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Electronic structure and phase transformation of alkali metal doped polyacetylene
ELSEVIER
Synthetic
Electronic
Department
structure
Metals
84 (1997)
and phase transformation
719-720
of alkali
Jiro Tanaka and Chizuko Faculty of Science, Kanagawa
of Chemistry,
metal
Tanaka University,
doped
Hdratsuka,
polyacetylene
259-12
Japan
Abstract The experimental studies on alkali metal doped polyacetylene have been reviewed. Molecular orbital calculations on model compounds of doped polyene are performed to show electronic and molecular structure doped species. Keywords ab initio quantum chemical methods and calculation, electrochemical doping, polyacetylene, photoelectron spectroscopy, metal-insulator phase transition, magnetic phase transition
of
1. Introduction
2.2 Electrical
The mechanism of insulator to metal transition in doped polyacetylene is a fundamental problem of conducting polymer research. We have been studying the transition by far infrared, Raman and ultraviolet pho toemission spectroscopy on doped polyacetylene and theoretical calculation on model compounds. The Xray structural, electrochemical and elecctron spin resonance were studied extensively on alkali metal doped polyacetylene, accordingly we have continued the electronic structural calculations on the model compounds of alkali metal doped polyene. In this report we will summarize earlier experimental researches and discuss on the calcualted results of molecular orbitals of the model compounds of the doped species.
Chung et al. [l] al so showed that the electrical conductivity increased rapidly at light doping level before the magnetic transition took place. They claimed that spinless charge transport is important in the light doping regime. Coustel et al. [3] confirmed the results of Heeger’s group by showing that the conductivity increased stepwise at y = 0.06 and y = 0.12. The temperature dependence of conductivity was studied by Galtier’s group and it showed rather little temperature dependence.[4]
2.
Summary
2.1 Electron
of experimental spin resonance
results and magnetic
transition
Chung et al. [l] first discussed magentic first order transiton on Na doped polyacetylene occurring at y > 0.06 (y is mole fraction of Na). However, their estimate of Na content seems too low from a view of our electronic and Raman spectral and chemical analysis data.[2] Comparing their spectra with ours, it appears that the magnetic transition to Pauli paramagetic state occurs at y > 0.12. 0379-6779/97/%17.00
PII s0379-6779(96)04117-3
63 1997
Elscvier
Science
S.k
All rights
reserved
conductivity
2.3 Electrochemical
potentials
Shacklette et al. [5] studied phase transformation in electrochemical doping of K and Na and found several phases appearing at y = 0.06 (stage 2), 0.12 (stage 1). Coustel et al. [3] carried out high resolution study of the cell potential and found additional transition at y = 0.15. Different cell potentials correspond to varied stage of dopnig. Accordingly we think that dissimilar doped chain unit correspond to these different potentials. 2.4 X-ray
structural
Structure models lene were investigated et al.[7] and Billaud
analysis of alkali metal by Baughman’s et al.[S]
doped poly- acetygroup [6], Fischer
720
J. Tanaka,
C. Tanaka
/Synthetic
In the light doping 0.06 < y < 0.12, the stage 2 structure was found by Murphy et al.[9], in which polyacetylene chain neighbors one metal column and four polyacetylene chains exist per alkali-metal column. For heavy doping y > 0.12, the tetragonal structure is formed [6], where each polyacetylene chain contacts with two metal columns and each metal column is surrounded by four polyacetylene chains. Saldi et al. [lo] found the metal-metal distance along the column 4.96 A for 0.12 > y > 0.06 and 3.98 A for y > 0.12 in K, Rb and Cs doped polyacetylene. 2.5 Raman spectra Raman spectra showed that the transition from the stage 2 to the stage 1 doped state occurs at y N 0.12.[2] In the highest doped state at 0.15 > y > 0.12, a characteristic Raman band was found at 1580 cm-l and it was assigned to the C-C stretching mode of the polson structure for CrsHr,r,Nas. [II]
Metals
84 (1997)
719-720
HOMO show different shape indicating that the charge carrier exists on the half of each unit with different spin. In (Na+)4(CssHss)4- two polson units are combined with an inversion symmetry and in (K+)4(CrzHr4)2polaron and antipolaron units are joined. These units may represent the carriers in the y N 0.14-0.15 and y N 0.166, respectively. In which has the constitution y = (K+)&~As)~-, 0.125, the charge and spin separation is not so obvious, indicating that the transition is just beginning. The model compound of poison, (Na+)4(C2sH2s)4-, has composition closest to the paramagnetic state, accordingly it may be the dominant structural unit at 0.16 > y > 0.12. However, coexistence of other species such as the polaron, (K+)4(CrzHr4)2-, or the charged soliton, (K+)z(C14H16)~-, are not completely excluded at y > 0.12. Acknowledgement We are grateful to the Computation Center of the Institute for Molecular Science.
