TEMPERATURE DEPENDENCE OF NMR T1 AND χ IN Br3, AsF6-DOPED (CH)x

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TEMPERATURE DEPENDENCE OF NMR T1 AND χ IN Br3, AsF6-DOPED (CH)x

K. Kume, K. Mizuno, K. Mizoguchi, K. Nomura, H. Takayama, S. Ishihara, J.

Tanaka, M. Tanaka, H. Fujimoto, H. Shirakawa

To cite this version:

K. Kume, K. Mizuno, K. Mizoguchi, K. Nomura, H. Takayama, et al.. TEMPERATURE DEPEN-

DENCE OF NMR T1 AND χ IN Br3, AsF6-DOPED (CH)x. Journal de Physique Colloques, 1983,

44 (C3), pp.C3-353-C3-356. �10.1051/jphyscol:1983371�. �jpa-00223032�

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JOURNAL DE PHYSIQUE

Colloque C3, supplément au n°6, Tome 44, juin 1983 page C3-353

TEMPERATURE DEPENDENCE OF NMR T1 AND X I N B r3, AsF6"DOPED ( C H ) x

K. Kume*, K. Mizuno*, K. Mizoguchi* , K. Nomura*, H. Takayama*, S. Ishihara*, J . Tanaka**, M. Tanaka** , H. Fujimoto**and H. Shirakawa***

* Department Phys. Tokyo Metropolitan Univ., 158 Japan Department Chem. Nagoya Univ., 464 Japan

***Inst. Materials Soi., Univ. of Tsukuba, 305 Japan

Résumé - Nous avons mesuré la dépendance en température du temps de relaxation Tj de RMN des protons et de la susceptibilité paramagnétique de Pauli X_

dans le polyacétylène (CH)x dopé avec Br^ et AsFg. Une évaluation de la conductibilité microscopique a^MR est obtenue à partir de ces résultats et comparée à la conductibilité à courant continu Opr-

Abstract - We have measured the temperature dependence of proton NMR Ti and Pauli paramagnetic susceptibility Xp in Br3- and AsF6-doped (CH)x. The microscopic conductivity O"NMR n a s been calculated from them, and compared with 0]x>

I t i s well known that polyacetylene (CH)x becomes metallic when doped with acceptor or donor molecules. In this work we have measured the temperature dependence of both proton NMR relaxation time Tj and paramagnetic susceptibility of AsF&- and Br3- doped trans-(CH)x. From the T-proportional metallic part of relaxation rates Tj-1 and the T-independent Pauli part Xp of magnetic susceptibility, we have calculated the diffusion constant DM and the microscopic conductivity cr^MR of metallic electrons along (CH)x chains.

The formula used to obtain D^M and Oj^^ were the same as used by Nechtschein et al in the room temperature proton NMR work on AsF6-doped (CH)x / l / . In the metallic s t a t e the proton NMR relaxation rate T^"^ i s connected with the Pauli susceptibility Xp through following equation.

V 1 = kBTXp [ - | - d2f ( a )n)+( a2 + ^ - d2) f ( c oe) ] ,

f(aO = (2D„ a))~1/2 . (1)

As the isotropic part of hyperfine field | a / ye| is known as 23.4 Gauss / 2 / , and the r a t i o of dipolar part d to isotropic one has been determined as d2/a2^0.25 / 1 , 3 / by the Overhauser experiment, we can obtain the diffusion constant D„ from eq. CI) • Using this D„, the mean free path Jl and the microscopic conductivity CNMR a r e

derived through following two equations / 4 , 5 / , I = 2irjrf XpD„c„ ,

0NMR = 2*1%l O ^ . W

where cN and 2 are the l a t t i c e constant and the cross-sectional area of a CCH)x chain, respectively. In following sections the obtained aj^R will be compared with

^DC which is measured with the usual four-terminal method on the same sample.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983371

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JOURNAL DE PHYSIQUE

I - AsF6-DOPED (CH) x

Figure 1 shows t h e experimental r e s u l t on t e m p e r a t u r e dependence of p r o t o n NMR r e - l a x a t i o n r a t e ~ ~i n AsFg-doped trans-(CH)x. - 1 The r e s u l t i s i n t e r p r e t e d as a super- p o s i t i o n of two c o n t r i b u t i o n s . The f i r s t i s a T - p r o p o r t i o n a l one due t o m e t a l l i c o r i g i n . This c o n t r i b u t i o n becomes dominant a t temperatures above 200 K . The second i s a hump of Bloembergen type below 200 K . That i s considerd t o be due t o t h e mod- u l a t i o n of p r o t o n magnetic d i p o l a r i n t e r a c t i o n , which i s caused by t h e molecular motion o f (CH)x c h a i n s . The peak p o s i t i o n was found t o s h i f t towards t h e low temperature s i d e , a s lowering t h e NMR Larmor frequency down t o 5.0 MHz.

