S!'p thetic llletals. 2 7 (1988) B439- B444 B43g

NEW CONDUCTING PARANITROPHENYLMALONONITRILE COMPLEXES WITH TTF AND TMTSF

K.D. TRUONG, A.D. BANDRAUK, J. ZAUHAR, M. AUBIN

Groupe de Recherche sur les Semiconducteurs et les Difilectriques, D@partements

de ehimie et de physiques, Facult~ des sciences, Universit@ de Sherbrooke,

Sherbrooke, QuEbec (Canada) JIK 2RI

ABSTRACT

The complexes of Paranitrophenylmalononitri]e (PNPMA) with TTF and TMTSF have

been synthesized. The electrical resistivity, thermoelectric power (TEP) and

ESR properties are studied and discussed. Both compounds are semiconductors

with the TTF ordered and the TMTSF disordered.

INTRODUCTION AND SYNTHESIS

The discovery of a superconducting ground state in the TMTSF2X compounds has

generated extensive interest in this class of materials [1,2]. In order to stu-

dy the influence of nonsymmetric anions on the conductivity of the TMTSF and TTF

salts, we have prepared paranitrophenylmalononitrile, an organic planar acceptor

and complexed it with TTF and TMTSF. This

0~. /CN O/N ~G~CN

new acceptor is similar to TCNQ in structu-

re but is clearly nonsymmetric, with the

NO 2 group being more electronegative than

the CN groups. (TMTSF)2(PNPMA) was preps-

red by a standard electrochemical method [3] in dried and doubly distilled me-

thylene chloride. A 5 ~A DC current was applied for 2½ days which resulted in

the deposit of thin shiny metallic gray crystals on the anode of various lengths

between 3 and i0 mm.

(TTF) 2(PNPMA) was obtained by mixing hot solutions containing equimolar quan-

tities of TTF and PNPMA prepared in specially purified acetonitrile. Upon slow

cooling and partial evaporation of the solvent over 24 hours, crystals of (TTF) 2

(PNPMA) of various length up to 3 mm having a yellow to light green metallic lus-

ter were obtained. The melting point is 129-138°C. (TTF)2(PNPMA) decomposed on

recrystallisation.

0379-6779/88/$3.50 © Elsevier Sequoia/Printed in The Netherlands

B440

RESULTS AND DISCUSSION

Transport properties

D.C. resistivities were measured using the conventional 4 probe technique,

the current through the sample was maintained at a low level of 1 x 10 -6 A.

Temperature dependences of the resistivity are shown in Figs i and 2. Both

2.

0 (:L

(:LI.

0

O.

e TMTSF2PN PMA

a I

e"

• ..t"," l I , , , , i

.15 .20 .25 T - i /5 ( K - I / 3 )

Q

I

I n i

30

Fig. i. Normalized resistivity as a function of temperature T -I/3 for sample

TMTSF2PNPMA.

%2. cL

cL

-9°1.

TTF2PNPMA o I

I •

e • I •

i •

i o

i e

®

.15 .16 i ! i

.17 .18 .19 T-I/3 (~V3)

Fig. 2. Normalized resistivity as a function of temperature T -I/3 for sample

TTF2PNPMA.

B441

compounds show semi-conductor behaviour. The resistivity of (TTF)2(PNPMA) is

.58 ~m at 280K. It is higher than in the case of (TMTSF)2(PNPMA) in which the

resistance is about 4 x 10 -4 ~m at room temperature. This difference may be ex-

plained by the disorder of the non centrosymmetric acceptor PNPMA in the TMTSF

salt [4]. This disorder accounts for its higher conductivity than in the TTF

salt where the acceptor PNPMA is ordered [4]. The PNPMA form an alternating

lattice of donnor molecules perpendicular to the TTFs and create a potential

with periodicity 4a, corresponding to 2k F. This potential is in part responsi-

ble for the semiconducting properties by the introduction of a pseudo Peierls

gap. The TTFs are also tetramerized, which also contributes to the nonmetallic

nature of the present system. There is no overlap between the PNPMA molecules

which indicates that the counter ion does not participate in the conductivity of

these systems. Only the donor chains of the TMISF or TTF participate in the

electron transport. The separation along the TTF chains are not all the same,

we can describe the aggregation of the TTF chains as tetrameric with the distan-

ces 3.48, 3.55, 3.48, 3.67 A ° between them (Fig. 3).

The electrical results in Figs 1 and 2 give evidence of transport via variable

range hopping (VRH) at low temperature. The nearest neighbour hopping conducti-

vity is given by [5]:

, = A exp (-~a) exp (-W/kT) (i)

near room temperature where W is the energy difference between nearest neighbour

-1 , states with a distance "a" apart, c~ zs the extent of the small polaron wave

function or the decay length of a localized state.

