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A novel metal–insulator transition in (EDO-TTF)2X (X=PF6, AsF6)
Synthetic Metals 133±134 (2003) 463±465
A novel metal±insulator transition in (EDO-TTF)2X (X PF6 , AsF6)$ A. Otaa,*, H. Yamochia,b, G. Saitoa a
Division of Chemistry, Graduate School of Science, Kyoto University, Kyoto, Japan b CREST, Japan Science and Technology Corporation (JST), Japan
Abstract Single crystals of cation radical salts of ethylenedioxy-tetrathiafulvalene (EDO), (EDO)2X (X PF6 , AsF6, ClO4) were prepared by electrocrystallization. Metal±insulator (MI) transitions with big molecular deformation were observed in (EDO)2PF6 at 280 K and in (EDO)2AsF6 at 268 K. Their crystal structures are almost the same at room temperature (RT), and in both cases, the calculated band structures show quasi-one-dimensional electronic nature. The magnetic susceptibility of (EDO)2PF6 and (EDO)2AsF6 showed the thermal hysteresis around 280 and 268 K, respectively, indicating the MI transition being the ®rst-order. Their crystal structures in the insulating phase are isostructural to each other and to that of (EDO)2ClO4 at RT. The structural analysis of these salts revealed that the mechanisms of the MI transitions observed in PF6 and AsF6 salts are the same, which show the cooperative nature of Peierls, charge-ordering (CO), and order±disorder (OD) one. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cation radical salt; Crystal structure; Conductivity; Metal±insulator transition; TTF; BEDO-TTF; EDO-TTF
1. Introduction
2. Results and discussion
In organic conductors, the low dimensionality and the strong electronic correlation cause various phenomenon including the formation of the density waves, charge-ordering (CO) and Mott transition, which exhibit the various aspects of electronic and structural properties of solids. However, because of the strong self-assembling property, in almost all cases, BEDO-TTF has afforded the stable metallic charge transfer complexes, in which the phase transitions are observed rarely [1]. To examine the partial suppression of the self-assembling property, we studied ethylenedioxy-tetrathiafulvalene (EDO), which is a low symmetric donor molecule derived from BO by removing one ethylenedioxy group. We have reported a novel metal±insulator (MI) transition in (EDO)2PF6 [2]. In this paper, the MI transition observed in the isostructural salt of (EDO)2AsF6 is reported.
EDO was synthesized according to the procedure by Iyoda et al. [3]. The AsF6 salt was prepared by the electrochemical method using ethanol as the solvent. The X-ray diffraction data were taken at room temperature (RT) and at 230 K. In the latter case, the sample was cooled slowly (2 K/h) prior to the measurement. The magnetic susceptibility was measured by SQUID magnetometer. In the crystal structure of (EDO)2AsF6 at RT (see Table 1), the donor layer consists of nearly uniform head-to-tail stacking of almost planar donor molecules. The counter anion, which shows huge thermal motion, is located on the center of inversion between the layers. The tight binding band calculation gave the quasi-one-dimensional Fermi surface mainly due to the intermolecular interactions along the stacking direction (Fig. 1a), that indicates this salt is susceptible to the modulation, which is relevant to the Peierls, CDW, or SDW mechanism. The metallic temperature dependence of the conductivity was observed for the single crystals of (EDO)2AsF6 down to 268 K by the conventional four-probe method (Table 2). All the crystals were broken into pieces at this temperature, though the compaction pellet made of the ground single crystals proved the occurrence of an MI transition by the distinct change of the activation energy (ea) at 268 K in the conductivity measurement. The static magnetic susceptibility
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Yamada Conference LVI, The Fourth International Symposium on Crystalline Organic Metals, Superconductors and Ferromagnets, ISCOM 2001ÐAbstract Number F25Mon. * Corresponding author. Tel./fax: 81-75-753-4035. E-mail address: [email protected] (A. Ota).
