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Phase transition in narrow-band organic metals (BEDT-ATD)2X(solvent) (X=PF6, AsF6, BF4; solvent=THF, DHF, DO)
Synthetic Metals 109 Ž2000. 33–37 www.elsevier.comrlocatersynmet
Phase transition in narrow-band organic metals žBEDT-ATD /2 X žsolvent / žX s PF6 , AsF6 , BF4 ; solvents THF, DHF, DO / Kyuya Yakushi ) , Mikio Uruichi, Yoshiro Yamashita Institute for Molecular Science, Myodaiji, Okazaki, 444-8585 Japan Received 26 June 1999; accepted 10 September 1999
Abstract We present the metal–insulator ŽMI. transition of the title compounds examined by reflection spectroscopy, X-ray diffraction, and magnetic susceptibility. Below the MI transition temperature, the space group changes from P21ra to Pa with 4k F lattice modulation in ŽBEDT-ATD. 2 PF6ŽDHF. and ŽBEDT-ATD. 2 BF4ŽTHF. which undergo MI transitions at 150 K and 200 K, respectively. On the other hand, the symmetry change is not observed at about 80 K in ŽBEDT-ATD. 2 PF6ŽDO. ŽTMI s 100 K., ŽBEDT-ATD. 2 PF6ŽTHF. ŽTMI - 50 K., and ŽBEDT-ATD. 2 AsF6ŽTHF. ŽTMI - 50 K.. The 4k F lattice modulation is supported by the magnetic susceptibility experiment as well. The precursor phenomenon of this structural change is observed already at room temperature in ŽBEDT-ATD. 2 PF6ŽDO.. All of these data suggest the view that ŽBEDT-ATD. 2 XŽsolvent. is a strongly correlated 1D metal. q 2000 Elsevier Science S.A. All rights reserved. Keywords: BEDT-ATD; Narrow-band organic metal; Charge-transfer salt; Reflection spectrum; Strong correlation; Phase transition
1. Introduction BEDT-ATD Ž4,11-bisŽ4X ,5X-ethylenedithio-1X ,3X-dithiol2 -ylidene.-4,11-dihydro-anthraw2,3-cxthiadiazole. is a non-planar molecule that has a small difference Ž90 mV. between the first and second oxidation potentials w1x. This potential difference is regarded as the measure of the on-site Žon-molecule. Coulomb energy U. The 2:1 chargetransfer salts of BEDT-ATD occludes several organic solvents in the crystal lattice, being expressed as ŽBEDTATD. 2 XŽsolvent.. These compounds are metallic near room temperature and undergo a metal-insulator ŽMI. transition. The MI transition temperature depends upon the counter anions ŽX. and occluded organic solvents. Among these charge-transfer salts, ŽBEDT-ATD. 2 PF6 ŽTHF. is claimed as a metal down to 3 K w2x. We have reported the temperature-dependent reflectance of ŽBEDT-ATD. 2 PF6ŽTHF. w3x, and pointed out the following results: Ž1. ŽBEDT-ATD. 2 PF6 ŽTHF. has a quasi-one-dimensional band structure with a narrow bandwidth of 0.24 eV, Ž2. The line shape of the intra-band transition suggests a X
) Corresponding author. Tel.: q81-564-55-7380; fax: q81-564-542254; e-mail: [email protected]
strong correlation effect, Ž3. the screw-axis symmetry begin to be broken at 100 K, which suggests the local structural modulation. In this paper, we expand the reflectance measurement to all the compounds in this family, and conduct the low-temperature ŽLT. X-ray diffraction experiment to confirm the breaking of symmetry at low temperature and to identify the type of the lattice modulation.
2. Experimental The crystals of BEDT-ATD salts with Xy anion were grown by an electrochemical crystallization in tetrahydrofuran ŽTHF., 2,5-dihydrofuran ŽDHF., and 1,3-dioxolane ŽDO.. The polarized reflection spectrum was obtained with two spectrometers combined with the microscope, Spectratech, IR-Plan: FT-IR spectrometer, Nicolet Magna 760 for 600–12 000 cmy1 region and multi-channel detection system, Atago Macs320 for 11 000–30 000 cmy1 region. The details of the experimental method were described in Ref. w3x. The crystal face was determined by X-ray diffraction method using Rigaku AFC-7R-2 four-circle diffractometer. The X-ray diffraction experiment at low temperature was conducted using the imaging-plate detec-
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 9 4 - 0
34
K. Yakushi et al.r Synthetic Metals 109 (2000) 33–37
Fig. 1. Temperature dependence of the optical conductivity spectrum of obtained by the Kramers–Kronig transformation of the reflectance of ŽBEDT-ATD. 2 PF6 ŽDHF.. Note the low-temperature enhancement of the vibronic modes.
tion system, Rigaku R-AXIS-4. The sample temperature is lowered spurting cold nitrogen gas over the sample crystal. The distance between the nozzle and crystal is 8 mm. The temperature sensor is set at the nozzle. The static magnetic susceptibility was measured on the SQUID magnetometer, Quantum Design MPMS-7.
