- Email: [email protected]
Conductivity from two energy levels in C60 (InCl3)2
Solid State Communications.Vol. 104. No. 2, pp. 91-93. 1998 0 1998 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/98 $I9.00+.00
Pergamon
PII: SOO38-1098(97)10249-6
CONDUCTIVITY FROM TWO ENERGY LEVELS IN C6a (InCl3)* M. Barati,+ P.K. Ummat and W.R. Datars* Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S 4M 1 (Received
27 October
1997; accepted
10 December
1997 by M.F. Collins)
The compound C60(InC13)2is prepared by the interaction of Cm and InCls. The resistivity is 150 Q cm-i at room temperature. The temperature dependence of the conductivity between 4.2 and 300 K is explained by the excitation from two energy levels. The excitation energies are 9 meV and 47 meV. 0 1998 Elsevier Science Ltd
The electrical conductivity of C6a compounds is of interest since the discovery of superconductivity in K& at 18 K [I]. All the known C6a superconductors have the same stoichiometry A&,. Other structurally known alkali metal doped fullerene materials such as the body centered tetragonal phase A4Cb0 and the body-centered cubic phase A&m are insulating [2]. The low ionization potential of the Cm molecule suggests that compounds or complexes with acceptor molecules can also be made. However, only a few of these materials have been reported and they are semiconductors with a single excitation energy [3]. The search for these compounds has the general aim of obtaining materials with important electrical properties. The purpose of this work is to establish C6a(InC1J2 as a material with interesting electrical properties. The conductivity is reasonably high at room temperature for a semiconductor and there is direct evidence of two energy levels. The synthesis was carried out with C6a powder prepared with the fullerene production system described previously [4] and InC13powder doubly sublimed under vacuum. Pure Cm and purified InCls weighed accurately for 1 : 2 stoichiometry were mixed well with a mortar and pestle in a dry box. This mixture was located into a Pyrex tube into which 1500 mbar of Cl1 gas was introduced before sealing it. The mixture was heated initially at 250°C and after l/2 h all the greenish-yellow Cl* gas disappeared. It was then heated at 300°C for 2 days after which there was a uniform, crystalline, black
mass. The reaction was allowed to proceed for 3 weeks at 400°C. When the Pyrex tube was opened in the dry
box the mass of the compound corresponded to the composition C&InCl,), 2C12. The samples cut from the masses formed in the reactions had dimensions of approximately 4 mm X 4 mm and electrical contacts were made with silver paste for the four-probe resistivity technique. The measurements were conducted by using a Keithley 224 programmable current source and a Keithley 181 nanovoltmeter. The temperature was monitored with platinum and carbon-glass thermometers. The resistance of a sample is shown as a function of temperature in Fig. 1. With a room temperature resistivity of 150 Q cm-i, it increases slowly with decreasing temperature down to 50 K. There is then a rapid increase as the temperature is lowered below 50 K. The resistivity is 2 X lo3 Qcm-’ at 77 K and 1 X lOI Q cm-’ at 4.2 K. Although the resistivity increases quickly with decreasing temperature below 60 K and appears like an insulator transition it can be shown that it follows a semiconducting characteristic conductivity is a=aoe
- Elkr
,
equation.
In this case, the
(1)
where E is the excitation energy. Figure 2(a) shows the fit of equation (1) to the low temperature data with an excitation energy of 10 meV. The resistivity is small above 77 K and appears to have little temperature dependence in Fig. 1. However the conductivity also has an exponential dependence on
reciprocal temperature and fits equation (1) with an excitation energy of 45 meV as shown in Fig. 2(b).
* Corresponding author. ’ Permanent address: Shiraz University, Shiraz Iran. 91
92
CONDUCTIVITY
IN Cm (InCl&
5
0
6
....................._-
0
i-
J ,
’
,
’
50
0
,
.,
.,
100
150
.,
200
.,
250
300
Fig. 1. Resistance
350
-
vs temperature
of C&InClj)2.
E,IkT + (J2 e - EdkT,
0.3-
(2)
a .o
;C b r Y
b a
o.2 O.l-
D.00
0.0 -
4
0.00
’
I
0.02
’
I
m
0.04
I.
I
0.06
0.08
’
-
II
0.10
(WV-’
0.002
0.004
0.006
0.15
0.20
0.25
Fig. 3. Conductivity vs temperature of C6,,(InC13)2. The dots are calculated from equation (2).
