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Electric and magnetic properties of a new conducting salt (DMTSA)2Cl (DMTSA = 2,3-dimethyl-tetraselenoanthracene)
ELSEVIER
SyntheticMetals79 (1996) 155-157
Electric and magnetic properties of a new conducting salt (DMTSA) $1 (DMTSA = 2,3-dimethyl-tetraselenoanthracene) M. Yanai a,*, K. Kawabata a, T. Sambongi a, Y. Aso b, K. Takimiya b, T. Otsubo b a Department of Physics, Graduate b Department of Applied Chemistry, Faculty
School of Science, Hokkaido University, Sapporo 060, Japan of Engineering, Hiroshima University, Higashi-Hiroshima 724, Japan
Received11 December1995;revised5 January
1996;
accepted8 January1996
Abstract A novel charge transfer salt (DMTSA)2CI was synthesized, where DMTSA is 2,3-dimethyl-tetraselenoanthracene. The salt belongs to a tetragonal space group, and lattice constants are a = b = 17.5 A, c = 5.06 A. Temperature dependences of the conductivity and of the magnetic susceptibility reveal that the system undergoes a metal-insulator transition around 240 K. A high-pressure experiment indicates that this transition does not arise from the one dimensionality of the electronic structure, such as the Peierls transition. Keywords:
2,3-Dimethyl-tetraselenoanthracene chloride;Electric properties;Magneticproperties
1. Introduction 9
Recently, radical cation salts of anthra[ 1,9-cd:4,10-c’d’] bisdichalcogenoles [ 1,2] were reported as a new family of conducting charge transfer complexes [ 11. DMTSA (2,3dimethyl-tetraselenoanthracene) and DMTTA (2,3dimethyl-tetrathioanthracene) are promising donors to synthesize highly conducting materials. Their structures are shown in Fig. 1. In particular, radical cation salts of (DMTSA) BF, and (DMTSA) NO3 exhibit high conductivity (4.5 X 10’ S cm- ‘) at room temperature. In this family, there are three types of crystal structure which exhibit different electric properties [2]. The type I crystal structure like
-se
tie-40 DMTSA
(DMTSA) BF, is orthorhombic and has segregated uniformly stacked columns. This type of crystal shows high conductivity along the stacking axis at room temperature. The type II structure like (DMTSA)ClO,
is monoclinic with
segregated dimerized stacked columns and is less conductive. Type III structure like (DMTTA) 2PF6 is orthorhombic with segregated dimerized
stacked columns and has very low
conductivity. In this work, we found a radical cation salt (DMTSA)$l which has a new type of crystal structure in this family as determined from X-ray examination and compositional analysis. Electrical
and magnetic measurements
metal-insulator
transition occurs around 240 K. Its origin is
discussed from the high-pressure
reveal that a
experiment.
* Correspondingauthor. 0379-6779/96/$15,000 1996Elsevier Science%A. All rights reserved PIIS0319-6119~9h~O?hz?-~
DM’lTA Fig. 1.Molecularstructures of DMTSA andDMTTA. 2. Experimental
The radical cation salt of (DMTSA),Cl by a standard electrocrystallizationmethod.
was synthesized The single crystal
had a black needle form with typical dimensions 1 X 0.1 X 0.1
mm. Electrical conductivity was measured by the four-probe d.c. method. Magnetic susceptibility was obtained by using a superconducting quantum interference device. In this measurement, orientation of the crystals was random in a basket.
156
M. Yanai et al. /Synthetic
Metals
79 (1996)
155-157
A constant magnetic field of 5 X lo4 G was applied. Pressure was generated by a 30 ton press with a piston-cylinder apparatus in which the sample was enclosed in a Teflon cell. The pressure was applied at room temperature and the apparatus was cooled down to liquid He temperature. A constant pressure load was automatically controlled during both cooling and heating processes in order to perform experiments under constant pressure.
