geometrical
salt conducting new, summary, a In 8-(BEDT-TTF)ZAuBrp has been synthesised and The relatively short AuBr2characterised. slightly packing molecular forces a anion found in other p-phase different from that counterions, triatomic with salts BEDT-TTF still network is BEDT-TTF the although remains two-dimensional. l 4K 8;iBEDT-TTF)2AuBr2 and ambient applied metallic to . Hysteretic behaviour in the pressures. conductivity at pressures up to 5.6 kbar and sharp transitions seen in the EPR linewidth and susceptibility near 20K suggest a low spin temperature structural phase transition. This work has been supported by the U.K. Science and Engineering Research Council.
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Crystal data for C2nHlsSIsBr2Au M 1126.2; space group pi (no.2). a,, = Triclinic c = 16.320Cl)A. a 9.020(l), b = 5.707(g), = 97.60(l), f3 = 92.12(l), y = 102.89(1P. 295K. Z=1;D,2.30gcm-3;U= F(000) 542; n(MoK,) J9.6cma ; 811.8(2)A3; A (graphite monochromator, Mo-X-Radiation) Final R(I$,,) 0.044 (0.044) for 0.71069A. absorption corrected independent 2556 intensities [28>5rP, 1>3o(I)] recorded on a diffractometer (w -28 P3m in Nicolet Structure solved by Patterson and scans). Fourier techniques and refined by blocked cascade least squares-l3 All non-hydrogen refined were with anisotropic atoms displacement parameters and all hydrogen were located and refined freely atoms Atomic coordinates, bond lengths (Uiso). angles, and atomic displacement and parameters, together with structure factor have been listings deposited at the Cambridge Crystallographic Data Centre. observed and The calculated structure amplitudes are retained by the Authors.
13.
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14.
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SALT B-(BEDT-TTFj2AuBr2
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