-I
a)
0.10 (T WI)-
The electrical conductivity is the sum of the carrier contribution from the two levels according to u=eie
L---,,....,%,.,
I,,,,,,,,,., 0.00 005
.,
T WI
-c 0
Vol. 106, No. 2
0.008
0.010
0.012
0.014
(T(K))-’
Fig. 2. Conductivity vs reciprocal temperature of C,(InCl&. The dots are calculated from equation (1) and the open circles are the data (a) for interval 10 < T < 60 K (b) for internal 77 < T < 280 K.
where u1 and uz depend on the concentration and mobility of the carriers from each level and El and E2 are the two excitation energies. Figure 3 shows the fit of equation (2) to the conductivity for 4.2 K < T < 300 K. The excellent fit yields El = 47 meV, E2 = 9 meV, ui = 2.4 X lo-* Qcm-’ and a2 = 2.6 X 10e3 f2 cm-‘. Thus, the conductivity of C,&nClj)2 is explained by a two level system. It is unlikely that the band structure is changed greatly from that of CM by the introduction of I&Is. Thus, the energy gap between the valence and conduction bands in C60(InC13)2 should be similar to the gap of 1.5 eV in C60 [6]. This means that the observed excitations are not across direct energy gaps. It is also unlikely that there are two energy gaps to give the semiconducting behaviour. Therefore it is likely that the excitations are from energy levels near the conduction band or the valence band. In conclusion it is established that C60(InCI& has important properties which warrant further study. The resistivity at room temperature is 150 Q cm-‘. The temperature dependence of the resistivity shows that there is carrier excitation thermally from two energy levels with energies of 9 meV and 47 meV. Acknowledgements-The research was supported financially by the Natural Sciences and Engineering Research Council of Canada. M. Barati acknowledges the summer research leave granted to him by Shiraz University. The assistance with the paper by P.A. Dube is gratefully acknowledged. REFERENCES 1.
Hebart, A.F., Rosseinsky, M.F., Haddon, R.C., Murphy, D.W., Glarum, S.H., Palstra, T.T.M.,
Vol. 106, No. 2
CONDUCTIVITY IN Cm (InCl&
Ramirez, A.P. and Kortan, A.R., Nature, 350,1991, 600. 2. Zhou, 0. and Cox, D.E., J. Phys. Chem. Solids, 53, 1992, 1373. 3. Datars, W.R., Palidwar, I.D. and Ummat, P.K., J. Phys. Chem. Solids, 57, 1996,917.
4.
93
Datars, W.R., Galts, S., Olech, T. and Ummat, P.K., Can. J. Phys., 73, 1995, 38. 5. Winkler, R., Pichler, T. and Kuzmas, H., Appl. Phys. Lat., 66, 1994, 1211. 6. Saito, S. and Oshiyama, A., Phys. Rev. Lett., 66, 1991,2637.
Pergamon
PII: SOO38-1098(97)10249-6
CONDUCTIVITY FROM TWO ENERGY LEVELS IN C6a (InCl3)* M. Barati,+ P.K. Ummat and W.R. Datars* Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S 4M 1 (Received
27 October
1997; accepted
10 December
1997 by M.F. Collins)
The compound C60(InC13)2is prepared by the interaction of Cm and InCls. The resistivity is 150 Q cm-i at room temperature. The temperature dependence of the conductivity between 4.2 and 300 K is explained by the excitation from two energy levels. The excitation energies are 9 meV and 47 meV. 0 1998 Elsevier Science Ltd
The electrical conductivity of C6a compounds is of interest since the discovery of superconductivity in K& at 18 K [I]. All the known C6a superconductors have the same stoichiometry A&,. Other structurally known alkali metal doped fullerene materials such as the body centered tetragonal phase A4Cb0 and the body-centered cubic phase A&m are insulating [2]. The low ionization potential of the Cm molecule suggests that compounds or complexes with acceptor molecules can also be made. However, only a few of these materials have been reported and they are semiconductors with a single excitation energy [3]. The search for these compounds has the general aim of obtaining materials with important electrical properties. The purpose of this work is to establish C6a(InC1J2 as a material with interesting electrical properties. The conductivity is reasonably high at room temperature for a semiconductor and there is direct evidence of two energy levels. The synthesis was carried out with C6a powder prepared with the fullerene production system described previously [4] and InC13powder doubly sublimed under vacuum. Pure Cm and purified InCls weighed accurately for 1 : 2 stoichiometry were mixed well with a mortar and pestle in a dry box. This mixture was located into a Pyrex tube into which 1500 mbar of Cl1 gas was introduced before sealing it. The mixture was heated initially at 250°C and after l/2 h all the greenish-yellow Cl* gas disappeared. It was then heated at 300°C for 2 days after which there was a uniform, crystalline, black
mass. The reaction was allowed to proceed for 3 weeks at 400°C. When the Pyrex tube was opened in the dry
box the mass of the compound corresponded to the composition C&InCl,), 2C12. The samples cut from the masses formed in the reactions had dimensions of approximately 4 mm X 4 mm and electrical contacts were made with silver paste for the four-probe resistivity technique. The measurements were conducted by using a Keithley 224 programmable current source and a Keithley 181 nanovoltmeter. The temperature was monitored with platinum and carbon-glass thermometers. The resistance of a sample is shown as a function of temperature in Fig. 1. With a room temperature resistivity of 150 Q cm-i, it increases slowly with decreasing temperature down to 50 K. There is then a rapid increase as the temperature is lowered below 50 K. The resistivity is 2 X lo3 Qcm-’ at 77 K and 1 X lOI Q cm-’ at 4.2 K. Although the resistivity increases quickly with decreasing temperature below 60 K and appears like an insulator transition it can be shown that it follows a semiconducting characteristic conductivity is a=aoe
- Elkr
,
equation.
