Preparation and characterization of CPP2I3-δ single crystals

Synthetic Metals, 55-57 (1993) 1735-1740

17 3 5

PREPARATION AND CHARACTERIZATION OF CPP213-5 SINGLE CRYSTALS

J. MORGADO, L. ALCACER Dep. de Eng. Quimica, Instituto Superior Trcnico, P-1096 Lisboa Codex, Portugal R. T. HENRIQUES, E. B. LOPES, M. ALMEIDA Dep. de Quimica, ICEN-LNETI, P-2686 Sacavrm Codex, Portugal M. FOURMIGUI~ Laboratoire de Physique des Solides, Universit6 Paris-Sud, 91405 Orsay Crdex, France

ABSTRACT CPP213-b single crystals ( where CPP = 1, 2, 7, 8 - tetrahydrodicyclopenta-(cd-lm)-perylene ) were prepared by diffusion and by electrocrystallisation techniques. By diffusion, only thin needles were obtained, while the galvanostatic process gives, in addition, diamond shaped crystals. X-ray diffraction studies on a diamond shaped crystal revealed a stoichiometry CPP2(I3)0.89, while the CHN microanalysis for thin needles gives 5 values from -0.1 to 0.4, in different batches. Thin needles are characterized by higher electrical conductivity, ORT = 200 ~)-lcm-1, and thermopower, SR1- = 30/aV/K, while the diamond shaped crystals have ORT - 0.5 xQ-lcm -1 and SRT = - 8 ~V/K. In all cases both electrical resistivity and thennopower show metallic behaviour at high temperatures down to - 60 K, where a metal-insulator transition occurs. EPR shows an almost isotropic line ( g=2.0044 ) with width of ~ 6 G, at room temperature, without significant differences for the two different types of crystals. The thin needles magnetic susceptibility shows an almost temperature independent contribution above the metal to insulator transition, ascribed to the conduction electrons in the CPP chains. INTRODUCTION Compounds with perylene and metal-bisdithiolene have been intensively investigated in our laboratories during the past [1,2]. Aiming at a better understanding of the relationship between chemical structure and electrical properties of these conductors, we decided to extend these studies to perylene derivatives, including CPP ( CPP = 1, 2, 7, 8 - tetrahydrodicyclopenta-(cd-lm)-perylene ), for which reports on conducting compounds appeared in literature [3]. In this communication we present preliminary results for iodine based CPP compounds. EXPERIMENTAL Sample preparation Single crystals of CPP213-b were prepared either by diffusion controlled reaction of CPP and iodine or by electrochemical 0379-6779/93/$6.00

oxidation

of CPP

in dichloromethane

solutions

with

© 1993- Elsevier Sequoia. All rights reserved

1736

tetrabutylammonium triiodide as electrolyte, on platinum electrodes ( current densities in the range 2 - 10 ~tA/cm2). CPP [4] and (C4H9)aNI3 [5] were synthesized according to known procedures. The diffusion technique gives thin needles ( ~- 3 x 0.02 x 0.02 mm 3 ) while the galvanostatic procedure gives two distinct shapes of crystals: needles as those obtained by diffusion and diamond shaped crystals ( = 1.5 x 0.2 x 0.08 mm 3 ), often with irregular and not so perfect faces. X-ray diffraction studies performed on a diamond shaped crystal indicate a monoclinic crystal structure, space group P21/n, with cell parameters a=4.3757(9)/~, b = 19.368(1)/~, c = 10.086(2) and 15=98.050(8) °. The structure consists of regular chains of CPP molecules stacked along a, and iodine, most probably as I3", is disordered in chains also parallel to a. The c o r r e s p o n d i n g stoichiometry is CPP2(I3)0.89. Full structure details will be published elsewhere [8]. The iodine content of the needle crystals, that were too thin for diffraction experiments, has been inferred from bulk CHN microanalysis. Different values were obtained, even for

the same

preparation batch. Usually, they revealed an iodine deficiency relatively to the CPP213 stoichiometry. Considering the formula CPP213-b, the obtained 5 values vary from - 0.1 to 0.4. Electrical measurements Electrical conductivity was measured in the temperature range 18 - 300 K, using four in-line contacts made with platinum paint. A low frequency current of I~A was used, and the voltage was measured with a lock in amplifier. In figure 1 we show representative results of electrical conductivity, obtained for several crystals of different preparation batches.