3. Phase transition of (BEDT-ATD)2 PF6 (DHF) 3.1. Room-temperature electronic properties The room-temperature ŽRT. crystal of ŽBEDTATD. 2 PF6 ŽDHF. is iso-structural to ŽBEDT-ATD. 2 PF6ŽTHF. w1x, the unit cell parameters being a s 27.866Ž1., ˚ b s 103.590Ž4.8, Z s 2, b s 7.926Ž1., c s 13.304Ž1. A,
with a space group of P21ra. The unit cell consists of two molecular columns, which develop along the b-axis and are likely to be weakly linked with the neighboring columns through a sulfur–sulfur short contact. However, the intermolecular interaction along the a-axis is blocked by the counter anions and solvent molecules. Two BEDT-ATD’s along the b-axis is connected by the screw axis symmetry. The polarized reflection spectra of E´ b and E´c resemble those of ŽBEDT-ATD. 2 PF6 ŽTHF. shown in Fig. 3 of Ref. w3x. The anisotropy in the mid-infrared region is very large, which means this compound is approximated as a one-dimensional Ž1D. conductor. Combining this result with the crystal structure, we expect a 3r4-filled 1D metallic band, if the electron correlation effect is weak. Actually Imaeda et al. w4x show the metallic transport and magnetic properties above 150 K. From the numerical
Fig. 2. Structure factors < Fo < with Ž0 k 0. indices of ŽBEDT-ATD. 2 PF6 ŽTHF. at room and low temperatures. The error bar denotes "3 s .
K. Yakushi et al.r Synthetic Metals 109 (2000) 33–37
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Fig. 3. Molecular arrangement in ŽBEDT-ATD. 2 PF6 ŽDHF. at 87 K viewed along the b-axis. Note the ferroelectrically oriented DHF molecules.
integration of the optical conductivity s Ž v . of E´ b calculated by Kramers–Kronig transformation of the reflectance, the plasma frequency is obtained as 3770 cmy1 , thereby the bandwidth is estimated to be 0.24 eV. In spite of such a narrow bandwidth, this compound is metallic, the reason of which may be ascribed to the small on-site Coulomb energy due to the large size of the molecule. 1 However, the line shape of s Ž v . looks like non-Drude like. As we discussed more in detail w3x, this unusual line shape come from the correlation effect. This result suggests the sight that this compound is located perhaps near the boundary between a metal and Mott insulator. 3.2. Symmetry change through phase transition According to Imaeda et al., the electrical resistivity and thermopower take upturns around 150 K w4x. The optical conductivity s Ž v . shown in Fig. 1 provides an information about the lattice distortion accompanied by the phase transition. As well as ŽBEDT-ATD. 2 PF6 ŽTHF., this compound shows a remarkable enhancement of the vibronic modes below 150 K. As we discussed before in Ref. w3x, the intensity of these vibronic modes is related to the lattice distortion such as generating a dimerized structure that breaks the screw-axis symmetry. Conversely the enhancement of the vibronic modes implies the evolution of the symmetry breaking through the phase transition. We have conducted the LT X-ray diffraction experiment to confirm the change of symmetry and to detect the superstructure. The main results are the following. Ž1. We
1 We calculate the molecular orbitals of BEDT-ATD and BEDT-TTF using the semi-empirical PM3 method, HOMO of BEDT-ATD is quite similar to BEDT-TTF but much more extended.
