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Hyperfine interaction of polyacetylene intercalated with iron halide anions: 57Fe Mössbauer and EPR spectroscopy studies
Solid State Communications, Printed in Great Britain.
HYPERFINE
INTERACTION
2. Kucharskix, x xx
xxx
Department
pp.397-399,
0038-1098/84 $3.00 + .OO Pergamon Press Ltd.
1984.
OF POLYACETYLENE INTERCALATED WITH IRON HALIDE ANIONS AND EPR SPECTROSCOPY STUDIES
A. Pron
Institute
Vo1.5O,No.5,
xx
, J. Suwalski
of Atomic Energy
of Chemistry,
x
-
, I. Kulszewicz
xx
Groupe
Fe M&SBAUER
, D. BillaudXXX and P. Bemier
05-400 Otwock-Swierk
xxxx
(Poland)
Technical University of Warsaw, 00664 Warsaw (Poland)
Noakowskiego
Laborataire de Chimie Minerale Appliquee, LA CNRS No 158, Universite B.P. 239, 54506 Vandoeuvre les Nancy Cedex (France) xxxx
57
:
3
de Nancy
I,
de Dynamique des Phases Condensees, LA CNRS, U.S.T.L. 34060 Montpellier (France) Received
18 September 1983, in revised form 23 January 1984 by E.F. Bertaut
57
Fe Mdssbauer spectroscopy and EPR spectroscopy were applied to study the chemical nature of the intercalant in polyacetylene oxidized (doped) chemically with FeC13 and FeBr3. For the (CH),/FeC13 system the Mossbauer lattice temperature (8, = 89 K) was estimated from the temperature dependence of the recoil-free fraction. The strong influence of the high FeIII state of the dopant is clearly evidenced through EPR spectra of both systems : while the chloride system shows a superposition of a narrow and a broad lines, the bromide one exhibits only a narrow line , whose intensity decreases with increasing dopant concentration.
tribution in the solid material, and about the stability of the polymer/dopant system upon exposure to air.
Introduction The simplest polyene, polyacetylene, can be intercalated with several electron accepting or electron donating compounds. The intercalation process results in a significant modification of its electrical properties leading to the conductivity increase even by a factor of 12 order of magnitude. Recently we have reported that (CH), can react with FeC13 1, but the determination of the structure after intercalation and the chemical nature of the intercalant is very difficult due to the poor crystallinity of the system studied. Mtissbauer spectroscopy is in general a powerful method for determining the chemical environment in molecules or complexes containing suitable Mdssbauer nucleus. From the values of the isomer shift and the guadrupole splitting, significant information concerning the geometrical arrangement of the coordination sphere and the symmetry of the charge distribution can be extracted. In addition from the temperature dependence of the recoil-free fraction (f) the Wssbauer lattice temperature (BM) characterizing the lattice dynamics can be calculated. In this communication we report the measurements of the Mdssbauer spectra of (CH)x/ FeC13 and (CHlx/FeBr3 over the temperature range 4.2 - 290 K. In addition for the chloride system the calculation of (SM) is reported. We also present EPR results obtained on the doped systems which confirm the interpretation of the Mdssbauer data and gives some information about the homogeneity of the dopant dis-
Experimental The reactions of polyacetylene with FeC13 and FeBr3 were carried out at RT in nitromethane solutions of-O.2 molarity and 0.3 molarity in the case of FeC13 and FeBr3 respectively. The resultant films were washed with dry nitromethane and pumped. All loading and transfer operations were carried out in a dry argon atmosphere. The Mossbauer effect (ME) measurements were performed in standard transmission geometry using a constant acceleration velocity spectrometer coupled to 57Co (Cr) source. For ME studies samples were transferred to the cryostat chamber in dry argon and then the cryostat was evacuated. The absorbers thickness was less than 0.1 mg/cm2 of 57Fe isotope. The spectra were recorded with a statistics of several million counts per channel to become reliable even in the case of low absorption coefficient and then fitted with Lorentz shape lines by the least square method with a computer. The EPR spectra were obtained using a Bruker ER 200 D spectrometer working at 9.5 GHz at RT. Doped samples were directly transferred in EPR tubes and then sealed under vacuum. Results FeC13 can be introduced to polyacetylene to high doping levels although above y = 0.07 2 397
HYPERFINE INTERACTIONOF POLYACETYLENE
398