10

10

4 I

\

2

\

2

1

2 t',.

kk

10

-2 4 i

0

100

i

200 Temperature ( K )

300

Figure 1- Temperature dependence of the resistivity for two diamond shaped crystals (1, 2) and two needle crystals ( 3, 4 ).

1737

Thin needles have a higher conductivity, ORT = 200 ~ - l c m - l , while diamond shaped crystals have ORT = 0.5 ~ l c m - 1 . A metal-like behaviour is displayed by all samples upon cooling from room temperature, with a broad resistivity minimum ( around 130 to 140 K for the thin needles and 170 K for the diamond like crystals ) which anticipates a rapid resistivity increase at low temperatures. An anomaly of the derivative d logp / d(1/[) is observed at temperature values = 60 K, with slight changes from sample to sample, which is taken as indication of a metal-insulator transition. T h e r m o p o w e r was measured in the same temperature range by a slow a.c. technique in an apparatus similar to the one described by P. M. Chaikin et al. [ 6 ], using gradients - I K. Absolute thermopower was obtained after correction of the results for the thermopower o f the gold leads, using the data of R. P. Huebner [ 7 ]. In order to avoid possible confusions due to sample to sample variations, the electrical resistivity was measured in the same sample used for thermopower, by placing two extra contacts. In figure 2 we show the absolute thermoelectric power for the samples whose conductivity results were shown in figure 1. Again a different behaviour is noticeable for the thin needles and the larger diamond shaped crystals. The needles have a positive value at room temperature = 30 pV/K, while the diamond like crystals show negative values = -8 ~JV/K. In all cases the temperature dependence in the high temperature range is metallic-like, in full agreement with conductivity results.

40 3

30 20 >

---.--""--

10

4

.. -f~

.'." .¢-

,~-

~. - 1 0 .

E ~- - 2 0 ~-

-...~--'"'"

.-'-'---

~" .......

" ......

----~---.

.......

--

,

--..

___

--,-L_,.

l ______

_________

./.'~ .'.-

.....

2

-30 -,

-40

.f .._..

..,-.y

-50 0

I

i

i

i

i

50

100

150

200

250

300

Temperature ( K ) Figure 2 - Absolute thermopower as a function of temperature of the same samples whose resistivity is shown in figure 1. Thermopower measurements reveal slight, but significant, sample dependent values, which are probably also associated with variations of the resistivity, hidden by the uncertainty in the absolute values. These sample dependent values suggest to us the existence of crystals with different iodine contents and/or different degrees of disorder. In all cases, evidence for possible loss of iodine with

1738

aging was never found and the thermopower was reproducible after several months or after cycling under vacuum, to slightly above room temperature. Magnetic measurements The magnetic susceptibility o f the thin needles was obtained in the temperature range 4 - 300 K, by the Faraday method, in an Oxford Instruments system; with a field of 5 T, applying forward and reverse gradient fields o f 5 T/m. The paramagnetic susceptibility obtained after a correction for diamagnetism, estimated as 4 . 0 5 x i 0 -4 emu/mol from tabulated Pascal constants and based on the exact stoichiometry CPP213, is shown in Figure 3.

1.5 4" •" ,

4".I.

4"

~

N

+

++ +++

1.0

( 1/Xp ) x 10 4

=

m, O

4"

4"

+

0.5

+

4-

+

1

/

~

0.0

+

4+"4"

4-

,

0

,

100

200 T(K)

300

44-

4"~4-4"-H-4"4"4"4"::,,,,!::+4-4"

0 0

4"

4"

4" 4"

4"

4"

4"

4"

4"

4"

i

i

100

200 Temperature

4"

4"

4"

.