carefully looked for the reflection at bUr2 in the oscillation photograph of the imaging plate, but found no streak or spot down to 87 K. Ž2. The temperature dependence of the lattice parameters shows no anomaly around 150 K. Ž3. At 87 K we found no super-lattice along the a- and c-axes as well. We collected the intensity of reflection at 87 K on the two crystals, which are cooled slowly Ž0.5 Krmin. and rapidly Ž6–8 Krmin.. From the examination of the systematic absence of h s 2 n y 1 for Ž h0 l . reflections, the glide-plane symmetry along the a-axis remains at this temperature in both cases. However, the screw-axis symmetry along the b-axis is broken in the slow-cooling case, as shown in Fig. 2. The reflections Ž030. and Ž050. appear weakly but they are significantly larger than 3 s ŽFo.. The weak reflection with an odd index suggests the view that the structural change is very small below this MI transition. It is concluded by the Ž0 k 0. reflection that the space group changes from P21ra to Pa or P2ra, where the screw-axis symmetry is lost at 87 K. This is consistent with the appearance of the vibronic modes in the mid-infrared region. The RT phase has a 3r4-filled metallic band in a single particle approximation, if we take the effective periodic unit as bX s br2. 2 This structural change, therefore, corresponds to the 4k F Žs bXU r2. lattice modulation. This kind of lattice modulation is consistent with the strongly correlated quasi-1D metal. In the Hubbard model, for example, the conduction band is split into upper and lower Hubbard band. In this case, the 3r4-filled conduction band is reduced to the 1r2-filled upper Hubbard band with 2 k F s bXU r2. This structural change opens a bandgap
2
X
The effective periodic unit along the b-axis is b s br2, since the molecules along this axis are connected by the screw axis.
K. Yakushi et al.r Synthetic Metals 109 (2000) 33–37
36
ature, the system approaches a diamagnetic state ŽspinPeierls state., thereby the paramagnetic susceptibility diminishes to zero on lowering temperature. However, such an indication is not observed in all of the salts at least down to 50 K. The defect-free single crystal is necessary to know the magnetic state below 50 K. Finally the strong correlation is suggested by the high paramagnetic susceptibility, which is slightly higher than the value ŽŽ0.5–0.6. = 10y3 emurmol. of highly correlated quasi-1D conductor ŽTMTTF. 2 X w5x, and almost comparable with the value Ž1 = 10y3 emurmol. of quasi1D Mott insulators bX-ŽBEDT-TTF. 2 X ŽX s ICl 2 , AuCl 2 . w6x.
Fig. 4. Schematic arrangement of DHF molecules in the doubled unit cell. An arrow denotes the dipole moment of DHF. XU
at the Fermi wave vector k F s b r4. Therefore this phase transition is considered as Peierls–Hubbard type. As we will discuss in Section 3.3, however, this phase transition has another aspect. The absence of the super-lattice with bUr2 is suggested by the static magnetic susceptibility as well. The paramagnetic susceptibility of ŽBEDT-ATD. 2 PF6 ŽDHF. is about 1.4 = 10y3 emurmol and almost temperature-independent. No indication is found around the phase transition temperature except the slight deflection at 130 K. In a 1D spin system with antiferromagnetic interaction, the electrons tend to make spin singlet pairs by making an alternate array. In this compound, one hole is contained in two BEDT-ATD’s, thereby the singlet pair of holes is formed by four molecules, which should generate the bUr2 super-lattice. If the singlet pairs are formed at low temper-
3.3. Low-temperature crystal structure of (BEDTATD)2 PF6 (DHF) The lattice parameters at 87 K are a s 27.640Ž2., b s ˚ b s 104.225Ž7.8. As we de7.834Ž2., c s 13.265Ž1. A, scribed in Section 3.2, the space group changes from P21ra to P2ra or Pa at 87 K. Since P2ra requires an orientational disorder for BEDT-ATD, we select the space group Pa and solve the structure using the slowly cooled crystal. The R factor is reduced to 0.056. As shown in Fig. 3, the center of symmetry is lost in this space group, thereby the orientationally disordered DHF is ferroelectrically ordered. The experiment to detect the spontaneous polarization or pyro-electricity will be able to confirm the orientation of DHF more directly. In Ž BEDTATD. 2 BF4ŽTHF. which shows a MI transition at 200 K, both BF4 and THF are orientationally disordered at room temperature. They are ordered at 90 K as well. This MI transition of Peierls–Hubbard type, therefore, accompanies the ordering of solvent molecule and counter anion.
Fig. 5. Comparison of the reflectance of ŽBEDT-ATD. 2 X Žsolvent. ŽX s PF6 , BF4 ; solvents PF6 , AsF6 , BF4 . at room temperature. The dips denoted by the circles indicate vibronic modes which suggest the structural fluctuation.
K. Yakushi et al.r Synthetic Metals 109 (2000) 33–37
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Table 1 Comparison of the properties of ŽBEDT-ATD. 2 XŽsolvent. XŽsolvent.