The p r o t o n m a g n e t i z a t i o n decay curve from which t h e above T i o f m e t a l l i c s t a t e was determined, was s i n g l e - e x p o n e n t i a l e x c e p t a s m a l l i n i t i a l drop. I n samples of semi- conducting s t a t e o f dopant c o n c e n t r a t i o n s below AsF6/CQ0.03, t h e decay curve was n o n - s i n g l e - e x p o n e n t i a l . This means t h a t t h e low c o n c e n t r a t i o n samples a r e inhomoge- neous

.

The degree of d e v i a t i o n from s i n g l e - e x p o n e n t i a l was h e a v i e r i n c i s - s t a r t e d sample t h a n i n t r a n s - s t a r t e d one. Because o f t h o s e non-single-exponential decay curves t h e T i r e s u l t f o r low c o n c e n t r a t i o n samples i s n o t shown i n F i g . 1.

The s p i n paramagnetic s u s c e p t i b i l i t y was measured with t h e Schumacher-Slichter ESR method / 6 / on t h e same sample a s NMR. The T-independent P a u l i p a r t

xp

was o b t a i n e d from a temperature dependence experiment. The r e s u l t i s shown i n F i g . 2 as a func- t i o n o f dopant AsF6 c o n c e n t r a t i o n , which was determined by a 1 9 ~ NMR experiment.

The measured

xp

was found t o change i n o r d e r o f (TIT)-1, a s n a t u r a l l y expected. The

xp

o b t a i n e d i n t h i s experiment was s i m i l a r t o t h o s e r e p o r t e d e a r l i e r / 7 , 8 / .

The microscopic c o n d u c t i v i t y UNMR was c a l c u l a t e d through e q s . ( I ) , (2) and (3) and l i s t e d i n Table I . I n t h i s Table i t i s a l s o shown ODC measured with t h e u s u a l f o u r - t e r m i n a l method. The r e a s o n why UNMR i s s u b s t a n t i a l l y l a r g e r than ODC, w i l l be due t o t h e f a c t t h a t t h e l a t t e r i s l i m i t e d by t h e i n t e r - f i b r i l hopping r a t e .

Figure 3 shows t h e temperature dependence o f p r o t o n NMR r e l a x a t i o n r a t e T ~ - ' i n Br3- doped trans-(CH)x. The f e a t u r e of t h i s r e s u l t i s t h a t t h e hump of Bloembergen type i s a b s e n t . That i s i n c o n t r a s t t o o t h e r cases such as AsFg-doped, 13-doped /9/ o r p r i s t i n e / 9 / trans-(CH)x. Because t h e hump i s a b s e n t i n our Br3-doped c a s e , t h e m e t a l l i c TlT=const-behaviour i s seen over t h e e n t i r e temperature range between 4.2 K

Fig. 1 - Proton NMR r e l a x a t i o n r a t e T ~ i n - ~ Fig. 2 - P a u l i paramagnetic s u s - AsFg-doped t r a n s - (CHI x vs temperature T . c e p t i b i l i t y

xp

(Shumacher-Slichter AsF6/C : o 4.0 %, 5 . 2 %, o 7 . 7 % . ) vs AsF6 c o n c e n t r a t i o n .

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Fig. 3

-

P r o t o n NMR r e l a x a t i o n r a t e T ~ - ' Fig. 4 - P a u l i paramagnetic s u s c e p t i b i l i - i n Br3-doped trans-(CH)x vs temperature t y

x

( s t a t i c Faraday balance) vs B r 3 T. Br3/C : o 7.3 % . c o n c e n t r a t i o n . P

and 300 K . The f i n i t e T ~ - ' a t T=O w i l l be due t o f i x e d paramagnetic i m p u r i t i e s . A s i m i l a r experimental r e s u l t on a d i f f e r e n t Brj-doped sample was a l r e a d y r e p o r t e d i n o u r p r e v i o u s paper / l o / .

