ELECTRICAL PROPERTIES OF SOLID SOLUTIONS OF THE KCl—KBr SYSTEM SUBJECTED TO PROTON BOMBARDMENT

Yu. M. Annenkov, V. A. Grishkov

(Presented by the scientific seminar of the Laboratory of Dielectrics and Semiconductors)

Irradiation of KBr and KJ crystals leads to the appearance of dielectric absorption in the range of audio frequencies, which can be described by the formulas of resonance absorption [1, 2]. A model was proposed of oscillating defects in the form of vacancy aggregates that had localized several holes on themselves [2]. However, similar dielectric absorption is not observed in pre-irradiated NaCl and KCl crystals. In connection with this circumstance, it is of interest, using solid solutions of the KCl—KBr system as an example and taking into account the specifics of their structure, to find the conditions necessary for the occurrence in alkali-halide crystals of radiation defects that cause resonance losses at low frequencies.

In the present work, thermal changes in electrical conductivity and dielectric losses were studied in the frequency range \((550—10000\ \text{Hz})\) for solid solutions of KCl—KBr. The crystals were irradiated with protons accelerated to \(4.5\ \text{MeV}\), with the same dose equal to \(1 \cdot 10^{15}\) protons/cm\(^2\).

In the temperature interval \(20—250^\circ\text{C}\), the electrical conductivity of the solid solutions varies with composition along a curve with a minimum at equal component content. The activation energy for migration of current carriers changes additively in going from KCl to KBr (Fig. 1). Dielectric losses in the solid solutions have the same character as in the pure components. The values of \(\operatorname{tg}\delta\), both at high and at low temperatures, deviate from the rule of additivity toward a decrease (Fig. 2). The conductivity of all the systems investigated decreases after irradiation. The temperature variation \(\lg \sigma_{\text{unirr.}} - \lg \sigma_{\text{irr.}}\) passes through a maximum at \(110^\circ\text{C}\). The maximum value of \(\lg \sigma_{\text{unirr.}} - \lg \sigma_{\text{irr.}}\) varies with the composition of the solid solution along a curve with positive nonadditivity (Fig. 2).

Thus, the data from the investigation of electrical conductivity indicate that in solid solutions, to a greater extent than in the components, vacancies are predisposed to aggregation. The increased ability of point defects in solid solutions to form vacancy bonds reduces the probability of formation of \(F\)-centers [3].

From the data presented in Figs. 1 and 2, it may be concluded that even with small additions of KBr to KCl the number of vacancy aggregates in the system

Fig. 1. Change in electrical conductivity and activation energy for migration of current carriers in KCl—KBr solid solutions as a function of composition.
1—electrical-conductivity values at 160°C,
2—electrical-conductivity values at 112°C,
3—activation-energy values for migration of current carriers.

Fig. 2. Dependence of $\lg \sigma_{\text{unirr.}} - \lg \sigma_{\text{irr.}}$ (curve 3) and $\operatorname{tg}\delta$ at a frequency of 100 Hz on the composition of the KCl—KBr solid solution.
1—values of $\operatorname{tg}\delta$ at 160°C,
2—values of $\operatorname{tg}\delta$ at 20°C.

Fig. 3. Temperature variation of $\operatorname{tg}\delta$ at a frequency of 50 Hz for unirradiated (curve 1) and proton-irradiated (curve 2) crystals of solid solutions of composition 86 mol% KCl—14 mol% KBr.

greater than in potassium chloride. Therefore, if the determining condition necessary for the creation of oscillating defects were a certain degree of development of aggregates, then resonance losses should also occur for compositions close to the equimolar composition on the KCl side. However, we have established that resonance dielectric absorption arises after irradiation only for compositions with a high content of Br ions (Figs. 3, 4). As the KCl content in KBr is decreased, the maximum of \(\operatorname{tg}\delta\) increases and shifts into the region of high temperatures.

Fig. 4. Dependence of \(\operatorname{tg}\delta\) on temperature for unirradiated (solid curves) and proton-irradiated (dashed curves) crystals of solid solutions of composition 29 mol% KCl—71 mol% KBr.
1—50 Hz, 2—100 Hz, 4—1000 Hz, 5—5000 Hz.

Upon irradiation of solid solutions with high-energy radiation, the probability of ionization of Br ions will be determined mainly by their concentration. Consequently, only localization on an aggregate of vacancies of holes in the form of Br atoms leads to the appearance of resonance absorption in KCl—KBr solid solutions.

It may be considered that the determining condition for the appearance of oscillators causing low-frequency resonance absorption in irradiated alkali-halide crystals is the special electrical properties of Br atoms, as well as iodine atoms, which chlorine atoms do not possess. We emphasize a similar regularity observed in attempts to color alkali-metal halides in halogen vapor [4].

References

1. Yu. M. Annenkov. Izvestiya TPI, vol. 140. Tomsk, 1965.
2. Yu. M. Annenkov. Izvestiya TPI, vol. 139. Tomsk, 1965.
3. T. S. Frangul’yan. Izvestiya TPI, vol. 140. Tomsk, 1965.
4. E. Mollwo. Ann. Phys., 29, 394, 1937.

1. Order 8076.