4"

4"

300 ( K )

Fig.3 - Static susceptibility of a polycrystalline sample o f CPP213-b thin needles. The inset is a representation of the inverse susceptibility v s . temperature. At room temperature, Xp = 1.2 x l 0 -4 emu/mol, slightly decreasing upon cooling until ~ 130 K and then starts increasing in a way that is dominated at low temperature by a large Curie tail corresponding to ~- 2.4% spin 1/2 impurities. A small local maximum is noticeable at = 79 K ( arrow in the figure ) that can be related to the temperature o f the metal-insulator transition. This is best shown in the inset plot o f the reciprocal susceptibility against temperature, where a clear "knee" is seen at this temperature. A small anomaly at = 45 K is attributed to oxygen contamination. EPR studies on thin needles from different preparation batches revealed an almost isotropic signal, when the field is applied perpendicular to the needle axis, and rotation is made around this axis. The temperature dependence o f the linewidth for needles and diamond shaped crystals is shown in figure 4. The g value at room temperature is ~' 2.0044, without significant temperature dependence. The integrated signal, estimated simply by the product of half the second derivative intensity times the

1739

0 0

%

X

+

~X

°I

3 .I

+

4

+

2,~x

.*

i

0

100

n

200

300

T e m p e r a t u r e (K) Figure 4 - Temperature dependence of the linewidth for three needles ( C l, C2 and C3, being C1 and C3 from the same preparation batch ) and two diamond shaped crystals ( L I and L2).

square of the linewidth, shows an almost steady decrease from room temperature down to -- 20K without significant changes near TMI.

CONCLUSIONS Our results show the ability of CPP to form conducting compounds with iodine. Thermopower and resistivity provide strong evidence for the existence of at least two phases in these compounds. In all cases, transport properties are dominated by small sample dependence variation, ascribed to variation in iodine content and associated disorder, as found in other iodine based conducting systems, such as TTT21319]. The clearly metallic behaviour of the diamond shaped crystals shown in both thermopower and resistivity data, suggest that the measured electric conductivity is a large underestimation of the intrinsic values, probably due to the large defects existing in the samples used for these measurements. Structural data on the "thin needles" is not yet available. However, data presently available on the diamond shaped crystals indicate only one type of regular chains. For the stoichiometry indicated, assuming iodine as I3-, a band close to 3/4 filled would be expected, for which a positive metallic thermopower is predictable at variance with experimental results. In this respect the thin needles, which exhibit higher metallic conductivity, are closer to these predictions of thermopower, and also in this case the room temperature paramagnetic susceptibility is of the order of that observed in other metallic systems, such as Per2M(mnt)2, for M=Au [ 1], Cu [ 10], and Co[ 11 ].

1740

ACKNOWLEDGEMENTS This work was partially supported by EEC under ESPRIT Basic Research Action 3121 and by Junta Nacional de Investigae~o Cientifica e Tecnol6gica ( contract 798/90/MPF ). REFERENCES 1 M. Almeida, V. Gama, R. T. Henriques, and L.Alcficer, Proceedings ofNATO ARW in R. M. Laine (ed.), Inorganic and Organometallic Polymers With Special Properties, Kluwer Academic Publishers, 1992 p.163. 2 R.T. Henriques, V. Gama, G. Bonfait, I. C. Santos, M. T. Duarte, L. Alcficer, and M. Almeida, these Proceedings. 3 R. Lapouyade, J. P. Morand, D. Chasseau, C. Hauw, and P. Delhaes, J. Phys. (Paris) Coll., 4___44(1983)

C3-1235.

4 N. Tanaka, and T. Kasai, Bull. Chem. Soc. Jpn., 54 (1981) 3026. 5 J.M. Williams, T. J. Emge, H. H. Wang, M. A. Beno, P. T. Copps, L. N. Hall, K.D. Carlson, and G. W. Crabtree, lnorg. Chem., 23 (1984) 2558. 6 P.M. Chaikin and J. F. Kwak, Rev. Sci. Instrum., 46 (1975) 218. 7 R.P. Huebner, Phvs. Rev.,135 (1964) 1281. 8 I.C. Santos, J. Morgado, P. P. Matias, M. T. Duarte, to be published. 9 See for instance S. K. Khanna, S. P. S. Yen, R. B. Somoano, P. M. Chaikin, C. Lowe Ma, R. Williams, S. Samson, Phys. Rev. B, 19 (1979) 655. 10 V. Gama, R. T. Henriques, G. Bonfait, M. Almeida, and L. Alcficer, Mol. Cryst. Liq. Crvst., in press. 11 V. Gama, R. T. Henriques, G. Bonfait, L. C. Pereira, J. C. Waerenborgh, I. C. Santos, M. T. Duarte, J. M. P. Cabral, and M. Almeida, lnorg. Chem., 31 (I 992) 2598.