self degradation of the product by the chlorination of the double bond occurs to some extend 3. In the (CH),/FeBr3 system doping levels of y = 0.07 can be achieved for long doping times however these doping levels are unstable with respect to pure nitromethane washing and after careful, repeated washing the (CH),/FeBr3 system always approaches the limiting, stable composition of y = 0.013. The Mdssbauer spectrum of a new (ICH)~/ FeBr3 system is presented in figure 1. The corresponding spectrum of (CH),/FeC13 can be found elsewhere 4. Mdssbauer parameters of both systems are collected in Table 1. In both systems, the intensity of the Mdssbauer lines strongly depends on the temperature. In figure 2 normalized absorptions are plot-
INTERCALATED
WITH IRON HALIDE ANIONS
ted versus temperature for various samples of polyacetylene doped with FeCl3 and FeBr3. For sample 1 several additional experimental points were taken between 78 and 290 K in order to check the linearity of the curve and to measure its slope with confidence. The EPR spectra observed with the chloride systems have been already described 5. They generally show the superposition of a narrow line (% 1 Gauss linewidth) and a very broad line (% 500 Gauss linewidth), characterizing the inhomogeneous dopant distribution in the film. At high doping level the spectra show the usual asymetric (Dysonian) shape due to the high conductivity of the system. The bromide systems present a different behaviour. No broad line is observed anytime, while a symmetric narrow line (g 1, 2.0026, linewidth 9 Gauss) disappears with increasing concentration. No resonance at‘all is observed for the highest dopant concentration (y = 0.07). Upon exposure to air, the above spectra do not present any significant evolution.
Discussion
Figure
1 - Mdssbauer spectrum of FeBr4- doped polyacetylene obtained at 4.2 K
Vol. 50, No.5
and Conclusions
The isomer shift and guadrupole splitting values obtained for both (CH),/FeC13 and (CH).J FeBr3 systems exclude the hypothesis that both halides retain their molecular identity upon intercalation. The IS and QS values differ significantly from those observed for neutral FeC13 and FeBr3 and are in the range typically obtained for monovalent ferric halide anions : F&14and FeBrq- 6 suggesting one electron transfer from the polymer chain per one iron halide species introduced. From the temperature dependence of the recoil-free fraction for thin absorbers the Mdssbauer lattice temperature can be calculated (assuming the Debye model in the high temperature limit 7, :
eM= _11.659
(- F)-1'2
= 104 K ;
HM T "-T
(1)
In the above formula the value M = 57 is used for the mass of the nucleus. However in the presence of covalency M should be replaced by Meff. which is different than the mass of "bare" 57Fe. The value of Meff_ can be estimated from the slope of the temperature dependence of the isomer shift. The above correction leads to the following formula 7 :
“ii = 0
100 200 TEMPERATURE
300 (K)
Figure 2 - Normalized absorptions versus l/T for (CH), doped chemically with FeCl and FeBr 3 3 1. IcH(F~c~~)~_~~~~~
3. ~cH(F~C~~)~~~~I~
2. ICH(FeC14)0.031x
4. ICH(FeBr3)0.031x
4.3203 x lo2 ,=,1/z
= 89 K
(2)
In the case of (cH),/F~B~~ system similar calculation could not be performed due to a very low absorption coefficient even at very low temperatures. Roughly estimated value of 8M is ca 120 K. The Mdssbauer lattice temperaturesare surprisingly low. The obtained data indicate "loose" packing of the intercalated anions and some degree of mobility even higher than in the case of FeC13 intercalated graphite whose f3M is 164 K 8,9. Inhomogeneous doping is a general feature in these systems, and is confirmed by the observation of superimposed EPR spectra for the chlori-
vol. 50, No.3
HYPERFINE
INTERACTION
OF POLYACETYLENE Table 57
Fe in parameters of and bromide complexes measured
Mdssoauer
Samples
e2gW2 INIlS
-1
)
4 0.051x
ICH(FeBr4)0.031x
(CH), doped at
-1
liquid
d 1nA -1
mms
IRON HALIDE ANIONS
iron
chloride
399
with helium
/ dT -1 deg
temperature
dIS/dT -1
%
e'M
nuns
K
4.2
0.30 +
0.54 +
0.03
0.03
0.32 +_
0.33 _ +
0.55 _ +
0.06
0.03
0.03
o.33 +_ 0.01
WITH
1
r/2 mm s
4.2 K
4.2 K
'CHfFeCl
IS mm s-1
INTERCALATED
de system. On the other hand the absence of a broad spectrum for the bromide system could be due to an increased relaxation rate of the Fe spin caused by the close Br atoms. The total absence of resonance in the most concentrated bromide system indicates that, if the charge carriers are also spin carriers, they are
-1.264.10
-2
-5.354.10
-4
104+5
89+8
120
coupled to the doping species in such a way that their own EPR signal is unobservable. The remarkable stability of these material5 (the bromide systems in particular) upon air exposure shows that both the dopant anion and the polymer carbocation keep their own identity in the presence of oxygen and/or moisture.
