EFFECT OF PROTON IRRADIATION ON THE ELECTRICAL CONDUCTIVITY OF KCl—KBr SOLID SOLUTIONS

V. A. POPOV, V. D. RUTSKOV

It is known that the principal contribution to the ionic conductivity of alkali-halide crystals is due to cation vacancies; thus, by measuring the electrical conductivity of crystals, it is possible to monitor the defectiveness of the cation sublattice [1].

In the works of Kobayashi, Smoluchowski, and Pearlstein it was shown that preliminary proton irradiation lowers the electrical conductivity of alkali-halide crystals [1, 2, 3].

There are at least two possible explanations of this phenomenon:

1. The decrease in conductivity is associated with the neutralization of the vacancies responsible for electrical conductivity by the capture of electrons and holes [1]. This hypothesis is based on a rapid electronic process that does not depend on temperature. It has not received experimental confirmation.

2. The decrease in electrical conductivity is associated with the formation of neutral complexes of cation and anion vacancies, as a result of which the number of free vacancies responsible for electrical conductivity is reduced [2]. This corresponds to the Varley mechanism. This mechanism provides an excess of anion vacancies. At sufficiently high temperatures, which ensure vacancy mobility, the formation of neutral complexes consisting of anion and cation vacancies becomes possible, as a result of which the conductivity decreases. With a further increase in temperature, the vacancy groups dissociate and the defects are annealed out [2]. This brings the crystal lattice back to its initial state, and the conductivity becomes equal to the conductivity of an unirradiated crystal.

In the present work, the effect of isodose proton irradiation of \(6 \cdot 10^{14}\) protons/cm\(^2\) on the electrical conductivity of KCl—KBr solid solutions was investigated. Single crystals of the solid solutions were grown by the Kyropoulos method. The electrical conductivity of the samples was measured no later than 10 days after growth of the single crystal. The electrical conductivity was measured under vacuum conditions using an amplifier of the “Kaktus” type. Irradiation of the samples was carried out with protons of energy \(4.0 \pm 0.1\) MeV on the cyclotron of the Tomsk Polytechnic ...

of the institute. During irradiation it was assumed that the coloring of the solutions changes additively with composition. The coloring depth was: KCl—270 μ, KBr—220 μ. The temperature of the specimens during irradiation did not exceed 40°C (proton-beam current 0.2 μA). The dependence

\[ \lg \sigma = f\!\left(\frac{10^{4}}{T}\right) \]

for unirradiated specimens had the form of straight lines. For irradiated crystals, in the linear dependence

\[ \lg \sigma = f\!\left(\frac{10^{4}}{T}\right) \]

a break is observed at different temperatures, depending on the composition.

The results of the study of the electrical conductivity of KCl—KBr solid solutions are presented in Fig. 1 in the form of the dependence \(\sigma_{0}/\sigma_{\mathrm{irr}} = f(t^\circ\mathrm{C})\).

Fig. 1. Dependence \(\sigma_0/\sigma_{\mathrm{irr}} = f(t^\circ\mathrm{C})\) for KCl—KBr solid solutions.
1. KBr; 2. KCl; 3. KCl—KBr 50—50%; 4. KCl—KBr 10—90%; 5. KCl—KBr 90—10%.

In the temperature range 20°—40°C the conductivity of the irradiated crystals is higher than that of the unirradiated ones. Above 40°C the conductivity of specimens of all compositions (except KBr) is already lower than that of the unirradiated ones. In irradiated KBr specimens the conductivity is lower than in unirradiated ones already at \(t > 56^\circ\mathrm{C}\). The ratio \(\sigma_0/\sigma_{\mathrm{irr}}\), depending on temperature, reaches a maximum for KCl and KBr crystals in the temperature interval from 58°C to 63°C. The magnitude of the maximum \(\sigma_0/\sigma_{\mathrm{irr}}\) for a KCl crystal is three times greater than for KBr. Thus, the greatest decrease in electrical conductivity after proton irradiation is observed in KCl crystals.

For solid solutions, the magnitude of the maximum \(\sigma_0/\sigma_{\mathrm{irr}}\) changes depending on the ratio of the components; namely, it increases with increasing KCl content in KBr. In addition, an increase in the KCl content in KBr causes a shift of the maximum of \(\sigma_0/\sigma_{\mathrm{irr}}\) toward higher temperatures.

The decrease in the conductivity of KCl—KBr solid solutions under isodose irradiation with protons of \(6 \cdot 10^{14}\) protons/cm² is apparently associated with the formation of neutral complexes [2]. In a study of dielectric losses in alkali-halide crystals carried out by graduate student Yu. M. Annenkov, relaxation maxima were observed as a function \(\tg \delta = f(t^\circ\mathrm{C})\), the occurrence of which is associated with the formation of neutral complexes.

In the dependence \(\operatorname{tg}\delta = f(t^\circ\mathrm{C})\), after proton irradiation, relaxation maxima are observed, the occurrence of which is explained by the formation of neutral complexes.

From the dependence \(\lg \sigma = f\!\left(\dfrac{10^4}{T}\right)\), the activation energy of the electrical-conductivity process was determined according to the formula

\[ \sigma = A \cdot e^{-\frac{U}{kT}} . \tag{1} \]

Table 1 gives the activation energies and the temperature of the break in the dependence \(\lg \sigma = f\!\left(\dfrac{10^4}{T}\right)\).

Table 1

The activation energy of the solid solutions is less than that of the pure components. With an increase in the KCl content in KBr, the activation energy of the electrical-conductivity process increases.

Conclusions

1. Qualitative data have been obtained on the effect of isodose proton irradiation on the electrical conductivity of KCl—KBr solid solutions: above \(40^\circ\mathrm{C}\), the conductivity of KCl—KBr solid solutions is lower than that of unirradiated specimens.

2. The magnitude of the maximum \(\sigma_0/\sigma_{\mathrm{irr}}\) in solid solutions increases with increasing KCl content in KBr.

3. The position of the maximum \(\sigma_0/\sigma_{\mathrm{irr}}\) shifts toward higher temperatures with increasing KCl content in KBr.

4. The activation energy of the electrical-conductivity process of KCl—KBr solid solutions, both unirradiated and after proton irradiation, is less than that of the pure components.

5. The activation energy of the electrical-conductivity process depends on the chemical composition of the solid solutions; with increasing KCl content in KBr, the activation energy increases.

References

1. K. Kobayashi. The Phys. Rev., vol. 102, 348, 1956.
2. R. Smolukhovskii. Report at the International Conference on the Peaceful Uses of Atomic Energy. Geneva, vol. 7, 1955.
3. Pearlstein E. A. Phys. Rev., vol. 29, No. 4, 1953.
4. Abstracts of papers at the XII Conference on Luminescence, Lviv, 30/I–5/II–64, p. 34, 1964.