AsF6 ŽTHF.
PF6 ŽTHF.
PF6 ŽDO.
PF6 ŽDHF.
BF4 ŽTHF.
T MI ŽK. Existence of systematic absence Ž0k0. RT structural fluctuation Paramagnetic susceptibility Žemurmol. RT unit cell ˚. volume ŽA
- 50 yes Ž75 K, rapid cool.
- 50 yes Ž100 K, rapid cool. small
150 yes Ž78 K, rapid cool. no Ž87 K, slow cool. extremely small
200 no Ž90 K, rapid cool.
intermediate
100 yes Ž76 K, rapid cool. yes Ž86 K, slow cool. large
Ž1.2–1.4. = 10y3
0.8 = 10y3
Ž0.7–0.8. = 10y3
Ž1.4–1.5. = 10y3
0.8 = 10y3
2890
2880
2874
2856
2838
To know that this ordered state is competitive or cooperative with the Peierls-Hubbard type MI transition, we calculate the difference of electrostatic energy between the two hypothetical arrangements A and B shown in Fig. 4. A is the actual LT structure, while B is the hypothetical one with antiferroelectric arrangement along the b-axis. The b-axis of the unit cell is doubled in this model to produce a periodic antiferroelectric arrangement. Since the solvent molecule is located at the center of symmetry, the inversion of the solvent molecule does not change the electrostatic energy with BEDT-ATDq0.5 and PF6y. Therefore, the energy difference between A and B is determined only by the dipole–dipole interaction within the solvent molecules. If we consider the nearest-neighbor dipoles along the b-axis, the arrangement B is more stable than A. To calculate the dipole sum, we calculate the charge population in DHF using the PM3 semi-empirical method in the HyperChem-R5.1 program system. The point charges on the oxygen, 2,5-carbon, and 3,4-carbon atoms are y0.278, 0.191, and y0.052, respectively, where the charge on hydrogen atoms are added to the adjacent carbon atom. 3 We calculate the infinite sum of the electrostatic energy between these point charges using the Ewald method w7x. The lattice sums per a DHF molecule are y0.136 meV Žy1.6 K. for case A and y0.153 meV Žy1.8 K. for case B. Both are much smaller than the MI transition temperature. This calculation suggests that the order–disorder transition is competitive with the Peierls–Hubbard type MI transition. However, the contribution of the dipole–dipole interaction of the DHF molecules seems to be negligible in this phase transition.
4. Comparison of (BEDT-ATD)2 X(solvent) Other compounds in this system show similar behaviors as to the reflectance, X-ray diffraction, and magnetic sus3 The dipole moment Ž1.67 D. of THF calculated based on the charge distribution by PM3 method agrees well with the experimental value Ž1.71 D..
small
ceptibility. It is well known that the vibronic modes are the very sensitive probe to detect the local lattice distortion which breaks the symmetry. Fig. 5 shows the comparison of the room-temperature reflectance. The vibronic modes encompassed by the circles clearly appear already at room temperature in ŽBEDT-ATD. 2 PF6 ŽDO.. The similar dips reflecting the structural fluctuation Žlocal distortion. are observed weakly in other compounds as well. A strong Ž030. reflection is found at 90 K in ŽBEDTATD. 2 BF4ŽTHF., whereas such Ž0 k 0. reflection with odd index is not found in ŽBEDT-ATD. 2 PF6 ŽTHF., ŽBEDTATD. 2 AsF6 ŽTHF., and ŽBEDT-ATD. 2 PF6 ŽDO.. These properties are listed in Table 1.
5. Conclusion ŽBEDT-ATD. 2 XŽsolvent. ŽX s PF6 , BF4 ; solvent s THF, DHF, DO. is a narrow-band quasi-1D metal with strong correlation, which is suggested by the conductivity spectrum s Ž v . and paramagnetic susceptibility. The MI transition of this system is Peierls–Hubbard type phase transition with 4k F modulation. This structural modulation is consistent with the strong correlation. This MI transition breaks the screw-axis symmetry and accompanies the ordering of the solvent molecules and counter anions.