In this case, the
(1)
where E is the excitation energy. Figure 2(a) shows the fit of equation (1) to the low temperature data with an excitation energy of 10 meV. The resistivity is small above 77 K and appears to have little temperature dependence in Fig. 1. However the conductivity also has an exponential dependence on
reciprocal temperature and fits equation (1) with an excitation energy of 45 meV as shown in Fig. 2(b).
* Corresponding author. ’ Permanent address: Shiraz University, Shiraz Iran. 91
92
CONDUCTIVITY
IN Cm (InCl&
5
0
6
....................._-
0
i-
J ,
’
,
’
50
0
,
.,
.,
100
150
.,
200
.,
250
300
Fig. 1. Resistance
350
-
vs temperature
of C&InClj)2.
E,IkT + (J2 e - EdkT,
0.3-
(2)
a .o
;C b r Y
b a
o.2 O.l-
D.00
0.0 -
4
0.00
’
I
0.02
’
I
m
0.04
I.
I
0.06
0.08
’
-
II
0.10
(WV-’
0.002
0.004
0.006
0.15
0.20
0.25
Fig. 3. Conductivity vs temperature of C6,,(InC13)2. The dots are calculated from equation (2).
-I
a)
0.10 (T WI)-
The electrical conductivity is the sum of the carrier contribution from the two levels according to u=eie
L---,,....,%,.,
I,,,,,,,,,., 0.00 005
.,
T WI
-c 0
Vol. 106, No. 2
0.008
0.010
0.012
0.014
(T(K))-’
Fig. 2. Conductivity vs reciprocal temperature of C,(InCl&. The dots are calculated from equation (1) and the open circles are the data (a) for interval 10 < T < 60 K (b) for internal 77 < T < 280 K.
where u1 and uz depend on the concentration and mobility of the carriers from each level and El and E2 are the two excitation energies. Figure 3 shows the fit of equation (2) to the conductivity for 4.2 K < T < 300 K. The excellent fit yields El = 47 meV, E2 = 9 meV, ui = 2.4 X lo-* Qcm-’ and a2 = 2.6 X 10e3 f2 cm-‘. Thus, the conductivity of C,&nClj)2 is explained by a two level system. It is unlikely that the band structure is changed greatly from that of CM by the introduction of I&Is. Thus, the energy gap between the valence and conduction bands in C60(InC13)2 should be similar to the gap of 1.5 eV in C60 [6]. This means that the observed excitations are not across direct energy gaps. It is also unlikely that there are two energy gaps to give the semiconducting behaviour. Therefore it is likely that the excitations are from energy levels near the conduction band or the valence band. In conclusion it is established that C60(InCI& has important properties which warrant further study. The resistivity at room temperature is 150 Q cm-‘. The temperature dependence of the resistivity shows that there is carrier excitation thermally from two energy levels with energies of 9 meV and 47 meV. Acknowledgements-The research was supported financially by the Natural Sciences and Engineering Research Council of Canada. M. Barati acknowledges the summer research leave granted to him by Shiraz University. The assistance with the paper by P.A. Dube is gratefully acknowledged. REFERENCES 1.
Hebart, A.F., Rosseinsky, M.F., Haddon, R.C., Murphy, D.W., Glarum, S.H., Palstra, T.T.M.,
Vol. 106, No. 2
CONDUCTIVITY IN Cm (InCl&
Ramirez, A.P. and Kortan, A.R., Nature, 350,1991, 600. 2. Zhou, 0. and Cox, D.E., J. Phys. Chem. Solids, 53, 1992, 1373. 3. Datars, W.R., Palidwar, I.D. and Ummat, P.K., J. Phys. Chem. Solids, 57, 1996,917.
4.
93
Datars, W.R., Galts, S., Olech, T. and Ummat, P.K., Can. J. Phys., 73, 1995, 38. 5. Winkler, R., Pichler, T. and Kuzmas, H., Appl. Phys. Lat., 66, 1994, 1211. 6. Saito, S. and Oshiyama, A., Phys. Rev. Lett., 66, 1991,2637.