Proceedings of the Tomsk Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov

Vol. 140　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1965

Effect of Deformation on the Kinetics of Accumulation of $F$-Centers in Pure KCl Crystals and in KCl Crystals Doped with Divalent Impurities

I. Ya. Melik-Gaikazyan, L. I. Roshchina, M. I. Ignat’eva

One of the possible mechanisms of radiation generation of vacancies at temperatures close to room temperature is the mechanism of dislocation jogs [1, 2]. Some authors [3–6] believe that the experimental fact of an increase in the growth rate of the density of $F$-centers in the linear stage of their accumulation in deformed crystals possessing a high dislocation density confirms the validity of dislocation mechanisms. A substantial shortcoming of these works [3–6] is an insufficiently clear separation of the effect of deformation on the change in the rate of accumulation of $F$-centers, first, on “pre-radiation” vacancies present also in unirradiated crystals (the so-called first stage of accumulation of $F$-centers), and second, on vacancies created by irradiation (the second stage of slow linear accumulation).

The aim of the present work is to attempt to explain the intensification of the process of accumulation of $F$-centers in the linear region in deformed crystals by the influence of redistribution of impurities, which are present even in “pure” crystals, and also to attempt to interpret the enhancement of the stage of rapid coloration on the basis of a comprehensive study of the radiation-induced change in optical properties and electrical conductivity.

Indeed, it has been established that plastic compression of crystals leads to an increase in the impurity content in the lattice, and not only to an increase in the dislocation density. This is evidenced by experimental facts on the luminescence of deformed rock-salt crystals [7], as well as by the increase obtained by us in the absorption coefficient at the maximum of the band $\lambda = 273\,m\mu$ of deformed KCl·Pb crystals; absorption in this wavelength region is attributed to lead ions located in the regular crystal lattice (Fig. 1). On the other hand, it is known that the addition of Sr impurity to alkali-halide crystals creates additional levels for localization of holes and causes an increase in the rate of accumulation of $F$-centers not only on “pre-radiation” vacancies but also on vacancies generated by radiation $(\alpha^*)$ [8].

Therefore one may expect that the additional introduction of the Sr impurity into the crystal lattice, caused by its deformation, will affect the increase in the generation rate of \(F\)-centers more strongly than the increase in the dislocation density. To test this assumption, we studied the effect of deformation on the value of \(a^*\) in KCl and KCl-Sr crystals.

Crystals were grown from the melt by the Kyropoulos method from chemically pure salts. Uniaxial compression was carried out at a constant rate of \(0.075\) mm/sec on a special machine for micromechanical tests. The samples were irradiated at room temperature in a URS-70 apparatus (W tube—anode, \(8\) mA, \(56\) kV, Al filter \(2\) mm).

Fig. 1. Activator absorption spectrum of a KCl·Pb crystal before (1) and after (2) plastic deformation by 6%.

At the irradiation intensity used by us, the generation rate of \(F\)-centers (\(a^*\)) in a “pure” KCl crystal is very low (Fig. 2, \(a\), Table 1). The addition of an Sr impurity to the crystal increases the rate of formation of \(F\)-centers on vacancies generated by irradiation. Plastic deformation leads to an enhancement of the generation rate of \(F\)-centers (\(a^*\)) predominantly in crystals containing strontium. From Table 1 and Fig. 2, \(a\), it is seen that even a smaller percentage deformation of the KCl·Sr crystal contributes to a greater increase in \(a^*\) than in pure KCl.

Fig. 2. Kinetic curves of accumulation of \(F\)-centers in KCl and KCl·Sr crystals.
\(a\)) 1. KCl undeformed. 2. KCl deformed by 3.6% after 20-hour irradiation with X-rays.
3. KCl + 0.022 mol % SrCl\(_2\), undeformed. 4. KCl + 0.022 mol % SrCl\(_2\), deformed by 1.8% after 20-hour irradiation.
\(b\)) The designations and crystals are the same; deformation of 6.5% was carried out before irradiation.

Apparently, the enhancement of the rate of formation of \(F\)-centers on vacancies generated by irradiation is caused not so much by an increase in the density

(according to the data of Fig. 2a)

Table 1

dislocations as by a redistribution of impurity caused by plastic compression. Therefore the increase in \(a^*\) in relatively “pure” deformed KCl crystals can be explained by the redistribution of those contaminating impurities in the crystal which are present in KhCh-grade salt. This agrees with the data of [9, 10], in which, for pure deformed specimens, no increase in the generation of \(F\)-centers was obtained on the linear portion of their accumulation. On the other hand, experiments on the increase, in deformed crystals, of the rate of accumulation of \(F\)-centers at “pre-radiation” vacancies (Fig. 2b) indicate a change, as a result of plastic compression, in the initial defectiveness of the crystals. Thus, we found that deformation after preliminary 20-hour irradiation with x rays leads to the secondary appearance of the first stage on the \(F\)-center accumulation curve, especially sharply expressed for KCl:Sr crystals (Fig. 2a). It may be assumed that this too is connected with the phenomenon of impurity redistribution as a result of plastic deformation [3].

However, intensification of rapid coloration was observed by Sibley [10] also in very pure crystals, and by Gordon and Bauer [11] in NaCl crystals even at a temperature of 10°K, when the action of the dislocation mechanism, as well as the influence of impurities, is excluded.