References
A. Pron, I. Kulszewicz, D. Billaud and J. Przyluski, J.C.S. Chem. Comm. 783 (1981). The parameter y describing the doping level is defined as the stoichiometric coefficient in the reaction product assuming FeC14- / or FeBr4/ as the intercalant ie. jCH(FeC14)yf, or!CH(FeBr4)y!, M. Przybylski, B. Bulka, I. Kulszewicz, A. Pron, Solid State Comm, 48, 893 (1983) A. Pron, M. Zagorska, Z. Kucharski, M. Lukasiak, J. Suwalski, Mat. Res. Bull. 17, 1505 (1982).
5. 6. 7. 8. 9.
H. Sakai, Y.. Maeda, T. Kobayashi, H, Shirakawa, Bull. Chem, Sot. Jap. 56, 1616 (1983). A. Pron, P, Bernier, D, Billaud, S. Lefrant, Solid State Comm. 46, 587 (1983). N.N, Greenwood, T.C. Gibb "Massbauer Spectroscopy" Chapman and Hall, London (1971). R.H. Herber, Y. Maeda, Physica 99 B, 352 (1980) Orsay M. Katada, R.H. Herber, J. Phys.Colloq. France 4OC2, 663 (1979). R.H. Herber, M. Katada, J, Inorg. Nucl. Chem. fll, 1097 (1979).
HYPERFINE
INTERACTION
2. Kucharskix, x xx
xxx
Department
pp.397-399,
0038-1098/84 $3.00 + .OO Pergamon Press Ltd.
1984.
OF POLYACETYLENE INTERCALATED WITH IRON HALIDE ANIONS AND EPR SPECTROSCOPY STUDIES
A. Pron
Institute
Vo1.5O,No.5,
xx
, J. Suwalski
of Atomic Energy
of Chemistry,
x
-
, I. Kulszewicz
xx
Groupe
Fe M&SBAUER
, D. BillaudXXX and P. Bemier
05-400 Otwock-Swierk
xxxx
(Poland)
Technical University of Warsaw, 00664 Warsaw (Poland)
Noakowskiego
Laborataire de Chimie Minerale Appliquee, LA CNRS No 158, Universite B.P. 239, 54506 Vandoeuvre les Nancy Cedex (France) xxxx
57
:
3
de Nancy
I,
de Dynamique des Phases Condensees, LA CNRS, U.S.T.L. 34060 Montpellier (France) Received
18 September 1983, in revised form 23 January 1984 by E.F. Bertaut
57
Fe Mdssbauer spectroscopy and EPR spectroscopy were applied to study the chemical nature of the intercalant in polyacetylene oxidized (doped) chemically with FeC13 and FeBr3. For the (CH),/FeC13 system the Mossbauer lattice temperature (8, = 89 K) was estimated from the temperature dependence of the recoil-free fraction. The strong influence of the high FeIII state of the dopant is clearly evidenced through EPR spectra of both systems : while the chloride system shows a superposition of a narrow and a broad lines, the bromide one exhibits only a narrow line , whose intensity decreases with increasing dopant concentration.
tribution in the solid material, and about the stability of the polymer/dopant system upon exposure to air.