2.6 Ultraviolet photoemission spectra (UPS) UPS of Na and K doped polyacetylene were studied by Miyamae et al.[12] The spectra changed stepwise. In the light doping, the population in the mid gap state was observed, and a density of states near EF was found when doping proceeds to y > 0.10. The spectra showed a typical pattern of strongly correlated electron system. This result is consistent with the Pauli paramagnetism in the heavily Na doped film.[l] 3. Molecular pounds
orbital
calculation
on model com-
To find the structure of the doped species, we used Gaussian 94 programs [13] at Computation Center of Institute for Molecular Science and our university. Here we will discuss on results on (K+)s(Cr4Hrs)2-, (K+)4(CrsHrs)2-, (Na+)h(CssHss)4- and (K+)4(CrsHr4)2-, where K or Na ions are placed at positions found in the crystalline state. We used the UHF method to find the carrier in the HOMOs of these complexes in the singlet state. In w h’rch reP resent the charged soliton (K+)2(C14H16)~-, structure dominating in the stage 2 of light doping, the shape of cy spin orbital and p spin orbital are exactly the same. It may mean that the charge transport in the lightly doped film is due to the spinlesscharged soliton wave.[l4] On th e contrary, in (Na+)4(CssHss)‘- and (K+)4(CmH14)2-, th e (Y spin HOMO and the p spin
References [l] T.-C. Chung, F. M oraes, J.D. Flood and A.J. Heeger, Phys. Rev.B, 29 (1984) 2341. [2] J. Tanaka, Y. Saito, M. Shimizu, 0. Tanaka and M. Tanaka, Bull. Chem. Sot. Jpn. 60, (1987) 1595. [3] N. Coustel, P. Bernier and J.E. Fischer, Phys. Rev.B, 43 (1991) 3147. [4] A.A. Benamara, M. Galtier and A. Montaner, Synth. Metals, 41-43 (1991) 45. [5] L.W. Shacklette and J.E. Toth, Phys. Rev.B, 32 (1985) 5892. [6] R.H. Baughman, N.S. Murthy and G.G. Miller, J. Chem. Phys., 79 (1983) 515. [7] J.E. Fischer, P.A. H einey and J. Ma, Synth. Metals, 41-43 (1991)
33.
[8] F. Saldi, M. Lelaurain and D. Billaud, Sol. State Con-m. 76 (1990) 595. [9] N.S. Murthy, L.W. Shacklette and R.H. Baughman, Phys. Rev.B, 41 (1990) 3708. [lo] F. Saldi, M.Lelaurain and D. Billaud, Synth. Metals, 41-43, (1991) 63. [ll] J. Tanaka and C. Tanaka, Synth. (1995) 647.
Metals, 69
[la] T. Miyamae, K. Kamiya, S. Hasegawa, K. Seki, C. Tanaka and J. Tanaka, Bull. Chem. Sot. Jpn., 68 (1995) 1897. [13] M.J. Frisch et al. G aussian Inc., Pittsburgh, PA, (1995) [14] E.M. Conwell and H.A. Mizes, Synth. Metals, 65 (1995)
203.
