Short Notes K55

phys. stat. sol. 3 3 , K55 (1W9)

Subject classification: 14.3 and 15; 22. 9

lnstitut Atr Physik der Piidagogischen Hochschule Potsdam

The Anisotropy of Electrical Conductivitv and Thermoelectric Power

inSingleCrvstalsof.the Anion Radical Salt r c H AsCHQ]+ TCNG L( 6-5 3-

BY E. WLLER, H. RITSCHEL, and H. HANSEL

The triphenylmethylarsonium 7, -7, 8, 8 - tetracyanoquinodimethan (As-TCNQ)

is an organic semiconductor with strongly anisotropic electrical conductivity and

thermoelectric power.

The significant anisotropy of the electrical conductivity in single crystals of

P-TCNQ has been reported recently (1).

Electric and other properties of As-TCNQ single crystals are very similar

to those of P-TCNQ.

The important similarity of numerous properties of the two substances is not

very surprising because 9f the isomorphism of the P-TCNQ and As-TCNQ molecular

crystals (2), and the far-reaching correlation in the structure of the complex

molecules setting up the crystal lattices (the very large complex molecule of

As-TCNQ differs from that of P-TCNQ only inasmuch as the phosphorus atom is

replaced by an arsenic atom).

Therefore it is intelligible that investigations on the position of the principal

axes of electrical conductivity and on the conductivity tensor in single crystals of

As-TCNQ (using the measuring methods which a re cited at the end of the present

paper (1)) show correlation in the position of the principal axes and negligible dif-

ferences between the electrical conductivities in corresponding directions in As-TCNQ

single crystals (see (1)).

More details about this will be reported in an other paper.

Repeated dissolution and crystallimtion of As-TCNQ out of the solution by

temperature decrease yields r ise of crystal perfection and purification. This be-

comes apparent in habit, size,and mechanical strength of the crystals of a higher

crystallization stage as well a s in the results of the mass spectrometric analyses

1) The electron attachment mass spectrometer diagrams were made in Forschungs- institut Manfred von Ardenne, Dresden, WeiBer Hirsch.

1)

K56 phyeica status eolidi 33

a

P

I (rw) a UtW) 1 Fig. 1. Especially selected orientations of the edge directions of samples having

the shape of a parallelepiped

(iodine already in crystals 01 the Xirat crystallization stage 18 barely detectable),

and in the results of the measurements of electrical conductivity and thermoelectric

power.

Bulk conductivity and thermoelectric power were measured on samples having

the shape of a parallelepiped with variable orientations (see Fig. l a and b). For

the sample preparation naturally grown As-TCNQ single crystals were treated in

a special mechanical manufacturing operation. Comparability of the results of

measurements on crystals of equal crystallization stage in the case of variable

orientation of the samples was especially warranted by cleavage of the crystals

for sample preparation, this cleavage always being into two pieces (cleavage plane

of the As-TCNQ crystals is parallel to the (010)-plane).

Fig. 2 shows the temperature dependence on the electrical conductivity of mono-

crystalline As-TCNQ samples of the zeroth crystallization stage (solid lines) as

well a s of the third crystallization stage (dotted lines), that can be described for

temperatures 0 O C < b < 100 OC and -150 OC < 3 < 0 OC for all three measuring

directions corresponding to Fig. l a by

d = 0' 0 exp(-E/kT) . (1)

A pronounced enhancement of electrical conductivity as a result of repeated

crystallization ip distinguishable.

The measued anisotropy relationship for the electrical conductivity of crystale

of the zeroth crystallization stage at room temperature is

ub+ : uc : da+ = 1800 : 450 : 1 .

Short Notes

~~ ~~

ooc < b< l0O0C -150 OC < a < 0 OC -

0. Crystallization Ea+ = Eb+ = EC Eb+ = E C + Ea+

w-l

3. Crystallization

stage

L 2 4 6 8

\

pK-F-

Ea+ = Eb+ = E C Eb+ = E C # Ea+

(0.30 eV) (0.25 eV) (0.20 ev)

Fig. 2

K57

P

Fig. 2. Temperature dependence on the electrical conductivity of As-TCNQ single

crystals: measuring directions according to Fig. la: solid lines: 0. crys-

tallization stage: dotted lines: 3. crystallization stage

Fig. 3. Temperature dependence on the thermoelectric power of As-TCNQ single

crystals; measuring directions according to Fig. l a

Table 1

Activation energies of As-TCNQ

I stage 1 (0.36 ev) 1 (0.25 eV) (0.22 eV)

K58 physica status solidi 33

Repeated crystallization has only little effect on thie relatiomhip.

Activation energy isotropy was determined at temperatures & > O C (Table 1).

An activation energy anisotropy affected by the crystallization stage, that was

determined in the range of low temperatures 3< 0 C, is striking. A simple ex-

planation for this anisotropy has been impossible up to now.

0

0

As-TCNQ single crystals have a considerable thermoelectric power anisotropy.

Such an anisotropy of organic substances has according to our information

hitherto not been described in the literature (apart from communication of a dia-

gram for the temperature range 20 to 150 C by Zosel and Ritschel (3) and several

data (4)).

0

Thermoelectric power and thermoelectric power anisotropy depend upon the

crystallization stage of Am-TCNQ in a complicated manner.

Details will be communicated in a later paper.

Fig. 3 shows the general behaviour of the thermoelectric power of As-TCNQ

single crystals as a function of temperature for differenct crystallographic direction

Acknowledgements

The authors a re grateful to Dr. D. Henning for substance preparation and to

W. Habel for crystal production.

References

(1) D. ZOSEL, H. RITSCHEL, and H. HANSEL, to be published.

(2) H. PREUSS, private communication.

(3) D. ZOSEL and H. RITSCHEL, Wissenschaft und Fortschritt g, 185 (1968). (4) DB-Patent, Int. C1. HOll; No. 1 211 722: 3.3. 1966.

(Received February 28, 1969)