THERMAL STABILITY OF COLOR CENTERS FORMED AT LIQUID NITROGEN TEMPERATURE IN KCl AND KBr SOLID-SOLUTION CRYSTALS

Yu. M. