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ESR of fluoranthenyl-radical-cation organic conductor crystals
Volume
77, ntimbcr
CHEMICAL
2
ESR OF FLUORANTHENYL-RADICAL-CATION H. EICHELE, M. SCHWOERER Lehrstuhl fiii Experzmerrtaiphysrk II, Umvwsltat
15 January
PHYSICS LlXl-CRS
ORGANIC
Bayreuth,
CONDUCTOR
D-8.580 Boyreuth.
1981
CRYSTALS
West Germany
and Ch. KROHNKE
and G. WEGNER
InsMut fir hfakromoiekLlare Chemre. Hermarlrl-Starrdlrlger-Haus. D-7800 melburg, West Germany Recewed
2 September
1980, in fiial form 14 October
Um I ersitat Fretburg
1980
The fist ESR mvestrgatlons on flUOr~thCnyI-radlC~-CdtlOn-~lt crystals (I-ARK), D II~‘H type of organic conductor, are reported. The anisotroprc spectrum consists of ESR hnes ns narrow .IS 20 mC, located atg = 2, wth J cornpIe\ thermal behawour paramagnetrc at low temperatures, thermnlly actwated at mtermedIatc tcmperntures and temperature-Independent
1. Introduction
2. Experimental
Fluoranthenyl-radlcal-catlon salts (FARK) consist of stacks of fluoranthene molecules with an interplane distance of only 3 3 A [I]. -
FARK-PF6 crystals were grown by electrocrystalhzatlon at -20°C with a current I = 1 5 mA for 6 h. FARK-SbFh crystals were grown at -30°C wth I = 4.5 mA. The freshly grown crystals were sealed m small glass tubes and stored m a refrigerator prior to measurement. Crystals were mounted on the crystal holder m a stream of dry mtrogen gas Durmg measurement the samples were III the hehum at,mosphere of a contmuous-flow cryostat which WBS inserted In a rectangular ESR cavity. To avoid lme broademng due to modulation sidebands a B. modulation frequency of 10 kHz was used
m 0
-
0
0
The stacks are orlented parallel to the crystahographlc a dIrectIon which is the needle axlS of the crystal. Each fluoranthene molecule bears an effective positive charge of 0.5 eo. Channels between the aromatic stacks are filled with anions. At room temperature the conductivity obtamed with polycrystalhne samples 1s of the order 1 SL-’ cm-l 3 ere IS evidence for a metallic conductivity above%: . Because of the chemical composition (fluoranthene); (PFg)- and the unique crystai structure, we are deahng with a onedimensional spin system with one spin per two aromatic molecules. In this letter we report our first results on the systems (fluoranthene)zpF& and (fluoranthene)$(SbF,)-.
0 009-2614/81/0000-0000/S
02.50
0 North-Holland
3. Results In fig. 1 the ESR spectra of a 1.5 X 0.2 X O-2 mm3 sized FARK-PF, crystal are shown for selected orlentations and temperatures. The three spectra m the left column were obtamed with the needle axis (a axis) oriented at IS,30 and 20” with respect to the B, magnetic field direction. For the three spectra m the Pubbshing
Company
Volume
77. number
2
CHEMICAL
PHYSICS
15 January
LETTERS
50 y” t
T/K
200 \
=+_-_~+g4++
iloI
*
z+*_*_ +
-\
Slope AE=OlS_
_+_ + *++z,
=-
32
I PFe
+‘+_ *c+
2olJ!3
“h*
of a FARK-PFe crystal at selected onenThe onentetron IS mdrcated m degrees between the needle axis and the B,-J magnetic field duectron Note the drfferent field scales AU spectra are located nearg = 2. The full wrdth between the points of maxunum slope of the upper-left spectrum IS 23 mC and
dependence
of a very small FARK-PFe change in linewrdth
of the ESR srgnal amphtude
crystal There IS no qnuficant
wrthm this temperature
range
temperatures.