References w1x Y. Yamashita, S. Tomura, K. Imaeda, Synth. Met. 71 Ž1995. 1965. w2x K. Imaeda, Y. Yamashita, S. Tanaka, H. Inokuchi, Synth. Met. 73 Ž1995. 107. w3x M. Uruichi, K. Yakushi, Y. Yamashita, J. Phys. Soc. Jpn. 68 Ž1999. 531. w4x K. Imaeda, J. Krober, C. Nakano, M. Tomura, S. Tanaka, Y. Yamashita, H. Kobayashi, H. Inokuchi, A. Kobayashi, Mol. Crys. Liq. Cryst. 296 Ž1997. 205. w5x C. Coulon, P. Delhaes, S. Flandrois, R. Lagnier, E. Bonjour, J.M. Fabre, J. Phys. 43 Ž1982. 1059. w6x N. Yoneyama, A. Miyazaki, T. Enoki, G. Saito, Bull. Chem. Soc. Jpn., in press. w7x R.M. Metzger, J. Chem. Phys. 57 Ž1972. 1870.
Phase transition in narrow-band organic metals žBEDT-ATD /2 X žsolvent / žX s PF6 , AsF6 , BF4 ; solvents THF, DHF, DO / Kyuya Yakushi ) , Mikio Uruichi, Yoshiro Yamashita Institute for Molecular Science, Myodaiji, Okazaki, 444-8585 Japan Received 26 June 1999; accepted 10 September 1999
Abstract We present the metal–insulator ŽMI. transition of the title compounds examined by reflection spectroscopy, X-ray diffraction, and magnetic susceptibility. Below the MI transition temperature, the space group changes from P21ra to Pa with 4k F lattice modulation in ŽBEDT-ATD. 2 PF6ŽDHF. and ŽBEDT-ATD. 2 BF4ŽTHF. which undergo MI transitions at 150 K and 200 K, respectively. On the other hand, the symmetry change is not observed at about 80 K in ŽBEDT-ATD. 2 PF6ŽDO. ŽTMI s 100 K., ŽBEDT-ATD. 2 PF6ŽTHF. ŽTMI - 50 K., and ŽBEDT-ATD. 2 AsF6ŽTHF. ŽTMI - 50 K.. The 4k F lattice modulation is supported by the magnetic susceptibility experiment as well. The precursor phenomenon of this structural change is observed already at room temperature in ŽBEDT-ATD. 2 PF6ŽDO.. All of these data suggest the view that ŽBEDT-ATD. 2 XŽsolvent. is a strongly correlated 1D metal. q 2000 Elsevier Science S.A. All rights reserved. Keywords: BEDT-ATD; Narrow-band organic metal; Charge-transfer salt; Reflection spectrum; Strong correlation; Phase transition
1. Introduction BEDT-ATD Ž4,11-bisŽ4X ,5X-ethylenedithio-1X ,3X-dithiol2 -ylidene.-4,11-dihydro-anthraw2,3-cxthiadiazole. is a non-planar molecule that has a small difference Ž90 mV. between the first and second oxidation potentials w1x. This potential difference is regarded as the measure of the on-site Žon-molecule. Coulomb energy U. The 2:1 chargetransfer salts of BEDT-ATD occludes several organic solvents in the crystal lattice, being expressed as ŽBEDTATD. 2 XŽsolvent.. These compounds are metallic near room temperature and undergo a metal-insulator ŽMI. transition. The MI transition temperature depends upon the counter anions ŽX. and occluded organic solvents. Among these charge-transfer salts, ŽBEDT-ATD. 2 PF6 ŽTHF. is claimed as a metal down to 3 K w2x. We have reported the temperature-dependent reflectance of ŽBEDT-ATD. 2 PF6ŽTHF. w3x, and pointed out the following results: Ž1. ŽBEDT-ATD. 2 PF6 ŽTHF. has a quasi-one-dimensional band structure with a narrow bandwidth of 0.24 eV, Ž2. The line shape of the intra-band transition suggests a X
) Corresponding author. Tel.: q81-564-55-7380; fax: q81-564-542254; e-mail: [email protected]
strong correlation effect, Ž3. the screw-axis symmetry begin to be broken at 100 K, which suggests the local structural modulation. In this paper, we expand the reflectance measurement to all the compounds in this family, and conduct the low-temperature ŽLT. X-ray diffraction experiment to confirm the breaking of symmetry at low temperature and to identify the type of the lattice modulation.