At lower temperature, the conduction is dominated by non nearest neighbours

and one can have variable range hopping:

~' (-~/na) (-W/n3kT) ~ = E A exp exp (2)

n=l

when T is small, one can demonstrate that:

I/4 ~ = A exp -(T°/T) (3)

4(~a 4W where T O = (~-)(~)

[n two dimensions the 1/4 power dependence becomes 1/3. Our complexes follow

the T -I/3 behavior at low temperature with T o in the order of 150c~,211K for

TMTSF and TTF salts respectively.

The TEP was measured by using a dynamic methcd to avoid stray voltages, as

previously described [6], except that the present system is completely automated.

The TEP of (TTF) 2PNPMA could not be obtained below 170K because of the high re-

B442

Fig. 3.

Y TTF2PNPMA

o / ~ i

548 ' ~1/

Projection along the z axls for the sample TTF2PNPMA.

e"

• e°

e•o • ,o •

O

%.

_IO

o0

Fig. 4.

I0 T M T S F 2 P N P M A

o o o o o e ° ° °

e • e °

o e

_20 ."

, I , I I J I ,

5.2 4.0 4 .8 5.6 6 .4 T I / 3 ( K I / 3 )

Seebeck c o e f f i c i e n t as a f u n c t i o n o f T - 1 / 3 for sample TMTSF2PNPMA.

sistance of the sample. Both salts (TMTSF) 2PNPMA, (TTF) 2PNPMA (Figs 4 and 5)

follow the T I/3 dependence characteristic of VRH [7-8]:

1 k kT)i/3 d in N S = ~ ~ (kT ° " dE (4)

At 295K, they are all p type semiconductors with the value of S positive.

Below room temperature, a change in sign is observed for the TEP indicating

mixed conductivity in this temperature range. Thus the resistivity and TEP

lead to the same conclusion regarding the conductivity mechanism.

Magnetic properties

ESR experiments have been carried out for (TMTSF)2PNPMA and for (TTF)2PNPMA

as a function of temperature. A broad ESR line is obtained at room temperature

B~43

Fig. 5.

0

_100 v -

_20o :=L

v

0" )_300

TTF2PNPMA OQO 000000001e e e o e

e • ee ee

I • e •

ee e

e e I

e e ee e e e

e e e~

1 I I I I I I

5.8 6.0 6 .2

TW3( K I/3)

_400 , I

5.6 6.4. 6.6

-]/3 Seebeck coefficient as a function of T for sample TTF2PNPMA.

for the two complexes. In the TTF salt the half width of ESR line is 8G, it is

narrower than in the case of the TMTSF salt (lOOG). We found also that the g

factors of TTF salts is 2.0069 nearer to the value of a free electron than in

the case of TMTSF salt (2.0209).

Integrated intensities were obtained on fine powders of the complexes between

150K and 360K. We assume in this case an activated form for the ESR intensity

which is characteristic of linear chain spins with an antiferromagnetic alterna-

ting interaction [9-10]:

I : (C/T) exp (-j/KT) (5)

where j is the exchange energy between neighbouring spins. On the other hand

the magnetic susceptibility of a localized singlet-triplet system is given by

[hi:

c [3 + exp (2j/kT)] -I XST = } (6)

This expression also describes an activated behavior but on the localized re-

gion we have used the antiferromagnetic equation (5) to interpret our results.

We therefore obtain for (TMTSF)2PNPMA:

-i j = .027 eV = 218 cm

and for (TTF)2PNPNA:

- 1 j = .09 eV = 726 cm

B444

The observed j values can be correlated with the intermolecular electronic

interactions between the radicals. The lack of hyperfine structures suggests

that the movement of the triplet exciton is so rapid that the triplet dipolar

structure is washed out [12,13] and that the antiferromagnetic expression (5)

is more appropriate.

CONCLUSIONS

(TMTSF)2PNPMA and (TTF)2PNPMA are all p type semiconductors. The resistivity

and TEP results indicate that they are two dimensional systems and the conducti-

vity mechanism can be described by a variable range hopping model at low temperature

The magnetic properties indicate considerable delocalisation of the spins. In

both series of compounds the anions are nonsymmetric. In the TTF complexes, the

anions are ordered and clearly contribute to the semiconducting via a pseudo

Peierls mechanism. In the TMTSF complexes, the anions are considerably disor-

dered, thus resulting in a higher conductivity.

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