3. Results and discussion 0
Crystal symmetry and the lattice parameters of the crystal were assigned by using X-ray oscillation and Weissenberg methods. Its stoichiometry was confirmed by compositional analysis, where chemical compositional analysis was made for C and H atoms and electron probe microanalysis for Se and Cl atoms. It was found that the crystals are (DMTSA)Jl and belong to a tetragonal space group. The lattice parameters are a = b = 17.5 A and c = 5.06 A. This crystal structure is quite different from those which have already been reported [2]. It is rather similar to (TSeT),Cl, where TSeT is tetraselenotetracene [ 31. The structure of (TSeT) *Cl is reported as tetragonal and its lattice parameters are a = b = 17.44 A and c=5.118 A. These results show that a new type of salt exists in the DMTSA family. Detailed crystal structure will be published separately. Fig. 2 shows the conductivity. The obtained salt has a high conductivity of 7.0 X lo* S cm- i at room temperature. It is metallic down to around 200 K, below which it becomes semiconducting. The apparent activation energy is very low and is estimated at 2.4 X 1O-2 eV. Fig. 3 (a) shows the magnetic susceptibility under a magnetic field of 5 X lo4 G, The core diamagnetic susceptibility of DMTSA was measured separately and obtained as - 1.9 X 10v4 e.m.u. mol-’ at room temperature. That of Clis - 2.6 X 10m5 e.m.u. mol-’ [4]. The observed susceptibility is independent of temperature above 240 K and decreases below this. At temperatures below 50 K the susceptibility
1
0
50
!
100
I
150
I
200
I
250
300
Fig. 3. Temperature dependences of the susceptibility (a) and the residual susceptibility after subtracting the core diamagnetism and the Curie paramagnetism (b). The dotted lines are guides to the eye. The solid line is the best fit to the Curie law below 50 K.
increases rapidly. Since the rapid increase of susceptibility can be fitted well to the Curie law, this increase is attributed to impurities. Impurity content is estimated at about 0.3 at.%, assuming that the effective number of the Bohr magneton of the impurity is equal to that of the electron. In Fig. 3 (b) , the susceptibility after subtracting core diamagnetic and Curie paramagnetic susceptibilities is replotted against temperature. The temperature-independent susceptibility above 240 K is interpreted as the Pauli spin susceptibility. The susceptibility gradually reduces below around 240 K and its reduction at low temperature, Ax, is estimated at 6 X 10F5 e.m.u. mol-‘. Reduction of carrier density, AN, in the crystal is obtained from the relation between Axand AN: AX= ANj.&knTn
l/T (K-l) Fig. 2. Temperature dependence of the conductivity.
where TF is the Fermi temperature. Then AN is obtained as 7 X 10”TF per mol by using the measured Ax of 6 X 10m5 e.m.u. mol- ‘. Since the electronic structure of this system is expected to be quasi-one dimensionaBom the needle shape of the crystal and the number of electrons is 0.5 per DMTSA molecule, AN is estimated at 1 X 102’ cms3 by using the density of 2.2 g crnw3, The reduction of x rather than x itself is used to estimate the carrier density at room temperature because of the probable errors in x. Thus, the carrier density in (DMTSA)&l at room temperature is larger than 1 X 10”
M. Yanai et al. /Synthetic
Metals
79 (1996)
155-157
157
(TMTSF) zPF6 undergoes a spin density wave transition and the pressure coefficient of TMa d( In T,,) ldP, is as large as - 0.5 per GPa at ambient pressure [ 61. Thus, the observed phase transition does not arise from the one dimensionality of the electronic structure. Further structural investigation at low temperature is needed in order to clarify the origin of the metal-insulator transition in the present system.
