Electronic and ionic conductions in molten Tl-chalcogen systems

] O U R N A L OF

Journal of Non-Crystalline Solids 156-158 (1993) 752-755 North-Holland

.,,,,aUI~iLrI~|'I]N~ ,lml'~lt, ul/~, ~1']~

Electronic and ionic conductions in molten Tl-chalcogen systems T. Usuki a, K. M a r u y a m a

a

and S. Tamaki b

a Graduate School of Science and Technology, Niigata University, Niigata 950-21, Japan b Department of Physics, Faculty of Science, Niigata University, Niigata 950-21, Japan

Electronic and ionic conductivities of liquid T1-Se alloys have been measured separately as a function of concentration and temperature. The system shows a deep minimum in the electronic conductivity at the stoichiometric composition (T12Se). An anomaly in the dependence of the ionic conductivity on the composition has been observed around the composition T12Se. A very unusual negative temperature dependence of the ionic conductivity has been confirmed in the Tl-rich range beyond the composition T12Se.

1. Introduction Very extensive investigations have been made for liquid solutions containing Te and Se [1-3]. Nakamura and Shimoji [3] measured the electrical conductivity and the thermoelectric power of the T1-Se and TI-Te systems near the stoichiometric composition. They reported a deep minimum in the electrical conductivity at the stoichiometric composition, and found that the thermoelectric power rapidly changed sign. This behaviour is sometimes described as a change from p-type to n-type conduction according to whether the thermoelectric power is positive or negative

[4]. It is an interesting problem how electronic and ionic conduction contributes to the anomalies in the electrical properties as a function of composition. Measurements of the electronic and ionic conductivities in the liquid state may give an answer to the problem. Analogous discussions have already been given for superionic conductors such as the Ag or Cu chalcogenides [5,6]. Endo et al. [7] have also reported a high ionic

Correspondence to: Dr T. Usuki, Graduate School of Science and Technology, Niigata University, Ikarashi, Niigata 950-21, Japan. Tel: + 81-25 262 6136. Telefax: + 81-25 263 3961.

conductivity in liquid Ag chalcogenides near the melting points. The main purpose of this paper is to report measurements of the ionic and electronic conductivity, tri and tre, for liquid TI-Se alloys. The results provide new insights of general importance for liquid semiconductors which show both ionic and electronic conductivity.

2. Experimental procedure Samples made from 99.99% purity T1 and 99.999% Se were sealed in a Pyrex glass tube under vacuum and melted and shaken for 24 h at 550°C. After quenching to room temperature, the sample was sealed in a cell. The cell of Pyrex glass with four tungsten electrodes used for the measurements is shown in fig. l(a). The cell constant was determined by using mercury at room temperature on a dc fourterminal technique. Then the electronic and ionic conductivities were measured using a two-terminal technique. The ionic conductivity, tri, can be obtained by the residual potential method [5,6,8]. We assume that the mobile species in liquid T1-Se alloys is only the T1+ ion. A direct current of constant intensity is sent to the specimen through the tungsten electrodes. We assume that the current intensity is so weak that no electroly-

0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

753

T. Usuki et al. / Electronic and ionic conductions (o)

B

D

method). The reproducibility of the data in heating and cooling runs was good. The experimental error in tri was < 10%. The relative error in o-e for the same sample was < 3%.

3.

sarn~e/

~./~

L~~eo

--~ t

ed

on

off

Fig. 1. (a) A sketch of the measurement cell of the electronic and ionic conductivities.(b) Potential differencevs. time.

sis takes place at the boundaries of the electrodes. Immediately after switching on (see fig. l(b)), all electrons and mobile ions in the specimen contribute to the conduction. After reaching a stationary state, the current is exclusively carried by electrons. When the current is switched off after such a stationary state is attained, a residual potential is left and it disappears as the polarization decays (decay process). For such processes, the electronic and ionic components of current density, Je and Ji, can be given by [6] Je = t r e ( d / d x ) ( (

~ e / e ) - qb),

(1)

and Ji = - ° ' i ( d ~ / d x ) ,

(2)

Results

Figure 2 shows an example of the decay process of the residual potential. The value denoted in the figure is a characteristic decay time which is calculated from the slope of the straight line shown in the figure, and it was found to decrease with temperature for liquid T12Se. This fact indicates that the diffusion constant of the mobile TI ions increases with temperature in liquid TI2Se. tre and o"i for the liquid T I - S e system are shown in figs. 3 and 4, respectively, against the reciprocal absolute temperature 1 / T . T h e values of tre for the alloys with x < 0.667 increase with temperature which is generally as characteristic for liquid semiconductors. Those with x > 0.667 (Tl-rich range), however, are much less dependent on temperature. This fact suggests that, since the valence band overlaps with the conduction band, the Fermi level lies above the bottom of the conduction band and then the concentration of carriers is essentially independent of temperature. That is to say, the electrons are degenerate and only those at the top of the conduction band can contribute to the transport process. The value of o"i for the alloys with x < 0.667 increases with temperature. However, an unusual negative temperature dependence of o-i has been

where ~'e is the electro-chemical potential of electrons and • is the electrostatic potential. For the stationary state, putting Ji = 0, we have j f l = cre Vs,

(3)

where l is the distance between two electrodes and V~ is the potential difference between two electrodes in the stationary state. Next, at the instant of switching off, we have the relation

~ / V s = O'i//(O"i q- O'e) ,

10

(4)

where Vr is the potential difference at the instant of switching off. Thus, ~ri can be obtained from V~/V~ if it is not too small (residual potential

I

I

I

I

[

I

I

I

I

8

~s

~=501 sec

v

E

TI2Se (410"C) I

t

I

I

I

I

1

t

t ( sec ) 965 Fig. 2. An example of the decay process of residual potential. The value of z is a characteristic decay time.