2. Experimental The crystals of BEDT-ATD salts with Xy anion were grown by an electrochemical crystallization in tetrahydrofuran ŽTHF., 2,5-dihydrofuran ŽDHF., and 1,3-dioxolane ŽDO.. The polarized reflection spectrum was obtained with two spectrometers combined with the microscope, Spectratech, IR-Plan: FT-IR spectrometer, Nicolet Magna 760 for 600–12 000 cmy1 region and multi-channel detection system, Atago Macs320 for 11 000–30 000 cmy1 region. The details of the experimental method were described in Ref. w3x. The crystal face was determined by X-ray diffraction method using Rigaku AFC-7R-2 four-circle diffractometer. The X-ray diffraction experiment at low temperature was conducted using the imaging-plate detec-
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 9 4 - 0
34
K. Yakushi et al.r Synthetic Metals 109 (2000) 33–37
Fig. 1. Temperature dependence of the optical conductivity spectrum of obtained by the Kramers–Kronig transformation of the reflectance of ŽBEDT-ATD. 2 PF6 ŽDHF.. Note the low-temperature enhancement of the vibronic modes.
tion system, Rigaku R-AXIS-4. The sample temperature is lowered spurting cold nitrogen gas over the sample crystal. The distance between the nozzle and crystal is 8 mm. The temperature sensor is set at the nozzle. The static magnetic susceptibility was measured on the SQUID magnetometer, Quantum Design MPMS-7.
3. Phase transition of (BEDT-ATD)2 PF6 (DHF) 3.1. Room-temperature electronic properties The room-temperature ŽRT. crystal of ŽBEDTATD. 2 PF6 ŽDHF. is iso-structural to ŽBEDT-ATD. 2 PF6ŽTHF. w1x, the unit cell parameters being a s 27.866Ž1., ˚ b s 103.590Ž4.8, Z s 2, b s 7.926Ž1., c s 13.304Ž1. A,
with a space group of P21ra. The unit cell consists of two molecular columns, which develop along the b-axis and are likely to be weakly linked with the neighboring columns through a sulfur–sulfur short contact. However, the intermolecular interaction along the a-axis is blocked by the counter anions and solvent molecules. Two BEDT-ATD’s along the b-axis is connected by the screw axis symmetry. The polarized reflection spectra of E´ b and E´c resemble those of ŽBEDT-ATD. 2 PF6 ŽTHF. shown in Fig. 3 of Ref. w3x. The anisotropy in the mid-infrared region is very large, which means this compound is approximated as a one-dimensional Ž1D. conductor. Combining this result with the crystal structure, we expect a 3r4-filled 1D metallic band, if the electron correlation effect is weak. Actually Imaeda et al. w4x show the metallic transport and magnetic properties above 150 K. From the numerical
Fig. 2. Structure factors < Fo < with Ž0 k 0. indices of ŽBEDT-ATD. 2 PF6 ŽTHF. at room and low temperatures. The error bar denotes "3 s .
K. Yakushi et al.r Synthetic Metals 109 (2000) 33–37
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Fig. 3. Molecular arrangement in ŽBEDT-ATD. 2 PF6 ŽDHF. at 87 K viewed along the b-axis. Note the ferroelectrically oriented DHF molecules.
integration of the optical conductivity s Ž v . of E´ b calculated by Kramers–Kronig transformation of the reflectance, the plasma frequency is obtained as 3770 cmy1 , thereby the bandwidth is estimated to be 0.24 eV. In spite of such a narrow bandwidth, this compound is metallic, the reason of which may be ascribed to the small on-site Coulomb energy due to the large size of the molecule. 1 However, the line shape of s Ž v . looks like non-Drude like. As we discussed more in detail w3x, this unusual line shape come from the correlation effect. This result suggests the sight that this compound is located perhaps near the boundary between a metal and Mott insulator. 3.2. Symmetry change through phase transition According to Imaeda et al., the electrical resistivity and thermopower take upturns around 150 K w4x. The optical conductivity s Ž v . shown in Fig. 1 provides an information about the lattice distortion accompanied by the phase transition. As well as ŽBEDT-ATD. 2 PF6 ŽTHF., this compound shows a remarkable enhancement of the vibronic modes below 150 K. As we discussed before in Ref. w3x, the intensity of these vibronic modes is related to the lattice distortion such as generating a dimerized structure that breaks the screw-axis symmetry. Conversely the enhancement of the vibronic modes implies the evolution of the symmetry breaking through the phase transition. We have conducted the LT X-ray diffraction experiment to confirm the change of symmetry and to detect the superstructure. The main results are the following. Ž1. We
1 We calculate the molecular orbitals of BEDT-ATD and BEDT-TTF using the semi-empirical PM3 method, HOMO of BEDT-ATD is quite similar to BEDT-TTF but much more extended.