4. Conclusions
T/K Fig. 4. Temperature dependences of the conductivity at ambient pressure (a) and under 0.3 GPa (b) and 0.6 GPa (c) The inset shows the conductivity as a function of l/T. The arrows indicate the positions of the maximum of conductivity.
cmw3, which is comparable with that in organic metals like ( BEDT-TTF)J3 [ 51. The temperature at which the susceptibility begins to decrease is in agreement with that at which the temperature derivative of conductivity changes its sign. The small deviation between the above two critical temperatures can be explained in terms of temperature-dependent mobility of the conduction electrons. The maximum of the conductivity is lower than the transition temperature, when the mobility increases with cooling temperature. Therefore, it is concluded that this salt is metal at room temperature and undergoes a metal-insulator transition at about 240 K. The temperature dependence of conductivity under several constant high pressures is shown in Fig. 4, in order to make clear the origin of the transition. Although the apparent activation energy at low temperature is lowered with increasing pressure, the transition temperature, Th?r,is almost independent of the pressure, where Th?ris defined conveniently as the temperature at which the conductivity shows the maximum. If this transition originates from the one dimensionality of the electronic structure such as the Peierls transition, TM, should be suppressed drastically by pressure. For example,
A new type of salt, (DMTSA),Cl, is synthesized in the DMTSA family. This crystal shows high conductivity at room temperature and a metal-insulator transition at about 240 K. We suggest from the high-pressure experiment that this transition does not originate in the low dimensionality of the electronic structure such as the Peierls transition.
Acknowledgements The authors are particularly indebted to Professor N. Mori and Dr H. Takahashi for technical support on the high-pressure experiment.
References [ 1] K. Takimiya, H. Miyamoto, Y. Aso, T. Otsubo and F. Ogura, C/tern. Lett., (1990) 567. [2] K. Takimiya, A. Ohnishi, Y. Aso, F. Ogura, K. Kawabata, K. Tanaka and M. Mizutani, Bull. Chem. Sot. Jpn., 67 (1994) 766. [3] R.P. Shibaeva and V.F. Kaminskii, Sov. Phys. Crystallogr., 23 (1978) 669. [4] Lundolt Btirnstein Tabellen, Neue Serie, Vol. II, Part2, Springer, Berlin, 1966. [5] M. Weger, K. Bender, T. Klutz, D. Schweitzer, F. Gross, C.-P. Heidmann, C. Probst and K. Andres, Synth. Met., 25 (1988) 49. [6] D. Jerome, Mol. Ctyst. Liq. @St., 79 (1982) 155; T. Ishiguro and K Yamaji, Organic Superconductors, Springer, Berlin, 1990.
SyntheticMetals79 (1996) 155-157
Electric and magnetic properties of a new conducting salt (DMTSA) $1 (DMTSA = 2,3-dimethyl-tetraselenoanthracene) M. Yanai a,*, K. Kawabata a, T. Sambongi a, Y. Aso b, K. Takimiya b, T. Otsubo b a Department of Physics, Graduate b Department of Applied Chemistry, Faculty
School of Science, Hokkaido University, Sapporo 060, Japan of Engineering, Hiroshima University, Higashi-Hiroshima 724, Japan
Received11 December1995;revised5 January
1996;
accepted8 January1996
Abstract A novel charge transfer salt (DMTSA)2CI was synthesized, where DMTSA is 2,3-dimethyl-tetraselenoanthracene. The salt belongs to a tetragonal space group, and lattice constants are a = b = 17.5 A, c = 5.06 A. Temperature dependences of the conductivity and of the magnetic susceptibility reveal that the system undergoes a metal-insulator transition around 240 K. A high-pressure experiment indicates that this transition does not arise from the one dimensionality of the electronic structure, such as the Peierls transition. Keywords:
2,3-Dimethyl-tetraselenoanthracene chloride;Electric properties;Magneticproperties
1. Introduction 9
Recently, radical cation salts of anthra[ 1,9-cd:4,10-c’d’] bisdichalcogenoles [ 1,2] were reported as a new family of conducting charge transfer complexes [ 11. DMTSA (2,3dimethyl-tetraselenoanthracene) and DMTTA (2,3dimethyl-tetrathioanthracene) are promising donors to synthesize highly conducting materials. Their structures are shown in Fig. 1. In particular, radical cation salts of (DMTSA) BF, and (DMTSA) NO3 exhibit high conductivity (4.5 X 10’ S cm- ‘) at room temperature. In this family, there are three types of crystal structure which exhibit different electric properties [2]. The type I crystal structure like
-se
tie-40 DMTSA
(DMTSA) BF, is orthorhombic and has segregated uniformly stacked columns. This type of crystal shows high conductivity along the stacking axis at room temperature. The type II structure like (DMTSA)ClO,
is monoclinic with
segregated dimerized stacked columns and is less conductive. Type III structure like (DMTTA) 2PF6 is orthorhombic with segregated dimerized
stacked columns and has very low
conductivity. In this work, we found a radical cation salt (DMTSA)$l which has a new type of crystal structure in this family as determined from X-ray examination and compositional analysis. Electrical
and magnetic measurements
metal-insulator
transition occurs around 240 K. Its origin is
discussed from the high-pressure
reveal that a
experiment.