0379-6779/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 2 9 5 - 3
464
A. Ota et al. / Synthetic Metals 133±134 (2003) 463±465
Table 1 Crystallographic data of EDO2X (X PF6 , AsF6, ClO4)
Ê) a (A Ê) b (A Ê) c (A a (8) b (8) g (8) Ê 3) V (A R a
AsF6 (RT)
PF6 (RT)
AsF6 (230 K)
PF6 (260 K)
ClO4 (RT)a
Triclinic P 1
Triclinic P 1
Triclinic P 1
Triclinic P 1
Triclinic P 1
7.224(2) 7.354(1) 12.093(2) 93.35(1) 74.822(9) 97.08(1) 615.1(2) 0.065
7.197(1) 7.343(1) 11.948(1) 93.454(7) 75.158(6) 97.405(7) 605.0(1) 0.087
9.771(8) 10.990(9) 11.341(8) 101.78(4) 100.06(4) 89.89(5) 1173.0(2) 0.085
9.822(1) 11.000(2) 11.487(2) 101.865(8) 99.128(8) 90.445(8) 1198.1(3) 0.048
9.760(1) 10.928(1) 11.221(1) 100.366(5) 99.404(5) 91.144(5) 1159.9(2) 0.058
The details of (EDO)2ClO4 will be reported elsewhere.
Table 2 Physical properties of (EDO)2X (X PF6 , AsF6, ClO4)
MI transition temperature, TMI (K) Conductivity at RT, sRT (S cm 1) Activation energy below TMI, ea (meV) w at RT (10 4 emu mol 1) Overlap integral at 230 K (10 3) (Fig. 1b) S1 S2 S3
PF6
AsF6
280 60 320 2.5
268 >337 165 3.8 10 300 500 2.3 ±
44.2 23.0 13.0
45.8 23.8 13.6
ClO4 5
46.2 22.8 13.4
(w) showed almost constant value derived from the Pauli para-magnetism down to the MI transition temperature (TMI). Below this temperature, w vanished, and the phase transition showed the thermal hysteresis around TMI. Comparing to that at RT, the unit cell was doubled along the donor column by the formation of tetramers, which consist of the bent donor molecules (B) and ¯at ones (F) at 230 K (Fig. 1b). Although the values of the overlap integrals (S) in the donor column are almost uniform at
RT, there observed large alternation of the S in the low temperature (LT) phase, which corresponds to the nesting of the Fermi surface at RT. This indicates that the MI transition contains the nature of Peierls transition. It is also shown that the B and F molecules are charged less and more than 0.5, respectively, from the analysis of the bond lengths. As a result, these B and F molecules form periodic stripes of the charges in the salt, indicating the occurrence of the CO. The counter anions are located in general positions at 230 K. In the LT phase, the temperature factor of ¯uorine atoms were largely reduced from the values at RT, and an axis of F±As±F is almost parallel to the donor stacking axis. These facts indicate that the nature of this transition also includes order± disorder (OD) feature. Summing these results up, it was revealed that the MI transition contains the cooperative nature of Peierls, CO, and OD ones with the molecular deformation. As mentioned earlier, the same type of MI transition has been observed at 280 K in (EDO)2PF6. The RT and LT phases of AsF6 salt are isostructural to the corresponding ones of PF6. The single crystals of (EDO)2ClO4 at RT are semiconductive with large ea up to 337 K, of which crystal structure is isostructural to those of (EDO)2X (X PF6 ,
Fig. 1. (a) Calculated band structure and Fermi surface of (EDO)2AsF6 at RT. (b) Donor stacking column and anions of (EDO)2AsF6 at 230 K viewed along the short molecular axis. The dotted lines represent the intermolecular atomic contacts, which are shorter than the sum of the van der Waals radii.
A. Ota et al. / Synthetic Metals 133±134 (2003) 463±465
AsF6) in the insulating phase. The TMI of (EDO)2ClO4 is, hence, regarded to be above 337 K. These facts suggest that these three salts have a common electronic structure with different TMI. Since the area and thickness per a donor molecule in the donor layers are expanded and shrunk below TMI, respectively, it is expected that large anions, which penetrate into the donor layer, obstruct the structural change of the donor layer.
465
References [1] S. Horiuchi, H. Yamochi, G. Saito, K. Sakaguchi, M. Kusunoki, J. Am. Chem. Soc. 118 (1996) 8604. [2] A. Ota, H. Yamochi, G. Saito, J. Mater. Chem. 12 (2002) 2600. [3] M. Iyoda, Y. Kuwatani, E. Ogura, K. Hara, H. Suzuki, T. Takano, K. Takeda, J. Takano, K. Ugawa, M. Yoshida, H. Matsuyama, H. Nishikawa, I. Ikemoto, T. Kato, N. Yoneyama, J. Nishijo, A. Miyazaki, T. Enoki, Heterocycles 54 (2001) 833.