carefully looked for the reflection at bUr2 in the oscillation photograph of the imaging plate, but found no streak or spot down to 87 K. Ž2. The temperature dependence of the lattice parameters shows no anomaly around 150 K. Ž3. At 87 K we found no super-lattice along the a- and c-axes as well. We collected the intensity of reflection at 87 K on the two crystals, which are cooled slowly Ž0.5 Krmin. and rapidly Ž6–8 Krmin.. From the examination of the systematic absence of h s 2 n y 1 for Ž h0 l . reflections, the glide-plane symmetry along the a-axis remains at this temperature in both cases. However, the screw-axis symmetry along the b-axis is broken in the slow-cooling case, as shown in Fig. 2. The reflections Ž030. and Ž050. appear weakly but they are significantly larger than 3 s ŽFo.. The weak reflection with an odd index suggests the view that the structural change is very small below this MI transition. It is concluded by the Ž0 k 0. reflection that the space group changes from P21ra to Pa or P2ra, where the screw-axis symmetry is lost at 87 K. This is consistent with the appearance of the vibronic modes in the mid-infrared region. The RT phase has a 3r4-filled metallic band in a single particle approximation, if we take the effective periodic unit as bX s br2. 2 This structural change, therefore, corresponds to the 4k F Žs bXU r2. lattice modulation. This kind of lattice modulation is consistent with the strongly correlated quasi-1D metal. In the Hubbard model, for example, the conduction band is split into upper and lower Hubbard band. In this case, the 3r4-filled conduction band is reduced to the 1r2-filled upper Hubbard band with 2 k F s bXU r2. This structural change opens a bandgap
2
X
The effective periodic unit along the b-axis is b s br2, since the molecules along this axis are connected by the screw axis.
K. Yakushi et al.r Synthetic Metals 109 (2000) 33–37
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ature, the system approaches a diamagnetic state ŽspinPeierls state., thereby the paramagnetic susceptibility diminishes to zero on lowering temperature. However, such an indication is not observed in all of the salts at least down to 50 K. The defect-free single crystal is necessary to know the magnetic state below 50 K. Finally the strong correlation is suggested by the high paramagnetic susceptibility, which is slightly higher than the value ŽŽ0.5–0.6. = 10y3 emurmol. of highly correlated quasi-1D conductor ŽTMTTF. 2 X w5x, and almost comparable with the value Ž1 = 10y3 emurmol. of quasi1D Mott insulators bX-ŽBEDT-TTF. 2 X ŽX s ICl 2 , AuCl 2 . w6x.
Fig. 4. Schematic arrangement of DHF molecules in the doubled unit cell. An arrow denotes the dipole moment of DHF. XU
at the Fermi wave vector k F s b r4. Therefore this phase transition is considered as Peierls–Hubbard type. As we will discuss in Section 3.3, however, this phase transition has another aspect. The absence of the super-lattice with bUr2 is suggested by the static magnetic susceptibility as well. The paramagnetic susceptibility of ŽBEDT-ATD. 2 PF6 ŽDHF. is about 1.4 = 10y3 emurmol and almost temperature-independent. No indication is found around the phase transition temperature except the slight deflection at 130 K. In a 1D spin system with antiferromagnetic interaction, the electrons tend to make spin singlet pairs by making an alternate array. In this compound, one hole is contained in two BEDT-ATD’s, thereby the singlet pair of holes is formed by four molecules, which should generate the bUr2 super-lattice. If the singlet pairs are formed at low temper-
3.3. Low-temperature crystal structure of (BEDTATD)2 PF6 (DHF) The lattice parameters at 87 K are a s 27.640Ž2., b s ˚ b s 104.225Ž7.8. As we de7.834Ž2., c s 13.265Ž1. A, scribed in Section 3.2, the space group changes from P21ra to P2ra or Pa at 87 K. Since P2ra requires an orientational disorder for BEDT-ATD, we select the space group Pa and solve the structure using the slowly cooled crystal. The R factor is reduced to 0.056. As shown in Fig. 3, the center of symmetry is lost in this space group, thereby the orientationally disordered DHF is ferroelectrically ordered. The experiment to detect the spontaneous polarization or pyro-electricity will be able to confirm the orientation of DHF more directly. In Ž BEDTATD. 2 BF4ŽTHF. which shows a MI transition at 200 K, both BF4 and THF are orientationally disordered at room temperature. They are ordered at 90 K as well. This MI transition of Peierls–Hubbard type, therefore, accompanies the ordering of solvent molecule and counter anion.