* Correspondingauthor. 0379-6779/96/$15,000 1996Elsevier Science%A. All rights reserved PIIS0319-6119~9h~O?hz?-~
DM’lTA Fig. 1.Molecularstructures of DMTSA andDMTTA. 2. Experimental
The radical cation salt of (DMTSA),Cl by a standard electrocrystallizationmethod.
was synthesized The single crystal
had a black needle form with typical dimensions 1 X 0.1 X 0.1
mm. Electrical conductivity was measured by the four-probe d.c. method. Magnetic susceptibility was obtained by using a superconducting quantum interference device. In this measurement, orientation of the crystals was random in a basket.
156
M. Yanai et al. /Synthetic
Metals
79 (1996)
155-157
A constant magnetic field of 5 X lo4 G was applied. Pressure was generated by a 30 ton press with a piston-cylinder apparatus in which the sample was enclosed in a Teflon cell. The pressure was applied at room temperature and the apparatus was cooled down to liquid He temperature. A constant pressure load was automatically controlled during both cooling and heating processes in order to perform experiments under constant pressure.
3. Results and discussion 0
Crystal symmetry and the lattice parameters of the crystal were assigned by using X-ray oscillation and Weissenberg methods. Its stoichiometry was confirmed by compositional analysis, where chemical compositional analysis was made for C and H atoms and electron probe microanalysis for Se and Cl atoms. It was found that the crystals are (DMTSA)Jl and belong to a tetragonal space group. The lattice parameters are a = b = 17.5 A and c = 5.06 A. This crystal structure is quite different from those which have already been reported [2]. It is rather similar to (TSeT),Cl, where TSeT is tetraselenotetracene [ 31. The structure of (TSeT) *Cl is reported as tetragonal and its lattice parameters are a = b = 17.44 A and c=5.118 A. These results show that a new type of salt exists in the DMTSA family. Detailed crystal structure will be published separately. Fig. 2 shows the conductivity. The obtained salt has a high conductivity of 7.0 X lo* S cm- i at room temperature. It is metallic down to around 200 K, below which it becomes semiconducting. The apparent activation energy is very low and is estimated at 2.4 X 1O-2 eV. Fig. 3 (a) shows the magnetic susceptibility under a magnetic field of 5 X lo4 G, The core diamagnetic susceptibility of DMTSA was measured separately and obtained as - 1.9 X 10v4 e.m.u. mol-’ at room temperature. That of Clis - 2.6 X 10m5 e.m.u. mol-’ [4]. The observed susceptibility is independent of temperature above 240 K and decreases below this. At temperatures below 50 K the susceptibility
1
0
50
!
100
I
150
I
200
I
250
300
Fig. 3. Temperature dependences of the susceptibility (a) and the residual susceptibility after subtracting the core diamagnetism and the Curie paramagnetism (b). The dotted lines are guides to the eye. The solid line is the best fit to the Curie law below 50 K.
increases rapidly. Since the rapid increase of susceptibility can be fitted well to the Curie law, this increase is attributed to impurities. Impurity content is estimated at about 0.3 at.%, assuming that the effective number of the Bohr magneton of the impurity is equal to that of the electron. In Fig. 3 (b) , the susceptibility after subtracting core diamagnetic and Curie paramagnetic susceptibilities is replotted against temperature. The temperature-independent susceptibility above 240 K is interpreted as the Pauli spin susceptibility. The susceptibility gradually reduces below around 240 K and its reduction at low temperature, Ax, is estimated at 6 X 10F5 e.m.u. mol-‘. Reduction of carrier density, AN, in the crystal is obtained from the relation between Axand AN: AX= ANj.&knTn
l/T (K-l) Fig. 2. Temperature dependence of the conductivity.