A novel metal±insulator transition in (EDO-TTF)2X (X PF6 , AsF6)$ A. Otaa,*, H. Yamochia,b, G. Saitoa a
Division of Chemistry, Graduate School of Science, Kyoto University, Kyoto, Japan b CREST, Japan Science and Technology Corporation (JST), Japan
Abstract Single crystals of cation radical salts of ethylenedioxy-tetrathiafulvalene (EDO), (EDO)2X (X PF6 , AsF6, ClO4) were prepared by electrocrystallization. Metal±insulator (MI) transitions with big molecular deformation were observed in (EDO)2PF6 at 280 K and in (EDO)2AsF6 at 268 K. Their crystal structures are almost the same at room temperature (RT), and in both cases, the calculated band structures show quasi-one-dimensional electronic nature. The magnetic susceptibility of (EDO)2PF6 and (EDO)2AsF6 showed the thermal hysteresis around 280 and 268 K, respectively, indicating the MI transition being the ®rst-order. Their crystal structures in the insulating phase are isostructural to each other and to that of (EDO)2ClO4 at RT. The structural analysis of these salts revealed that the mechanisms of the MI transitions observed in PF6 and AsF6 salts are the same, which show the cooperative nature of Peierls, charge-ordering (CO), and order±disorder (OD) one. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cation radical salt; Crystal structure; Conductivity; Metal±insulator transition; TTF; BEDO-TTF; EDO-TTF
1. Introduction
2. Results and discussion
In organic conductors, the low dimensionality and the strong electronic correlation cause various phenomenon including the formation of the density waves, charge-ordering (CO) and Mott transition, which exhibit the various aspects of electronic and structural properties of solids. However, because of the strong self-assembling property, in almost all cases, BEDO-TTF has afforded the stable metallic charge transfer complexes, in which the phase transitions are observed rarely [1]. To examine the partial suppression of the self-assembling property, we studied ethylenedioxy-tetrathiafulvalene (EDO), which is a low symmetric donor molecule derived from BO by removing one ethylenedioxy group. We have reported a novel metal±insulator (MI) transition in (EDO)2PF6 [2]. In this paper, the MI transition observed in the isostructural salt of (EDO)2AsF6 is reported.
EDO was synthesized according to the procedure by Iyoda et al. [3]. The AsF6 salt was prepared by the electrochemical method using ethanol as the solvent. The X-ray diffraction data were taken at room temperature (RT) and at 230 K. In the latter case, the sample was cooled slowly (2 K/h) prior to the measurement. The magnetic susceptibility was measured by SQUID magnetometer. In the crystal structure of (EDO)2AsF6 at RT (see Table 1), the donor layer consists of nearly uniform head-to-tail stacking of almost planar donor molecules. The counter anion, which shows huge thermal motion, is located on the center of inversion between the layers. The tight binding band calculation gave the quasi-one-dimensional Fermi surface mainly due to the intermolecular interactions along the stacking direction (Fig. 1a), that indicates this salt is susceptible to the modulation, which is relevant to the Peierls, CDW, or SDW mechanism. The metallic temperature dependence of the conductivity was observed for the single crystals of (EDO)2AsF6 down to 268 K by the conventional four-probe method (Table 2). All the crystals were broken into pieces at this temperature, though the compaction pellet made of the ground single crystals proved the occurrence of an MI transition by the distinct change of the activation energy (ea) at 268 K in the conductivity measurement. The static magnetic susceptibility
$
Yamada Conference LVI, The Fourth International Symposium on Crystalline Organic Metals, Superconductors and Ferromagnets, ISCOM 2001ÐAbstract Number F25Mon. * Corresponding author. Tel./fax: 81-75-753-4035. E-mail address: [email protected] (A. Ota).