Fig. 5. Comparison of the reflectance of ŽBEDT-ATD. 2 X Žsolvent. ŽX s PF6 , BF4 ; solvents PF6 , AsF6 , BF4 . at room temperature. The dips denoted by the circles indicate vibronic modes which suggest the structural fluctuation.
K. Yakushi et al.r Synthetic Metals 109 (2000) 33–37
37
Table 1 Comparison of the properties of ŽBEDT-ATD. 2 XŽsolvent. XŽsolvent.
AsF6 ŽTHF.
PF6 ŽTHF.
PF6 ŽDO.
PF6 ŽDHF.
BF4 ŽTHF.
T MI ŽK. Existence of systematic absence Ž0k0. RT structural fluctuation Paramagnetic susceptibility Žemurmol. RT unit cell ˚. volume ŽA
- 50 yes Ž75 K, rapid cool.
- 50 yes Ž100 K, rapid cool. small
150 yes Ž78 K, rapid cool. no Ž87 K, slow cool. extremely small
200 no Ž90 K, rapid cool.
intermediate
100 yes Ž76 K, rapid cool. yes Ž86 K, slow cool. large
Ž1.2–1.4. = 10y3
0.8 = 10y3
Ž0.7–0.8. = 10y3
Ž1.4–1.5. = 10y3
0.8 = 10y3
2890
2880
2874
2856
2838
To know that this ordered state is competitive or cooperative with the Peierls-Hubbard type MI transition, we calculate the difference of electrostatic energy between the two hypothetical arrangements A and B shown in Fig. 4. A is the actual LT structure, while B is the hypothetical one with antiferroelectric arrangement along the b-axis. The b-axis of the unit cell is doubled in this model to produce a periodic antiferroelectric arrangement. Since the solvent molecule is located at the center of symmetry, the inversion of the solvent molecule does not change the electrostatic energy with BEDT-ATDq0.5 and PF6y. Therefore, the energy difference between A and B is determined only by the dipole–dipole interaction within the solvent molecules. If we consider the nearest-neighbor dipoles along the b-axis, the arrangement B is more stable than A. To calculate the dipole sum, we calculate the charge population in DHF using the PM3 semi-empirical method in the HyperChem-R5.1 program system. The point charges on the oxygen, 2,5-carbon, and 3,4-carbon atoms are y0.278, 0.191, and y0.052, respectively, where the charge on hydrogen atoms are added to the adjacent carbon atom. 3 We calculate the infinite sum of the electrostatic energy between these point charges using the Ewald method w7x. The lattice sums per a DHF molecule are y0.136 meV Žy1.6 K. for case A and y0.153 meV Žy1.8 K. for case B. Both are much smaller than the MI transition temperature. This calculation suggests that the order–disorder transition is competitive with the Peierls–Hubbard type MI transition. However, the contribution of the dipole–dipole interaction of the DHF molecules seems to be negligible in this phase transition.
4. Comparison of (BEDT-ATD)2 X(solvent) Other compounds in this system show similar behaviors as to the reflectance, X-ray diffraction, and magnetic sus3 The dipole moment Ž1.67 D. of THF calculated based on the charge distribution by PM3 method agrees well with the experimental value Ž1.71 D..
small
ceptibility. It is well known that the vibronic modes are the very sensitive probe to detect the local lattice distortion which breaks the symmetry. Fig. 5 shows the comparison of the room-temperature reflectance. The vibronic modes encompassed by the circles clearly appear already at room temperature in ŽBEDT-ATD. 2 PF6 ŽDO.. The similar dips reflecting the structural fluctuation Žlocal distortion. are observed weakly in other compounds as well. A strong Ž030. reflection is found at 90 K in ŽBEDTATD. 2 BF4ŽTHF., whereas such Ž0 k 0. reflection with odd index is not found in ŽBEDT-ATD. 2 PF6 ŽTHF., ŽBEDTATD. 2 AsF6 ŽTHF., and ŽBEDT-ATD. 2 PF6 ŽDO.. These properties are listed in Table 1.
5. Conclusion ŽBEDT-ATD. 2 XŽsolvent. ŽX s PF6 , BF4 ; solvent s THF, DHF, DO. is a narrow-band quasi-1D metal with strong correlation, which is suggested by the conductivity spectrum s Ž v . and paramagnetic susceptibility. The MI transition of this system is Peierls–Hubbard type phase transition with 4k F modulation. This structural modulation is consistent with the strong correlation. This MI transition breaks the screw-axis symmetry and accompanies the ordering of the solvent molecules and counter anions.
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