Introduction The simplest polyene, polyacetylene, can be intercalated with several electron accepting or electron donating compounds. The intercalation process results in a significant modification of its electrical properties leading to the conductivity increase even by a factor of 12 order of magnitude. Recently we have reported that (CH), can react with FeC13 1, but the determination of the structure after intercalation and the chemical nature of the intercalant is very difficult due to the poor crystallinity of the system studied. Mtissbauer spectroscopy is in general a powerful method for determining the chemical environment in molecules or complexes containing suitable Mdssbauer nucleus. From the values of the isomer shift and the guadrupole splitting, significant information concerning the geometrical arrangement of the coordination sphere and the symmetry of the charge distribution can be extracted. In addition from the temperature dependence of the recoil-free fraction (f) the Wssbauer lattice temperature (BM) characterizing the lattice dynamics can be calculated. In this communication we report the measurements of the Mdssbauer spectra of (CH)x/ FeC13 and (CHlx/FeBr3 over the temperature range 4.2 - 290 K. In addition for the chloride system the calculation of (SM) is reported. We also present EPR results obtained on the doped systems which confirm the interpretation of the Mdssbauer data and gives some information about the homogeneity of the dopant dis-
Experimental The reactions of polyacetylene with FeC13 and FeBr3 were carried out at RT in nitromethane solutions of-O.2 molarity and 0.3 molarity in the case of FeC13 and FeBr3 respectively. The resultant films were washed with dry nitromethane and pumped. All loading and transfer operations were carried out in a dry argon atmosphere. The Mossbauer effect (ME) measurements were performed in standard transmission geometry using a constant acceleration velocity spectrometer coupled to 57Co (Cr) source. For ME studies samples were transferred to the cryostat chamber in dry argon and then the cryostat was evacuated. The absorbers thickness was less than 0.1 mg/cm2 of 57Fe isotope. The spectra were recorded with a statistics of several million counts per channel to become reliable even in the case of low absorption coefficient and then fitted with Lorentz shape lines by the least square method with a computer. The EPR spectra were obtained using a Bruker ER 200 D spectrometer working at 9.5 GHz at RT. Doped samples were directly transferred in EPR tubes and then sealed under vacuum. Results FeC13 can be introduced to polyacetylene to high doping levels although above y = 0.07 2 397
HYPERFINE INTERACTIONOF POLYACETYLENE
398
self degradation of the product by the chlorination of the double bond occurs to some extend 3. In the (CH),/FeBr3 system doping levels of y = 0.07 can be achieved for long doping times however these doping levels are unstable with respect to pure nitromethane washing and after careful, repeated washing the (CH),/FeBr3 system always approaches the limiting, stable composition of y = 0.013. The Mdssbauer spectrum of a new (ICH)~/ FeBr3 system is presented in figure 1. The corresponding spectrum of (CH),/FeC13 can be found elsewhere 4. Mdssbauer parameters of both systems are collected in Table 1. In both systems, the intensity of the Mdssbauer lines strongly depends on the temperature. In figure 2 normalized absorptions are plot-
INTERCALATED
WITH IRON HALIDE ANIONS
ted versus temperature for various samples of polyacetylene doped with FeCl3 and FeBr3. For sample 1 several additional experimental points were taken between 78 and 290 K in order to check the linearity of the curve and to measure its slope with confidence. The EPR spectra observed with the chloride systems have been already described 5. They generally show the superposition of a narrow line (% 1 Gauss linewidth) and a very broad line (% 500 Gauss linewidth), characterizing the inhomogeneous dopant distribution in the film. At high doping level the spectra show the usual asymetric (Dysonian) shape due to the high conductivity of the system. The bromide systems present a different behaviour. No broad line is observed anytime, while a symmetric narrow line (g 1, 2.0026, linewidth 9 Gauss) disappears with increasing concentration. No resonance at‘all is observed for the highest dopant concentration (y = 0.07). Upon exposure to air, the above spectra do not present any significant evolution.