right column the crystal orientations were 105, 120 and 1 IO”. The azimuth angle is not known. At some
other arbitrary onentatrons the spectrum looks much more comphcated with linewrdths as small as 20 mG. On lowering the temperature below 300 K the lures first broaden and then a second, even broader background ESR signal 1s superimposed (see fig. 1). Note the different field scales of the two lower spectra. If the f3, field is rotated in the plane perpendicular to the crystal’s needle axis rt is not possible to reduce the spectrum to a smgle iine. The ESR intensity of the broad background signal, shown in fig. 1, lower line, shows Curie behavrour where-
as the sharp line supenmposed does not. Fig. 2 shows the temperature dependence 05 the ESR signal amplitude of a 20 pg FARK-PF6 crystal. The crystal was oriented to get a smgle line ESR spectrum. In the temperature range 130 < T < 300 K no significant changes in the linewidth were observed. Above 170 K the ESR signal amplitude is temperature independent. At lower temperatures a thermally activated signal 1s observed; the activation energy AE = 0.15 + 0.1 tV. From saturation experiments a nearly perfect homogeneous ESR signal 1s observed at temperatures above 135 K. A much more comphcated dependence of the spectra on the microwave power is observed at 4.8 K. At very low microwave powers an almost symmetric single line is observed. At very high microwave powers 312
50
LO
FIN. 2. Temperature
tahons
016 eV
L+. FARK
El
Fig. 1. ESR spectra
135
IS0
175
I
E20 c#l
z
250
1981
(150 mW) thrs hne drsappears and a different, almost lure appears which IS shrfted to lower magnetic fields by Ml.5 G. At mtermediate microwave powers both Tines coexist; however, the h&-field component shows the nearly triangular shape whrch is also seen in fig. 1 in the lower-nght spectrum. Fig. 3 shows the ESR signal of a FARK-SbF, crystal at an orientatron where the ESR spectrum cons&s of a single line. Here the needle axis is onented at 60° with respect to the B,-, magnetic field duectron. However the line&rape analysis shows that this smgle hne 1s composed of two lorentzian lines (fig. 3). T’h~s analysis yrelds the temperature dependence of the linewidth shown m fig. 4. Below 70 K, the data were taken from the lmeshape analysrs. .4bove 70 K, simply the peak-to-peak drstance was taken with a mmimum value symmetnc,
It
FARKISbFa .55K
-
Frg. 3. ESR spectrum of a FARK-SbFe crystal at 4.5 K. The cry&d IS onented for a smgle-lure spectrum. The line shape is perfectly fitted by two lorentzian lures. The spectrum IS shown together with the fit and the mdmdual components.
Volume
77, number
2
CHEhllCAL
PHYSICS
lf.,2_
II x=“*
lo-
= = =x.x
x
I x.x
FARK I SbFs x o Fit-Dota + ABpp direct xx
aO8\
x
EO6m
x
T/K
-
Trg. 4. Tempernturc dependence of the ESR hncwdth of the FARK-SW6 crystal Below 70 K, data from a hncshape analys~s, above
70 K, ihe peak-to-peak
hnewldth IS 70 mG.
dwance
IS used
The smallest
of 70 mG. The lorentman hneshape is also confirmed by saturation experiments, yieldmg a nearly homogeneous saturation behavlcur. The g value of component 1 m fig. 3 IS temperature mdependent with g ==z 2.00195 whereas the g value of component 3 m fig. 3 IS shghtly temperature dependent: g 3 2.0023 1 at 4.5 K and g = 2.00221 at 60 K. The integrated ESR mtensity of FARK-SbF6 shows a Curie behavlour below -70 K and a temperature-activated behavlour above with an activation energy of -30 meV. There IS some evidence for the ESR mtensity to become temperature mdependent above T = 300 K.
LEI-XRS
15 January 1981
shows some interestmg aspects of thus new famdy of organic conductors. Though no thorough determmation of the spm concentration has been made, it may be concluded by comparison with sohd DPPH that the number of spins at room temperature IS of the order of the number of aromatic molecules. The extremely narrow ESR hnes mdlcate a rapid exchange or rapid motion of the spms wlthm the aromatic system Because the mterplane distance UI the aromatic stacks 1s the same m both the FARK-SbF, and the FARK-PF, crystals, the larger hnewdth m the SbF, system IS probably due to a stronger spm-orblt couplmg. The analysis of the ESR lineshapes of the lowtemperature spectra of FARK--PE6 yields evldencc but no proof that a part of the broad ESR hne is absent when the B,-, field crosses the resonance field of the sharp line superunposed (see fig. 1, lower-left spectrum). There a part of the ESR lme seems to be cut out and the remammg parts pushed together. Thus may be eyplamed by a sudden change in the local magnetic field when the spms causmg the narrow lme are m resonance_ If thus explanation IS indeed true, certamly collective excitations of the spm system do exist. For this conjecture there IS some additional experimental evidence
of a shape anisotropy. Since the ESR mtenslty certam crystal
IS thermally temperature range, It is unclear IS a semiconductor or a metal
activated m a whether the _
References 4. Discussion The selection of expenmental
111 Ch. Krohnke,
results presented above
V Ilnkclmann and G Wegner. submltted for publwation. 13-1 V rnkclmann, pwate communlcabon
Anger
Chem
313
,
77, ntimbcr
CHEMICAL
2
ESR OF FLUORANTHENYL-RADICAL-CATION H. EICHELE, M. SCHWOERER Lehrstuhl fiii Experzmerrtaiphysrk II, Umvwsltat
15 January
PHYSICS LlXl-CRS
ORGANIC
Bayreuth,
CONDUCTOR
D-8.580 Boyreuth.