T. Usuki et al. / Electronic and ionic conductions

754

f

I

i

Iv6

1

8 I

/

• 67

_ _

'

'

'

I

'

'

'

!

2

-I

a 50

f¢~______~

'

-I.

O ~.7 • 60

"1

•A~'O 20

. ~l

1

O'e

1

'i

"r~..0~ 01

-2

12

113

17 1000 1 T

-3

'

0

'

I

'

I

50

'

'

I

'

100

Fig. 3. Electronic conductivity,tre, for liquid TI-Se system as a function of temperature.

Se ctt .% TI TI Fig. 5. Concentration variation of cre and (7i at 440°C.

recognized in the Tl-rich range. In this range the situation can be described as follows. Since an enlargement of the thermal vibration of the conduction electrons is remarkable with

increasing temperature, the conduction of mobile T1 ions is abstracted by such vibrations, i.e., the value of the cross-conductivity, o-ie, which represents an ionic conduction caused by the conduction of electrons, increases with temperature. Then a negative temperature dependence of o"i is observed. Such a dependence is more pronounced with increasing T1 content. The concentration variations of tre and o-i at 440°C are shown in fig. 5. o-e shows a deep minimum at the composition of TleSe, with value of about 417 f I - a cm-1. Since the valence electrons in T12Se are localized, the transport should be predominantly due to electrons thermally excited to the conduction b a n d near the mobility edges [9]; a relatively high electronic conductivity may be obtained at this composition. It is of particular interest that o-i increases appreciably at this composition. The value of ~i at 440°C is about 0.32 f1-1 cm -1, only one order of magnitude smaller than that of tre. This fact shows that, because of the appreciable decrease of the electronic conduction, the mobile T1 ions diffuse smoothly. Therefore, it is expected that the value of o-i~ will decrease drastically at this composition. We consider that such 'highly ionic conduc-

0

I

I

I

I

I v 68 • 67 o 66.7 •

60

,. 55 'I 50 A z,O • 20

'---1

"r E U

v

ff 0

-3

I

1.2

1.3

I

I

1.4 1.5 1000 / T

I

I

1.6

1.7

Fig. 4. Ionic conductivity, tri, for liquid TI-Se system as a function of temperature.

T. Usuki et al. / Electronic and ionic conductions

tion' is generally confirmed for other liquid semiconductors.

4. Discussion After the transition from a non-metallic to a metallic state, the ionic conduction will be disturbed by the conduction electrons or the crossconduction; then ~ri decreases with increasing T1 content. On the other hand, with increasing Se content beyond the stoichiometric composition (0.5 < x < 0.667), ~e increases slightly. In this range, the situation can be described as follows. If the conventional semiconductor band model is still valid in the liquid as in heavily doped crystalline semiconductors, an excess of the more electronegative constituent (Se atom) acts as an acceptor (p-type). Then, with doping by small amounts of excess Se in T12Se, the liquid behaves as a p-type semiconductor, i.e., the electronic conducion is caused by positive holes. This feature is supported by the result of thermoelectric power measurements [3]. In this range, an appreciable decrease of ori is observed. This feature is caused by the reason that the diffusion of the mobile T1 ions is limited by the impurity atoms (Se atoms); conduction by T1 ions is sensitive to the impurity atoms. In the more Se-rich range (x < 0.5), the concentration of covalent Se-chain cluster increases rapidly with addition of Se atoms, and the liquid structure indicates a pseudo-binary mixture of TleSe and Se-chain clusters [10]. Therefore, it may be assumed that two hopping-type conductiv-

755

ity mechanisms exist: intra- and inter-cluster hopping. We consider that the non-linear behaviour of log O ' e / ( 1 / T ) is caused by the presence of these two activation processes. In this range, since the conduction path of the T1 ions is completely blocked by Se-chain clusters, % falls to zero.

5. Conclusions Electronic and ionic conductivities of liquid T1-Se alloys have been measured separately as a function of concentration and t e m p e r a t u r e . Highly ionic conductivity was observed at the stoichiometric composition T12Se because of the decrease of the electronic conduction. The authors are grateful to Professor H. Okazaki for stimulating discussions for this study.

References [1] See, for example, M. Cutler, Liquid Semiconductors (Academic press, New York, 1977). [2] H. Rasolondramanita and M. Cutler, Phys. Rev. B29 (1984) 5694. [3] Y. Nakamura and M. Shimoji, Trans. Faraday Soc. 65 (1969) 1509. [4] M. Cutler and C.E. Mallon, Phys. Rev. 144 (1966) 642. [5] S. Miyatani, J. Phys. Soc. Jpn. 10 (1955) 786. [6] S. Miyatani, J. Phys. Soc. Jpn. 13 (1958) 341. [7] H. Endo, M. Yao and K. Ishida, J. Phys. Soc. Jpn. 48 (1980) 235. [8] I. Yokota, J. Phys. Soc. Jpn. 16 (1961) 2213. [9] M. Cutler, Phys. Rev. B9 (1974) 1762. [10] T. Usuki, Y. Shirakawa and S. Tamaki, J. Phys. Soc. Jpn. 61 (1992) 2805.