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INVESTIGATION OF ELECTRIC CHARGES
CARRIED BY MOVING DISLOCATIONS IN NaCl
SINGLE CRYSTALS
A. Tóth, J. Sárközi
To cite this version:
c7-604 JOURNAL DE PHYSIQUE Colloque C7, suppliment au no 12, Tome 37, Dicembre 1976
INVESTIGATION OF ELECTRIC CHARGES CARRIED
BY MOVING DISLOCATIONS IN NaCl
SINGLE CRYSTALS
A. TOTH and J. SARKOZI
Department for experimental Physics, University of Technology, 1521 Budapest, Hungary
RBsumB.
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Les charges portBes par des dislocations mobiles ont BtB mesurees dans des mono- cristaux de NaCl dopes d'impuretks cationiques bivalentes, et le rapport entre les charges et la concentration des sites cationiques vacants libres du cristal a 6tB Ctudit. Nous avons constate que : 1. les dislocations portaient des charges negatives ; 2. IYintensit6 des charges variait proportionnel- lement B la concentration des sites vacants cationiques libres independamment de la manikre (dopage cationique bivalent, traitement thermique, changernent de la tempbature de mesure) dont la concentration des sites vacants Btait changee.Abstract.
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The charge carried by moving dislocations was measured in NaCl single crystals doped with divalent cation impurities, and the connection between the charge and the concentration of the free cation vacancies of the crystal was investigated. According to the experimental results the dislocations carry negative charge and the magnitude of the charge changes proportionally to the concentration of the free cation vacancies independently of the way (divalent cation doping, heat treatment, change of the measuring temperature) the vacancy concentration was changed.1. Introduction. - It is a well known fact that if an ionic crystal is subjected to plastic deformation potential differences appear between different points of the crystal. It is generally supposed that his phenome- non is due to the moving dislocations produced during plastic deformation, because the carry electric charges in these crystals. The charges on the moving disloca- tions probably have their origin in the sweeping up of some charged point defects of the crystal [l], 121. Since in our investigations the sodium chloride crystals were doped with divalent cation impurities these point defects were probably cation vacancies, consequently we discuss only this case.
Two possibilities by which the moving dislocations may sweep up cation vacancies in these crystals should be considered : either dislocations sweep up free cation vacancies or the dislocations sweep up vacancies from impurity-vacancy pairs as suggested by Whitworth [3]. The aim of the present communication is to inves- tigate more closely the change of electric charge carried by moving dislocations by changing the concentration of the free cation vacancies in sodium chloride single crystals.
2. Experimental details. - The crystals used were prepared by a special method [4] and practically contained only impurities introduced intentionally into the crystals. The concentration of other impurities was less than 2 X 10-7 mol/mol. The dopants and their
approximate concentrations used in different cases are shown in table 1.
Crystal Dopant Concentration (mol/mol)
-
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NaCl Ca2 + 2 X I O - ~ , 1 0 ~ , 1 0 - ~ , I O - ~ NaCl Ba2+ 1 0 - ~ 1 0 - ~ NaCl Pb2 + I O - ~-
I O - ~ NaCl Sr2'
1 0 ~I O - ~ ~ NaCl Mg2 + I O - ~-
I O - ~ NaCl Mn2*
I O - ~ I O - ~ The charge, carried by moving dislocations was determined as described earlier by measuring the potential difference U due to plastic deformation realised by the indentation of the { 100 ) planes of the crystals, the depth of indentation, and the length of the dislocation rosette around the indentation [5, 61. The measuring scheme is shown in figure 1. ( C is the total capacitance of the measuring system.)indenfer
1
FIG. 1. - Schematic diagram of the measuring system to inves- tigate the charges carried by dislocations.
ELECTRIC CHARGES CARRIED BY DISLOCATIONS C7-605
In order to avoid surface effects large indentations were made. The data for the estimation of the disloca- tion charge were obtained in every case at the same indentation depth of 20 pm.
The concentration of the free cation vacancies has been calculated from the measured values of the ionic conductivity of the crystals [7]. The ionic conductivity was measured in the usual way with a vibrating conden- ser electrometer in a vacuum of 10-3 torr.