Synthetic
Electronic
Department
structure
Metals
84 (1997)
and phase transformation
719-720
of alkali
Jiro Tanaka and Chizuko Faculty of Science, Kanagawa
of Chemistry,
metal
Tanaka University,
doped
Hdratsuka,
polyacetylene
259-12
Japan
Abstract The experimental studies on alkali metal doped polyacetylene have been reviewed. Molecular orbital calculations on model compounds of doped polyene are performed to show electronic and molecular structure doped species. Keywords ab initio quantum chemical methods and calculation, electrochemical doping, polyacetylene, photoelectron spectroscopy, metal-insulator phase transition, magnetic phase transition
of
1. Introduction
2.2 Electrical
The mechanism of insulator to metal transition in doped polyacetylene is a fundamental problem of conducting polymer research. We have been studying the transition by far infrared, Raman and ultraviolet pho toemission spectroscopy on doped polyacetylene and theoretical calculation on model compounds. The Xray structural, electrochemical and elecctron spin resonance were studied extensively on alkali metal doped polyacetylene, accordingly we have continued the electronic structural calculations on the model compounds of alkali metal doped polyene. In this report we will summarize earlier experimental researches and discuss on the calcualted results of molecular orbitals of the model compounds of the doped species.
Chung et al. [l] al so showed that the electrical conductivity increased rapidly at light doping level before the magnetic transition took place. They claimed that spinless charge transport is important in the light doping regime. Coustel et al. [3] confirmed the results of Heeger’s group by showing that the conductivity increased stepwise at y = 0.06 and y = 0.12. The temperature dependence of conductivity was studied by Galtier’s group and it showed rather little temperature dependence.[4]
2.
Summary
2.1 Electron
of experimental spin resonance
results and magnetic
transition
Chung et al. [l] first discussed magentic first order transiton on Na doped polyacetylene occurring at y > 0.06 (y is mole fraction of Na). However, their estimate of Na content seems too low from a view of our electronic and Raman spectral and chemical analysis data.[2] Comparing their spectra with ours, it appears that the magnetic transition to Pauli paramagetic state occurs at y > 0.12. 0379-6779/97/%17.00
PII s0379-6779(96)04117-3
63 1997
Elscvier
Science
S.k
All rights
reserved
conductivity
2.3 Electrochemical
potentials
Shacklette et al. [5] studied phase transformation in electrochemical doping of K and Na and found several phases appearing at y = 0.06 (stage 2), 0.12 (stage 1). Coustel et al. [3] carried out high resolution study of the cell potential and found additional transition at y = 0.15. Different cell potentials correspond to varied stage of dopnig. Accordingly we think that dissimilar doped chain unit correspond to these different potentials. 2.4 X-ray
structural
Structure models lene were investigated et al.[7] and Billaud
analysis of alkali metal by Baughman’s et al.[S]
doped poly- acetygroup [6], Fischer
720
J. Tanaka,
C. Tanaka
/Synthetic
In the light doping 0.06 < y < 0.12, the stage 2 structure was found by Murphy et al.[9], in which polyacetylene chain neighbors one metal column and four polyacetylene chains exist per alkali-metal column. For heavy doping y > 0.12, the tetragonal structure is formed [6], where each polyacetylene chain contacts with two metal columns and each metal column is surrounded by four polyacetylene chains. Saldi et al. [lo] found the metal-metal distance along the column 4.96 A for 0.12 > y > 0.06 and 3.98 A for y > 0.12 in K, Rb and Cs doped polyacetylene. 2.5 Raman spectra Raman spectra showed that the transition from the stage 2 to the stage 1 doped state occurs at y N 0.12.[2] In the highest doped state at 0.15 > y > 0.12, a characteristic Raman band was found at 1580 cm-l and it was assigned to the C-C stretching mode of the polson structure for CrsHr,r,Nas. [II]
Metals
84 (1997)
719-720
HOMO show different shape indicating that the charge carrier exists on the half of each unit with different spin. In (Na+)4(CssHss)4- two polson units are combined with an inversion symmetry and in (K+)4(CrzHr4)2polaron and antipolaron units are joined. These units may represent the carriers in the y N 0.14-0.15 and y N 0.166, respectively. In which has the constitution y = (K+)&~As)~-, 0.125, the charge and spin separation is not so obvious, indicating that the transition is just beginning. The model compound of poison, (Na+)4(C2sH2s)4-, has composition closest to the paramagnetic state, accordingly it may be the dominant structural unit at 0.16 > y > 0.12. However, coexistence of other species such as the polaron, (K+)4(CrzHr4)2-, or the charged soliton, (K+)z(C14H16)~-, are not completely excluded at y > 0.12. Acknowledgement We are grateful to the Computation Center of the Institute for Molecular Science.