1981
CRYSTALS
West Germany
and Ch. KROHNKE
and G. WEGNER
InsMut fir hfakromoiekLlare Chemre. Hermarlrl-Starrdlrlger-Haus. D-7800 melburg, West Germany Recewed
2 September
1980, in fiial form 14 October
Um I ersitat Fretburg
1980
The fist ESR mvestrgatlons on flUOr~thCnyI-radlC~-CdtlOn-~lt crystals (I-ARK), D II~‘H type of organic conductor, are reported. The anisotroprc spectrum consists of ESR hnes ns narrow .IS 20 mC, located atg = 2, wth J cornpIe\ thermal behawour paramagnetrc at low temperatures, thermnlly actwated at mtermedIatc tcmperntures and temperature-Independent
1. Introduction
2. Experimental
Fluoranthenyl-radlcal-catlon salts (FARK) consist of stacks of fluoranthene molecules with an interplane distance of only 3 3 A [I]. -
FARK-PF6 crystals were grown by electrocrystalhzatlon at -20°C with a current I = 1 5 mA for 6 h. FARK-SbFh crystals were grown at -30°C wth I = 4.5 mA. The freshly grown crystals were sealed m small glass tubes and stored m a refrigerator prior to measurement. Crystals were mounted on the crystal holder m a stream of dry mtrogen gas Durmg measurement the samples were III the hehum at,mosphere of a contmuous-flow cryostat which WBS inserted In a rectangular ESR cavity. To avoid lme broademng due to modulation sidebands a B. modulation frequency of 10 kHz was used
m 0
-
0
0
The stacks are orlented parallel to the crystahographlc a dIrectIon which is the needle axlS of the crystal. Each fluoranthene molecule bears an effective positive charge of 0.5 eo. Channels between the aromatic stacks are filled with anions. At room temperature the conductivity obtamed with polycrystalhne samples 1s of the order 1 SL-’ cm-l 3 ere IS evidence for a metallic conductivity above%: . Because of the chemical composition (fluoranthene); (PFg)- and the unique crystai structure, we are deahng with a onedimensional spin system with one spin per two aromatic molecules. In this letter we report our first results on the systems (fluoranthene)zpF& and (fluoranthene)$(SbF,)-.
0 009-2614/81/0000-0000/S
02.50
0 North-Holland
3. Results In fig. 1 the ESR spectra of a 1.5 X 0.2 X O-2 mm3 sized FARK-PF, crystal are shown for selected orlentations and temperatures. The three spectra m the left column were obtamed with the needle axis (a axis) oriented at IS,30 and 20” with respect to the B, magnetic field direction. For the three spectra m the Pubbshing
Company
Volume
77. number
2
CHEMICAL
PHYSICS
15 January
LETTERS
50 y” t
T/K
200 \
=+_-_~+g4++
iloI
*
z+*_*_ +
-\
Slope AE=OlS_
_+_ + *++z,
=-
32
I PFe
+‘+_ *c+
2olJ!3
“h*
of a FARK-PFe crystal at selected onenThe onentetron IS mdrcated m degrees between the needle axis and the B,-J magnetic field duectron Note the drfferent field scales AU spectra are located nearg = 2. The full wrdth between the points of maxunum slope of the upper-left spectrum IS 23 mC and
dependence
of a very small FARK-PFe change in linewrdth
of the ESR srgnal amphtude
crystal There IS no qnuficant
wrthm this temperature
range
temperatures.
