FORMATION OF $M$ CENTERS IN ALKALI-HALIDE CRYSTALS WITH A DIVALENT IMPURITY

V. M. LISITSYN, I. Ya. MELIK-GAIKAZYAN

It is known that irradiation of alkali-halide crystals with ionizing radiation, or excitation of a crystal containing $F$ centers by light of the $F$ band, leads to the formation of $M$, $R$-, and $N$ centers. It has been suggested that these centers are the product of coagulation of $F$ centers [1]. This assumption was confirmed by many experiments on the polarized luminescence of crystals excited by $F$ light, on optical bleaching, and on the ratio of $F$ and $M$ centers in the crystal [1—3]. At present the most probable model of the $M$ center appears to be that according to which the $M$ center consists of two covalently bound centers oriented along the (110) axis.

Attempts have been made to explain the mechanism of formation of $M$ and more complex centers [4—6] and, proceeding from this mechanism, to clarify certain details of the processes occurring during optical bleaching and during irradiation of crystals with ionizing radiation. The most probable mechanism is the following. Excitation of the crystal by light of the $F$ band, or the action of radiation, leads to the formation of $\alpha$ and $F'$ centers, which have a lifetime at $0^\circ\mathrm{C}$ of approximately $10^{-2}$ sec [4]. The presence of thermal peaks, as a result of the action of radiation, annihilation of an electron and a hole, and capture of an electron at an $\alpha$ center, leads to a change in the local energetic vibrations of the lattice and to migration of $\alpha$ centers, since their mobility is greater than the mobility of $F$ and $F'$ centers. And the existence of Coulomb interaction forces between $\alpha$ and $F'$ centers and of monopole–dipole interaction between $\alpha$ and $F$ centers leads to directed migration of $\alpha$ centers. As a result of the action of the two processes mentioned, condensation of $F$ centers occurs in certain regions, where the probability of formation of $M$ centers increases.

Such a mechanism for the formation of complex centers makes it possible to explain the process of bleaching of a crystal upon its excitation by light of the $F$ band and the process of accumulation of $M$ centers under the action of radiation. When ionizing radiation acts on a crystal, as $F$ centers are formed their condensation occurs. With the growth of the concentration of $F$ centers, the rate of formation of $M$ centers increases to a certain value determined by the thermodynamic equilibrium between $F$ and $M$ centers [7], [9]. In crystals subjected to deformation, na-

a different character of accumulation of \(M\)-centers is observed [5]. At the beginning of irradiation a rapid increase in their concentration is observed, which subsequently changes to accumulation of centers according to a linear law. The rate of accumulation in the deformed crystal is several times greater than in the undeformed one. The difference in accumulation is apparently connected with a change in the dislocation structure of the crystal. As shown in the review by I. Ya. Melik-Gaikazyan and S. U. Gol’denberg [10], deformation leads to an increase in the density of vacancies and to the entry of impurities into the lattice. Therefore it seems of interest to study the influence of impurity on the kinetics of accumulation of \(M\)-centers.

The present work is devoted to the study of the influence of Sr impurity on the accumulation of \(M\)-centers in KCl and KBr crystals. The crystals were grown from the melt by the Kyropoulos method. Irradiation with protons of energy \(4.1\) MeV was carried out using the TPI cyclotron, which makes it possible to obtain doses sufficiently large for studying the kinetics of accumulation of \(M\)-centers. Measurements of absorption spectra on an SF-4 spectrophotometer and irradiation on the cyclotron were carried out at room temperature. The impurity concentration in the crystals was determined by the complexometric method using an FEK-56 colorimeter–nephelometer.