2.6 Ultraviolet photoemission spectra (UPS) UPS of Na and K doped polyacetylene were studied by Miyamae et al.[12] The spectra changed stepwise. In the light doping, the population in the mid gap state was observed, and a density of states near EF was found when doping proceeds to y > 0.10. The spectra showed a typical pattern of strongly correlated electron system. This result is consistent with the Pauli paramagnetism in the heavily Na doped film.[l] 3. Molecular pounds
orbital
calculation
on model com-
To find the structure of the doped species, we used Gaussian 94 programs [13] at Computation Center of Institute for Molecular Science and our university. Here we will discuss on results on (K+)s(Cr4Hrs)2-, (K+)4(CrsHrs)2-, (Na+)h(CssHss)4- and (K+)4(CrsHr4)2-, where K or Na ions are placed at positions found in the crystalline state. We used the UHF method to find the carrier in the HOMOs of these complexes in the singlet state. In w h’rch reP resent the charged soliton (K+)2(C14H16)~-, structure dominating in the stage 2 of light doping, the shape of cy spin orbital and p spin orbital are exactly the same. It may mean that the charge transport in the lightly doped film is due to the spinlesscharged soliton wave.[l4] On th e contrary, in (Na+)4(CssHss)‘- and (K+)4(CmH14)2-, th e (Y spin HOMO and the p spin
References [l] T.-C. Chung, F. M oraes, J.D. Flood and A.J. Heeger, Phys. Rev.B, 29 (1984) 2341. [2] J. Tanaka, Y. Saito, M. Shimizu, 0. Tanaka and M. Tanaka, Bull. Chem. Sot. Jpn. 60, (1987) 1595. [3] N. Coustel, P. Bernier and J.E. Fischer, Phys. Rev.B, 43 (1991) 3147. [4] A.A. Benamara, M. Galtier and A. Montaner, Synth. Metals, 41-43 (1991) 45. [5] L.W. Shacklette and J.E. Toth, Phys. Rev.B, 32 (1985) 5892. [6] R.H. Baughman, N.S. Murthy and G.G. Miller, J. Chem. Phys., 79 (1983) 515. [7] J.E. Fischer, P.A. H einey and J. Ma, Synth. Metals, 41-43 (1991)
33.
[8] F. Saldi, M. Lelaurain and D. Billaud, Sol. State Con-m. 76 (1990) 595. [9] N.S. Murthy, L.W. Shacklette and R.H. Baughman, Phys. Rev.B, 41 (1990) 3708. [lo] F. Saldi, M.Lelaurain and D. Billaud, Synth. Metals, 41-43, (1991) 63. [ll] J. Tanaka and C. Tanaka, Synth. (1995) 647.
Metals, 69
[la] T. Miyamae, K. Kamiya, S. Hasegawa, K. Seki, C. Tanaka and J. Tanaka, Bull. Chem. Sot. Jpn., 68 (1995) 1897. [13] M.J. Frisch et al. G aussian Inc., Pittsburgh, PA, (1995) [14] E.M. Conwell and H.A. Mizes, Synth. Metals, 65 (1995)
203.