Discussion
Figure
1 - Mdssbauer spectrum of FeBr4- doped polyacetylene obtained at 4.2 K
Vol. 50, No.5
and Conclusions
The isomer shift and guadrupole splitting values obtained for both (CH),/FeC13 and (CH).J FeBr3 systems exclude the hypothesis that both halides retain their molecular identity upon intercalation. The IS and QS values differ significantly from those observed for neutral FeC13 and FeBr3 and are in the range typically obtained for monovalent ferric halide anions : F&14and FeBrq- 6 suggesting one electron transfer from the polymer chain per one iron halide species introduced. From the temperature dependence of the recoil-free fraction for thin absorbers the Mdssbauer lattice temperature can be calculated (assuming the Debye model in the high temperature limit 7, :
eM= _11.659
(- F)-1'2
= 104 K ;
HM T "-T
(1)
In the above formula the value M = 57 is used for the mass of the nucleus. However in the presence of covalency M should be replaced by Meff. which is different than the mass of "bare" 57Fe. The value of Meff_ can be estimated from the slope of the temperature dependence of the isomer shift. The above correction leads to the following formula 7 :
“ii = 0
100 200 TEMPERATURE
300 (K)
Figure 2 - Normalized absorptions versus l/T for (CH), doped chemically with FeCl and FeBr 3 3 1. IcH(F~c~~)~_~~~~~
3. ~cH(F~C~~)~~~~I~
2. ICH(FeC14)0.031x
4. ICH(FeBr3)0.031x
4.3203 x lo2 ,=,1/z
= 89 K
(2)
In the case of (cH),/F~B~~ system similar calculation could not be performed due to a very low absorption coefficient even at very low temperatures. Roughly estimated value of 8M is ca 120 K. The Mdssbauer lattice temperaturesare surprisingly low. The obtained data indicate "loose" packing of the intercalated anions and some degree of mobility even higher than in the case of FeC13 intercalated graphite whose f3M is 164 K 8,9. Inhomogeneous doping is a general feature in these systems, and is confirmed by the observation of superimposed EPR spectra for the chlori-
vol. 50, No.3
HYPERFINE
INTERACTION
OF POLYACETYLENE Table 57
Fe in parameters of and bromide complexes measured
Mdssoauer
Samples
e2gW2 INIlS
-1
)
4 0.051x
ICH(FeBr4)0.031x
(CH), doped at
-1
liquid
d 1nA -1
mms
IRON HALIDE ANIONS
iron
chloride
399
with helium
/ dT -1 deg
temperature
dIS/dT -1
%
e'M
nuns
K
4.2
0.30 +
0.54 +
0.03
0.03
0.32 +_
0.33 _ +
0.55 _ +
0.06
0.03
0.03
o.33 +_ 0.01
WITH
1
r/2 mm s
4.2 K
4.2 K
'CHfFeCl
IS mm s-1
INTERCALATED
de system. On the other hand the absence of a broad spectrum for the bromide system could be due to an increased relaxation rate of the Fe spin caused by the close Br atoms. The total absence of resonance in the most concentrated bromide system indicates that, if the charge carriers are also spin carriers, they are
-1.264.10
-2
-5.354.10
-4
104+5
89+8
120
coupled to the doping species in such a way that their own EPR signal is unobservable. The remarkable stability of these material5 (the bromide systems in particular) upon air exposure shows that both the dopant anion and the polymer carbocation keep their own identity in the presence of oxygen and/or moisture.
References
A. Pron, I. Kulszewicz, D. Billaud and J. Przyluski, J.C.S. Chem. Comm. 783 (1981). The parameter y describing the doping level is defined as the stoichiometric coefficient in the reaction product assuming FeC14- / or FeBr4/ as the intercalant ie. jCH(FeC14)yf, or!CH(FeBr4)y!, M. Przybylski, B. Bulka, I. Kulszewicz, A. Pron, Solid State Comm, 48, 893 (1983) A. Pron, M. Zagorska, Z. Kucharski, M. Lukasiak, J. Suwalski, Mat. Res. Bull. 17, 1505 (1982).
5. 6. 7. 8. 9.
H. Sakai, Y.. Maeda, T. Kobayashi, H, Shirakawa, Bull. Chem, Sot. Jap. 56, 1616 (1983). A. Pron, P, Bernier, D, Billaud, S. Lefrant, Solid State Comm. 46, 587 (1983). N.N, Greenwood, T.C. Gibb "Massbauer Spectroscopy" Chapman and Hall, London (1971). R.H. Herber, Y. Maeda, Physica 99 B, 352 (1980) Orsay M. Katada, R.H. Herber, J. Phys.Colloq. France 4OC2, 663 (1979). R.H. Herber, M. Katada, J, Inorg. Nucl. Chem. fll, 1097 (1979).