Dielectric Properties of Single Crystals of KCl—KBr—SrCl₂ Solid Solutions Irradiated with Protons

V. A. Grishukov

It is shown that, depending on the concentration of the introduced strontium impurity in the solid solutions, it has different effects on the radiation-induced change in dielectric losses in these crystals. Whereas small impurity concentrations (up to \(3 \cdot 10^{-3}\) mol. %) cause the appearance of strong dielectric absorption, a further increase in the impurity content in the solid solutions leads to a considerable decrease in this absorption. Mechanisms are proposed that explain the decrease in dielectric absorption.

It is demonstrated that divalent impurities show different preference on radiation change of dielectric loss in the dependence from introduced concentrations impurities in solid solution crystals. A small concentration impurities \((3 \cdot 10^{-3}\) mol. %) evokes appearance of the paved dielectric absorption. Further increase of maintenance divalent impurities in crystals brings to considerable decrease of the absorption. It is proposed mechanism explavning decrease of dielectric absorption.

The study of irradiated crystals of the KCl—KBr solid solution [1] showed that, in compositions with a high KBr content and in the KBr component, dielectric absorption is observed which disappears completely after annealing to 200°C. These results indicate that either the radiation creates new defects capable of producing such intense dielectric absorption, or it only increases the electrical activity of existing defects.

Optical and thermal bleaching [2] showed that color centers apparently are not a constituent part of the radiation defects responsible for dielectric absorption in KBr.

It is known that alkaline-earth ions are acceptors for holes generated by radiation. Therefore, if localized holes participate in the formation of radiation defects responsible for dielectric absorption, then the introduction of strontium ions into the crystal should change the amplitude of the dielectric absorption peak. To clarify the nature of the observed absorption, crystals of the solid solution with a strontium impurity were investigated.

The investigated crystals of the solid solution with various strontium additions were grown from the melt. The impurity concentration in each investigated sample was determined by a complexometric method using an FEK-56 colorimeter-nephelometer.

Irradiation was carried out with protons of energy 4.5 MeV from both sides of the sample, the crystal thickness being chosen so as to ensure the most uniform coloration throughout the volume. The integral dose was \(1 \cdot 10^{15}\) protons/cm².

The introduction of strontium impurities at concentrations of \(3 \cdot 10^{-3}\) mol.% into KBr and solid-solution compositions with a high KBr content led to the appearance of strong dielectric absorption in the temperature region around \(120^\circ\mathrm{C}\). At low frequencies (300 Hz) a splitting of the dielectric-absorption peak is observed. In the temperature dependence of the dielectric constant of these crystals, a sharp increase in \(\varepsilon\) and a shift of the maximum with increasing frequency toward the high-temperature region are observed.

The splitting of the peak is especially clear in an irradiated and optically bleached KBr crystal (Fig. 1). It is seen from Fig. 1 that the low-temperature peak has a clearly relaxation character, with the change in the real part of the dielectric constant characteristic of relaxation processes. The activation energy of the relaxation polarization is \(1.2\) eV.

Fig. 1. Temperature dependence of \(\tg \delta\) and \(\varepsilon\) for KBr \(+\ 3 \cdot 10^{-3}\) mol.% SrCl\(_2\) crystals irradiated with protons and bleached with white light. 1 — 300 Hz, 2 — 1000 Hz, 3 — 5000 Hz.

The dielectric-absorption peak on the high-temperature side has a sharp drop, and the minimum of \(\tg \delta\) at all frequencies occurs at one and the same temperature. This decrease in \(\tg \delta\) on the high-temperature side suggests that, probably, annealing of the radiation defects responsible for the dielectric absorption is taking place. At present it is not yet clear whether there are indeed two closely located dielectric-absorption peaks, or whether this splitting of the peak is due to other causes. It has been found that the concentration of impurity in the crystal has a strong influence on the dielectric absorption. Increasing the concentration of strontium impurity in the crystals led to a sharp lowering of the dielectric-absorption peak and shifted it into the low-temperature region (Fig. 2). Thus, on the one hand, the introduction of small additions of impurity into KBr crystals and compositions with a high KBr content leads to the appearance of one or two regions of strong dielectric absorption, considerably exceeding the absorption in “pure” crystals, and on the other hand—an increas-

the introduction of the impurity leads to a sharp decrease in the absolute magnitude of dielectric absorption. This, at first glance, contradiction can be explained by the following reasons.

It may be assumed that the thermal stability of radiation defects depends strongly on the impurity concentration in the crystal. In this case, at low strontium concentrations in the crystal, the temperature range in which radiation defects begin to relax and the region of their stable state coincide, and strong dielectric absorption is observed. As the impurity concentration increases, the temperature of their destruction shifts into the region of low temperatures, while the relaxation time either decreases more slowly or does not change at all. In this case the radiation defects are annealed before they have time to manifest themselves in relaxation processes.

Fig. 2. Temperature dependence of $tg\delta$ for proton-irradiated crystals of composition 10 M% KCl—90 M% KBr at a frequency of 300 Hz and impurity concentrations: 1—$3 \cdot 10^{-3}$ mol.% SrCl$_2$, 2—$12 \cdot 10^{-3}$ mol.% SrCl$_2$, 3—$18 \cdot 10^{-3}$ mol.% SrCl$_2$.

Another mechanism explaining the decrease in absorption is also possible. Proceeding from the premise of a dual distribution of impurities in crystals, the indisputable fact is that, with increasing impurity concentration in crystals, the fraction of impurities at dislocations, mosaic blocks, etc., will increase in comparison with the fraction of impurity dissolved in the lattice. In this case, if it is assumed that the model of the relaxators is a $V_2$ center localized on the cation vacancy of an excited complex [2], then, with increasing impurity concentration at special sites, the probability increases of localization of holes on impurity ions located on mosaic blocks, dislocations, etc.; this leads to a sharp decrease in the concentration of dipoles responsible for relaxation.

The set of studies carried out showed that impurities of divalent metal ions play an important role in the radiation-induced change of the dielectric properties of KCl—KBr solid solutions.

In conclusion, I take this opportunity to thank E. K. Zavadovskaya and Yu. M. Annenkov for proposing the topic and for their constant attention to the work, and V. Voropaev for carrying out the measurements.

References

1. Yu. M. Annenkov, V. A. Grishukov. Izvestiya TPI, vol. 139, Tomsk, 1965.
2. Yu. M. Annenkov. Izv. TPI, vol. 139, Tomsk, 1965.