These facts indicate the appearance of an additional number of “pre-radiation” vacancies as a result of plastic deformation. It is known that an increase in the rate of accumulation of \(F\)-centers at defects already present in a crystal can occur due to an increase in the concentration of:

1) anion vacancies \(v^-\), i.e., an increase in the number of electron-capture centers leading to the formation of \(F\)-centers;

2) cation vacancies \(v^+\), which increase the radiation stability of \(F\)-centers through the formation of hole \(V_2\)-centers including a cation vacancy, which would explain the intensification of rapid coloration at room temperature, but not at a temperature of 10°K, at which formation of \(V_2\)-centers does not occur;

3) pairs \(v^-v^+\), which decompose under the action of x rays.

Which of the listed defects are formed preferentially in the crystal as a result of plastic deformation? From works of recent years it is known that dislocation lines in alkali-halide crystals may be charged, i.e., may contain predominantly ions of one sign. This gives grounds to suppose that, during the motion of dislocations in deformation, generation of vacancies also predominantly of one sign may occur. According to Seitz [1], the probability of formation of a cation vacancy is greater, since the energy of its forma-

…is smaller than for an anion vacancy. This assumption is consistent with the results of [12, 13, 14] on the increase in the electrical conductivity of crystals during plastic deformation and immediately after its cessation. However, as shown in one of Fröhlich’s recent works [15], the increase in the current through a crystal at the moment of deformation is associated with the motion not of ions, but of dislocations. This current arises in the crystal even without the application of an external electric field.

To resolve the question of the possibility of radiation generation of vacancies by the mechanism of dislocation jogs, it is important to measure the residual change in defectiveness caused by deformation, since it is known that dislocations created in the process of plastic compression remain in the crystal for a long time. The change in defectiveness can be monitored by measurements of ionic conductivity in the low-temperature region, in which, as is known, the current carriers in alkali-halide crystals are cations moving over free cation vacancies; that is, the value of the ionic conductivity responds to changes in the defectiveness of the cation sublattice. If, during plastic deformation of a crystal, cation vacancies \(v^{+}\) are generated predominantly, its conductivity should increase; if the principal point defects generated by deformation are \(v^{+}v^{-}\) pairs, the conductivity should not change appreciably. Generation of an additional number of anion vacancies \(v^{-}\) at room temperature will lead to a decrease in the concentration of isolated cation vacancies through the formation of \(v^{+}v^{-}\) pairs and, consequently, to a decrease in conductivity.

We found that the conductivity of deformed KCl crystals in the temperature interval \((50 \div 150)^\circ\mathrm{C}\) decreases (Fig. 3, \(a\), curve 2) according to a linear law with increasing percentage deformation (Fig. 3, \(b\)). The electrical conductivity was measured one day after deformation.

From the above arguments it follows that the decrease in the electrical conductivity of deformed crystals is caused by the formation, during deformation, of anion vacancies with subsequent association into \(v^{+}v^{-}\) pairs. However, at a temperature of \(300^\circ\mathrm{C}\) the effect of a decrease in crystal conductivity due to deformation is removed (Fig. 2, \(a\), curve 2), while the pairs are dissociated at a higher temperature; moreover, upon repeated measurement of the deformed, now annealed crystal, its electrical conductivity almost coincides with the electrical conductivity of the undeformed crystal (Fig. 3, \(a\), curve 3). It remained to assume that, as a result of deformation, there occurs a decrease in the concentration of cation vacancies, and that deformation-induced changes in the defectiveness of the sublattices may be connected with the experimental fact of electrically charged dislocations. We therefore turned to the numerous works on determining the charge of dislocations. It should be noted that the results are contradictory. In the works of some authors [16, 17], dislocations formed during deformation proved to be positively charged; in others [18—20], negatively charged. Fröhlich [15] uncovered the reasons for the contradiction in the data and showed that uncharged dislocation lines arising during deformation, moving through the crystal, interact with lattice defects and acquire a charge. Depending on the concentration of vacancies of one sign or the other, dislocations may be either a sink or a source of vacancies. The sign of the dislocation charge depends on the type of excess vacancies. In relatively pure alkali-halide crystals there is always…

divalent ions Sr and Ca are present in small quantities, and consequently also an excess number of cation vacancies. Therefore a moving dislocation acquires, as a rule, a negative charge. The magnitude and sign of the charge of dislocations can be changed by increasing or decreasing the concentration of cation and anion vacancies. Thus, for example, the addition of Ca increases the negative charge; the addition of OH changes the sign of the charge to positive. Dissolution of cation vacancies and evaporation of anion vacancies from dislocation steps will lead to the appearance of a negative sign of the dislocation charge.

Fig. 3.

a. Dependence of the logarithm of electrical conductivity on temperature.
1. KCl. 2. KCl deformed by 10%. 3. KCl deformed by 10%, measured again.
b. Dependence of the logarithm of electrical conductivity on the percentage of deformation for KCl crystals at a temperature of 80°C.

Consequently, during plastic compression there occurs a decrease in the concentration of cation vacancies and an increase in the concentration of anion vacancies, which is consistent with the experimental fact obtained by us: a decrease in electrical conductivity and an increase in the colorability of deformed KCl crystals.

Conclusions

1. The experimental fact of an increase in the rate of generation of $F$-centers at radiation vacancies in deformed crystals is not a direct confirmation of the validity of the dislocation mechanism.

2. The intensification, in deformed crystals, of the process of formation of $F$-centers at vacancies generated by x-ray radiat-

is caused not so much by a change in the dislocation density as by a redistribution of the impurity between the defective sites of the crystal and its lattice, brought about by plastic compression.

1. The change, upon deformation, in the defectiveness of the cation and anion sublattices accounts for the decrease in electrical conductivity and the increase in colorability of alkali-halide crystals.

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