right column the crystal orientations were 105, 120 and 1 IO”. The azimuth angle is not known. At some
other arbitrary onentatrons the spectrum looks much more comphcated with linewrdths as small as 20 mG. On lowering the temperature below 300 K the lures first broaden and then a second, even broader background ESR signal 1s superimposed (see fig. 1). Note the different field scales of the two lower spectra. If the f3, field is rotated in the plane perpendicular to the crystal’s needle axis rt is not possible to reduce the spectrum to a smgle iine. The ESR intensity of the broad background signal, shown in fig. 1, lower line, shows Curie behavrour where-
as the sharp line supenmposed does not. Fig. 2 shows the temperature dependence 05 the ESR signal amplitude of a 20 pg FARK-PF6 crystal. The crystal was oriented to get a smgle line ESR spectrum. In the temperature range 130 < T < 300 K no significant changes in the linewidth were observed. Above 170 K the ESR signal amplitude is temperature independent. At lower temperatures a thermally activated signal 1s observed; the activation energy AE = 0.15 + 0.1 tV. From saturation experiments a nearly perfect homogeneous ESR signal 1s observed at temperatures above 135 K. A much more comphcated dependence of the spectra on the microwave power is observed at 4.8 K. At very low microwave powers an almost symmetric single line is observed. At very high microwave powers 312
50
LO
FIN. 2. Temperature
tahons
016 eV
L+. FARK
El
Fig. 1. ESR spectra
135
IS0
175
I
E20 c#l
z
250
1981
(150 mW) thrs hne drsappears and a different, almost lure appears which IS shrfted to lower magnetic fields by Ml.5 G. At mtermediate microwave powers both Tines coexist; however, the h&-field component shows the nearly triangular shape whrch is also seen in fig. 1 in the lower-nght spectrum. Fig. 3 shows the ESR signal of a FARK-SbF, crystal at an orientatron where the ESR spectrum cons&s of a single line. Here the needle axis is onented at 60° with respect to the B,-, magnetic field duectron. However the line&rape analysis shows that this smgle hne 1s composed of two lorentzian lines (fig. 3). T’h~s analysis yrelds the temperature dependence of the linewidth shown m fig. 4. Below 70 K, the data were taken from the lmeshape analysrs. .4bove 70 K, simply the peak-to-peak drstance was taken with a mmimum value symmetnc,
It
FARKISbFa .55K
-
Frg. 3. ESR spectrum of a FARK-SbFe crystal at 4.5 K. The cry&d IS onented for a smgle-lure spectrum. The line shape is perfectly fitted by two lorentzian lures. The spectrum IS shown together with the fit and the mdmdual components.
Volume
77, number
2
CHEhllCAL
PHYSICS
lf.,2_
II x=“*
lo-
= = =x.x
x
I x.x
FARK I SbFs x o Fit-Dota + ABpp direct xx
aO8\
x
EO6m
x
T/K
-
Trg. 4. Tempernturc dependence of the ESR hncwdth of the FARK-SW6 crystal Below 70 K, data from a hncshape analys~s, above
70 K, ihe peak-to-peak
hnewldth IS 70 mG.
dwance
IS used
The smallest
of 70 mG. The lorentman hneshape is also confirmed by saturation experiments, yieldmg a nearly homogeneous saturation behavlcur. The g value of component 1 m fig. 3 IS temperature mdependent with g ==z 2.00195 whereas the g value of component 3 m fig. 3 IS shghtly temperature dependent: g 3 2.0023 1 at 4.5 K and g = 2.00221 at 60 K. The integrated ESR mtensity of FARK-SbF6 shows a Curie behavlour below -70 K and a temperature-activated behavlour above with an activation energy of -30 meV. There IS some evidence for the ESR mtensity to become temperature mdependent above T = 300 K.
LEI-XRS
15 January 1981
shows some interestmg aspects of thus new famdy of organic conductors. Though no thorough determmation of the spm concentration has been made, it may be concluded by comparison with sohd DPPH that the number of spins at room temperature IS of the order of the number of aromatic molecules. The extremely narrow ESR hnes mdlcate a rapid exchange or rapid motion of the spms wlthm the aromatic system Because the mterplane distance UI the aromatic stacks 1s the same m both the FARK-SbF, and the FARK-PF, crystals, the larger hnewdth m the SbF, system IS probably due to a stronger spm-orblt couplmg. The analysis of the ESR lineshapes of the lowtemperature spectra of FARK--PE6 yields evldencc but no proof that a part of the broad ESR hne is absent when the B,-, field crosses the resonance field of the sharp line superunposed (see fig. 1, lower-left spectrum). There a part of the ESR lme seems to be cut out and the remammg parts pushed together. Thus may be eyplamed by a sudden change in the local magnetic field when the spms causmg the narrow lme are m resonance_ If thus explanation IS indeed true, certamly collective excitations of the spm system do exist. For this conjecture there IS some additional experimental evidence
of a shape anisotropy. Since the ESR mtenslty certam crystal
IS thermally temperature range, It is unclear IS a semiconductor or a metal
activated m a whether the _
References 4. Discussion The selection of expenmental
111 Ch. Krohnke,
results presented above
V Ilnkclmann and G Wegner. submltted for publwation. 13-1 V rnkclmann, pwate communlcabon
Anger
Chem
313
,