0379-6779/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 2 9 5 - 3
464
A. Ota et al. / Synthetic Metals 133±134 (2003) 463±465
Table 1 Crystallographic data of EDO2X (X PF6 , AsF6, ClO4)
Ê) a (A Ê) b (A Ê) c (A a (8) b (8) g (8) Ê 3) V (A R a
AsF6 (RT)
PF6 (RT)
AsF6 (230 K)
PF6 (260 K)
ClO4 (RT)a
Triclinic P 1
Triclinic P 1
Triclinic P 1
Triclinic P 1
Triclinic P 1
7.224(2) 7.354(1) 12.093(2) 93.35(1) 74.822(9) 97.08(1) 615.1(2) 0.065
7.197(1) 7.343(1) 11.948(1) 93.454(7) 75.158(6) 97.405(7) 605.0(1) 0.087
9.771(8) 10.990(9) 11.341(8) 101.78(4) 100.06(4) 89.89(5) 1173.0(2) 0.085
9.822(1) 11.000(2) 11.487(2) 101.865(8) 99.128(8) 90.445(8) 1198.1(3) 0.048
9.760(1) 10.928(1) 11.221(1) 100.366(5) 99.404(5) 91.144(5) 1159.9(2) 0.058
The details of (EDO)2ClO4 will be reported elsewhere.
Table 2 Physical properties of (EDO)2X (X PF6 , AsF6, ClO4)
MI transition temperature, TMI (K) Conductivity at RT, sRT (S cm 1) Activation energy below TMI, ea (meV) w at RT (10 4 emu mol 1) Overlap integral at 230 K (10 3) (Fig. 1b) S1 S2 S3
PF6
AsF6
280 60 320 2.5
268 >337 165 3.8 10 300 500 2.3 ±
44.2 23.0 13.0
45.8 23.8 13.6
ClO4 5
46.2 22.8 13.4
(w) showed almost constant value derived from the Pauli para-magnetism down to the MI transition temperature (TMI). Below this temperature, w vanished, and the phase transition showed the thermal hysteresis around TMI. Comparing to that at RT, the unit cell was doubled along the donor column by the formation of tetramers, which consist of the bent donor molecules (B) and ¯at ones (F) at 230 K (Fig. 1b). Although the values of the overlap integrals (S) in the donor column are almost uniform at
RT, there observed large alternation of the S in the low temperature (LT) phase, which corresponds to the nesting of the Fermi surface at RT. This indicates that the MI transition contains the nature of Peierls transition. It is also shown that the B and F molecules are charged less and more than 0.5, respectively, from the analysis of the bond lengths. As a result, these B and F molecules form periodic stripes of the charges in the salt, indicating the occurrence of the CO. The counter anions are located in general positions at 230 K. In the LT phase, the temperature factor of ¯uorine atoms were largely reduced from the values at RT, and an axis of F±As±F is almost parallel to the donor stacking axis. These facts indicate that the nature of this transition also includes order± disorder (OD) feature. Summing these results up, it was revealed that the MI transition contains the cooperative nature of Peierls, CO, and OD ones with the molecular deformation. As mentioned earlier, the same type of MI transition has been observed at 280 K in (EDO)2PF6. The RT and LT phases of AsF6 salt are isostructural to the corresponding ones of PF6. The single crystals of (EDO)2ClO4 at RT are semiconductive with large ea up to 337 K, of which crystal structure is isostructural to those of (EDO)2X (X PF6 ,
Fig. 1. (a) Calculated band structure and Fermi surface of (EDO)2AsF6 at RT. (b) Donor stacking column and anions of (EDO)2AsF6 at 230 K viewed along the short molecular axis. The dotted lines represent the intermolecular atomic contacts, which are shorter than the sum of the van der Waals radii.
A. Ota et al. / Synthetic Metals 133±134 (2003) 463±465
AsF6) in the insulating phase. The TMI of (EDO)2ClO4 is, hence, regarded to be above 337 K. These facts suggest that these three salts have a common electronic structure with different TMI. Since the area and thickness per a donor molecule in the donor layers are expanded and shrunk below TMI, respectively, it is expected that large anions, which penetrate into the donor layer, obstruct the structural change of the donor layer.
465
References [1] S. Horiuchi, H. Yamochi, G. Saito, K. Sakaguchi, M. Kusunoki, J. Am. Chem. Soc. 118 (1996) 8604. [2] A. Ota, H. Yamochi, G. Saito, J. Mater. Chem. 12 (2002) 2600. [3] M. Iyoda, Y. Kuwatani, E. Ogura, K. Hara, H. Suzuki, T. Takano, K. Takeda, J. Takano, K. Ugawa, M. Yoshida, H. Matsuyama, H. Nishikawa, I. Ikemoto, T. Kato, N. Yoneyama, J. Nishijo, A. Miyazaki, T. Enoki, Heterocycles 54 (2001) 833.