where TF is the Fermi temperature. Then AN is obtained as 7 X 10”TF per mol by using the measured Ax of 6 X 10m5 e.m.u. mol- ‘. Since the electronic structure of this system is expected to be quasi-one dimensionaBom the needle shape of the crystal and the number of electrons is 0.5 per DMTSA molecule, AN is estimated at 1 X 102’ cms3 by using the density of 2.2 g crnw3, The reduction of x rather than x itself is used to estimate the carrier density at room temperature because of the probable errors in x. Thus, the carrier density in (DMTSA)&l at room temperature is larger than 1 X 10”
M. Yanai et al. /Synthetic
Metals
79 (1996)
155-157
157
(TMTSF) zPF6 undergoes a spin density wave transition and the pressure coefficient of TMa d( In T,,) ldP, is as large as - 0.5 per GPa at ambient pressure [ 61. Thus, the observed phase transition does not arise from the one dimensionality of the electronic structure. Further structural investigation at low temperature is needed in order to clarify the origin of the metal-insulator transition in the present system.
4. Conclusions
T/K Fig. 4. Temperature dependences of the conductivity at ambient pressure (a) and under 0.3 GPa (b) and 0.6 GPa (c) The inset shows the conductivity as a function of l/T. The arrows indicate the positions of the maximum of conductivity.
cmw3, which is comparable with that in organic metals like ( BEDT-TTF)J3 [ 51. The temperature at which the susceptibility begins to decrease is in agreement with that at which the temperature derivative of conductivity changes its sign. The small deviation between the above two critical temperatures can be explained in terms of temperature-dependent mobility of the conduction electrons. The maximum of the conductivity is lower than the transition temperature, when the mobility increases with cooling temperature. Therefore, it is concluded that this salt is metal at room temperature and undergoes a metal-insulator transition at about 240 K. The temperature dependence of conductivity under several constant high pressures is shown in Fig. 4, in order to make clear the origin of the transition. Although the apparent activation energy at low temperature is lowered with increasing pressure, the transition temperature, Th?r,is almost independent of the pressure, where Th?ris defined conveniently as the temperature at which the conductivity shows the maximum. If this transition originates from the one dimensionality of the electronic structure such as the Peierls transition, TM, should be suppressed drastically by pressure. For example,
A new type of salt, (DMTSA),Cl, is synthesized in the DMTSA family. This crystal shows high conductivity at room temperature and a metal-insulator transition at about 240 K. We suggest from the high-pressure experiment that this transition does not originate in the low dimensionality of the electronic structure such as the Peierls transition.
Acknowledgements The authors are particularly indebted to Professor N. Mori and Dr H. Takahashi for technical support on the high-pressure experiment.
References [ 1] K. Takimiya, H. Miyamoto, Y. Aso, T. Otsubo and F. Ogura, C/tern. Lett., (1990) 567. [2] K. Takimiya, A. Ohnishi, Y. Aso, F. Ogura, K. Kawabata, K. Tanaka and M. Mizutani, Bull. Chem. Sot. Jpn., 67 (1994) 766. [3] R.P. Shibaeva and V.F. Kaminskii, Sov. Phys. Crystallogr., 23 (1978) 669. [4] Lundolt Btirnstein Tabellen, Neue Serie, Vol. II, Part2, Springer, Berlin, 1966. [5] M. Weger, K. Bender, T. Klutz, D. Schweitzer, F. Gross, C.-P. Heidmann, C. Probst and K. Andres, Synth. Met., 25 (1988) 49. [6] D. Jerome, Mol. Ctyst. Liq. @St., 79 (1982) 155; T. Ishiguro and K Yamaji, Organic Superconductors, Springer, Berlin, 1990.