To o b t a i n t h e P a u l i paramagnetic s u s c e p t i b i l i t y

xp,

we f i r s t a p p l i e d t h e Schumacher- S l i c h t e r ESR method. However, t h e o b t a i n e d v a l u e was a s s m a l l as ? . 1 ~ 1 0 - ~ emu/mole- carbon. The o ~ ~ ~ which was c a l c u l a t e d from t h i s v a l u e was much s m a l l e r t h a n a D c . Next, we a p p l i e d t h e method o f s t a t i c Faraday b a l a n c e . Five samples were prepared, o f which c o n c e n t r a t i o n s were determined by weight uptake. To prevent t h e outgoing of dopant B r , t h e samples were packed i n t h i n p o l y e t h l e n e b a g s . By s e p a r a t i n g t h e inverse-T Curie p a r t from experimental d a t a of temperature dependence, t h e T-independent p a r t was determined. When t h e T-independent p a r t was p l o t t e d a g a i n s t t h e c o n c e n t r a t i o n , we found a r a t h e r abrupt i n c r e a s e i n i t between t h e two samples of Br3/C=0.022 and 0.030, b e s i d e s t h e s t e a d y decrease p r o p o r t i o n a l t o t h e dopant con- c e n t r a t i o n , which was a s c r i b e d t o diamagnetism o f dopants. Between t h e two samples we n o t i c e d a change of c o l o u r i n d u l l s u r f a c e of (CH)x f i l m . We i d e n t i f i e d t h e change a s a t r a n s i t i o n t o m e t a l l i c s t a t e . The i n c r e a s e i n t h e s u s c e p t i b i l i t y when t h e t r a n s i t i o n i s crossed w i l l n a t u r a l l y be a s c r i b e d t o t h e appearance of P a u l i paramagnetic s u s c e p t i b i l i t y . I n Fig. 4 t h u s o b t a i n e d

x

was p l o t t e d . For t h e sample of Br3/C=0.073, on which NMR T1 was measured, t h e P

xp

was 4 x 1 0 - ~ emu/mole- carbon.

The c a l c u l a t e d oNMR u s i n g t h i s

xp

i s shown i n Table I . I t i s remarkable t h a t ONMR

i s s i m i l a r i n b o t h Br3- and AsFg-doped c a s e s . The 'NF~R i s s u b s t a n t i a l l y l a r g e r than

ODC a l s o i n t h i s Br3-doped c a s e . The cause w i l l be a t t r i b u t e d t o i n t e r - f i b r i l hopping r a t e .

TABLE I

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C3-356 JOURNAL DE PHYSIQUE

For t h e c a s e of 13-doped (CH)x, E p s t e i n e t a1 has o b t a i n e d ~ ~ % 3 x 1 0 - ~ emu/mole-carbon w i t h t h e method o f Faraday b a l a n c e /XI/. With t h e ESR method no d e t e c t a b l e

xp

has been observed /12/. This d i f f e r e n c e i s s i m i l a r t o o u r Brg-doped case. The dopant c o n c e n t r a t i o n where t h e t r a n s i t i o n t o m e t a l l i c s t a t e occurs i s a l s o s i m i l a r i n t h e Br3- and 13-doped /11/ c a s e s . I n t h e 13-doped case O N M ~ can a l s o be calcu- l a t e d with t h e p r o t o n NMR TI r e p o r t e d b Masin e t a1 /9/.

r

The r e s u l t i s ONMR"J 2 x 1 0 ~ 0-1,-1. This i s l a r g e r t h a n 0 ~ ~ " J 3 ~ 1 0 /13/.

I11 - CONCLUDING REMARKS

I n t h i s p a p e r we were mainly concerned w i t h ONMR i n t h e m e t a l l i c s t a t e which was c a l c u l a t e d from t h e p r o t o n NMR TI and t h e P a u l i paramagnetic s u s c e p t i b i l i t y

xp.

The o b t a i n e d ONMR was l a r g e r t h a n ODC measured w i t h t h e u s u a l f o u r - t e r m i n a l method.

The d i f f e r e n c e was a t t r i b u t e d t o t h e f a c t t h a t t h e ODC i s l i m i t e d by t h e i n t e r - f i b r i l hopping r a t e .

However, t h e p r e s e n t experiment has a f f o r d e d o t h e r new remarkable f a c t s t o be ex- p l a i n e d i n t h e f u t u r e . One o f them i s t h a t i n t h e Br3-doped case t h e

x

^measured

with t h e resonance method o f ESR was s m a l l e r t h a n t h a t with t h e s t a t i c g r a d a y method by more t h a n two o r d e r s o f magnitude. This d i f f e r e n c e w i l l be e x p l a i n e d by t h e presence of some broad component i n ESR s p e c t r a . S i m i l a r anomaly h a s a l s o been r e p o r t e d i n t h e 13-doped c a s e . The o t h e r i s t h a t t h e hump o f Bloembergen type i n t h e temperature dependence of p r o t o n NMR TI i s not observed i n the Brj-doped case.

This w i l l b e a t t r i b u t e d t o t h e d i f f e r e n c e i n doped s t r u c t u r e . To s o l v e t h e above q u e s t i o n s f u r t h e r s t u d i e s w i l l be r e q u i r e d .

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