If the distribution of the generated \(F\)-centers in the crystal is taken to be random, then the probability of formation of two \(F\)-centers side by side, capable of creating an \(M\)-center, is equal to the product of the probabilities of formation of two \(F\)-centers, i.e., to the square of the probability of formation of an \(F\)-center. Hence the density of \(M\)-centers in the crystal must be proportional to the square of the concentration of \(F\)-centers [2]. And since this proportionality should not change during the accumulation of \(F\)- and \(M\)-centers, the dependence of the density of \(M\)-centers on the square of the concentration of \(F\)-centers must be expressed by a straight line. Such a dependence, obtained for a KBr—Sr crystal, is shown in Fig. 1. The linear dependence is distorted in the region of high concentrations of \(F\)-centers, probably because of the influence on the accumulation kinetics of the process of conversion of \(F\)- and \(M\)-centers into \(R\)-centers.

Fig. 1. Dependence of the density of \(M\)-centers on the square of the concentration of \(F\)-centers in KBr—Sr crystals irradiated with protons at an intensity of \(2.0 \cdot 10^7\) erg/cm\(^2\) min.

The accumulation curves of \(M\)-centers are shown in Figs. 2 and 3 for KCl and KBr crystals with Sr impurity. The curves have a clearly pronounced two-stage character, with both stages depending on the concentration of impurity in the crystal. The first stage is a rapid increase in the concentration of \(M\)-centers at the beginning of irradiation; the second stage is slow linear accumulation of centers. With an increase in the impurity content in the crystal, the magnitude of the step of the first stage of \(M\)-center accumulation increases, and the rate of accumulation in the second stage increases. This means that \(F\)-centers are generated in certain defective sites associated with the impurity, with a high local density at the first stage.

accumulation and coagulation of \(F\)-centers proceeds more intensively in crystals with a higher impurity content.

Studies of the dependence of the concentration of \(M\)-centers on the impurity content (Figs. 4, 5) showed that increasing the impurity concentration

Fig. 2. Accumulation curves of \(M\)-centers in KBr crystals with different Sr impurity contents.

at low impurity content leads to a sharp increase in the density of these centers, whereas at high impurity content its influence on the formation of \(M\)-centers is insignificant. The dependence was obtained under isodose irradiation of crystals with different impurity contents. The selected dose corresponds to the beginning of the second stage of accumulation of \(M\)-centers. The dependence can be explained, by analogy with the similar dependence for \(F\)-centers, by the dual distribution of the impurity in the crystal: the location of the impurity at sites of the crystal lattice and at the boundaries of grains and blocks [8]. From the obtained dependence it is seen that the first stage of accumulation of \(M\)-centers depends on the amount of impurity that has entered

Fig. 3. Accumulation curves of \(M\)-centers in KCl crystals with different Sr impurity contents.

in the sites of the crystal lattice. The presence of the first stage of rapid accumulation of \(M\)-centers indicates the formation, at the initial moment of irradiation of the crystal, of a high local concentration of \(F\)-centers, which is probably due to a high local concentration of impurity.

Fig. 4. Dependence of the density of \(M\)-centers on the concentration of Sr impurity in a KCl crystal under isodose irradiation \(40 \cdot 10^7\) erg/cm\(^2\).

Fig. 5. Dependence of the density of \(M\)-centers on the concentration of Sr impurity in a KBr crystal under isodose irradiation \(40 \cdot 10^7\) erg/cm\(^2\).

The mechanism of formation of \(M\)-centers described above assumes, as the principal act of the process, the directed migration of \(\alpha\)-centers. In crystals containing divalent impurities of alkaline-earth metals, strong permanent dipoles are formed—impurity–cation vacancy—which can affect the migration of \(\alpha\)-centers, i.e., the condensation of \(F\)-centers. Since the presence of impurity in the crystal leads to an enhancement of the condensation of \(F\)-centers, whose generation occurs without connection with the impurity, this process leads to an increase in the rate of accumulation of \(M\)-centers in the second stage.

Thus, the alkaline-earth-metal impurities introduced into the crystal actively participate in the process of creation and accumulation of \(M\)-centers in alkali-halide crystals under irradiation.

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