The concentration of the free cation vacancies was changed in the course of the experiments by the follow- ing methods :
1) crystals doped with different concentration of the same divalent impurity (Ca2+) or else,
2) crystals doped with different divalent impurities but at about the same concentration were used,
3) crystals doped with mol/mol Ca2+ were annealed for 3 hours at different temperatures and quenched to room temperature (the annealing tempe- rature varied from 50 to 600 OC),
4) measurements of charge and ionic conductivity were carried out at different temperatures with crystals containing 10-5 mol/mol Ca2+ in the temperature range of 25 to 200 OC.
In the first three cases the measurements were carried out at room temperature.
3 . Results and discussion. - The changes of the dislocation charge q by changing the concentration of the free cation vacancies in crystals are depicted in figure 2. The quantity denoted with n, is proportional to the concentration of the free cation vacancies. The points denoted with different symbols show the results obtained with crystals whose cation vacancy concentra- tion was changed in different ways.
As can be seen from this figure the charge carried by moving dislocations is negative in these crystals, and changes proportionally to the concentration of the free cation vacancies independently of the way the vacancy concentration is influenced.
Since in our investigations the dislocations move rapidly diffusion effects may be neglected, and for the explanation of the experimental results obtained the
0
5
10
15
20
25
n, (arbitrary units
1
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FIG. 2.
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The dislocation charge (g) as a function of the con- centration of free cation vacancies (nu). The following symbols are used : 0-crystals doped with different amounts of Ca2+ (from left to right : 2 X 10-7, 10-5, 10-4, and 10-3 mol/mol) ;.-heat treated crystals containing 10-3 mol/mol Ca2+ ; A-crys- tals doped with 10-5 mol/mol Ca2+ measured at different tem- peratures (the measuring temperature rises from left to right) ;
A-crystals doped with different divalent cations (from left to right : Ba2+, Pb2+, Sr2+, Mg2+, Mn2+, Ca2+).
swept up by the dislocation is equal to the rate at which they are lost from it.
If it is further assumed that the dislocations sweep up free cation vacancies and the mechanism of the loss i f
cation vacancies is independent of the point defects of the crystal, the equilibrium charge ought to change proportionally to the concentration of the cation vacancies. In this case one may draw the conclusion from the temperature dependence of the charge that the activation energy of charge transport by dislocations is near to zero [7].
However it is possible, that the experimental results obtained may be explained by assuming that disloca- tions sweep up vacancies also from the impurity- vacancy pairs and that the mechanism of loss of charge from dislocations is related to the point defects of the crystal, though such an explanation appears to be rather difficult at present because of the numerous unknown parameters of the model.
simple model proposed by Whitworth [3] can be used. Acknowledgments. - The authors wish to thank According to this model the equilibrium charge on a Dr. R. Voszka for growing the single crystals, moving dislocation may be obtained from the condi- Dr. Z. Morlin and Dr. E. Hartmann for the valuable tion that the rate at which the cation vacancies are discussions.
References
[l] URUSOVSKAYA, A. A., Uspekhi fiz. Nauk 96 (1968) 39. [5] T ~ T H , A. and KISS, J. L., Phys. Stat. Sol. (a) 19 (1973)
[2] WHITWORTH, R. W., Adv. Phys. 24 (1975) 203. K61.
[3] WHITWORTH, R. W., Phil. Mag. 15 (1967) 305. [6] T ~ T H , A. and SARKOZI, J., Phys. Stat. Sol. (a) 28 (1975)
[4] VOSZKA, R., T A R J ~ N , I., BERKES, L. and KRAJSOVSZKY, J., K93.
DISCUSSION
R. W. WHITWORTH. - The problem of calculating the dislocation charge from the electrical signal in an indentation test is a very difficult one (see WHITWORTH, R. W., Adu. Phys. 24 (1975) 203). Until we understand more about the distribution of charge in an indentation there must be some doubt about the validity of the charges you quote. This affects not only the absolute magnitude but also the relative values in crystals of different purity.
A. T ~ T H . -The charge distribution under the indenter may be very complicated indeed, but the
experiments show that in a given crystal the calculated charge does not depend on the depth of indentation, though the charge distribution is probably different at different indentations. As to the measurements in crystals of different purity, from the results of Kolo- miitsev obtained in NaCl crystals by changing the concentration of the Cd impurity (see Fiz. tverd. TeEa
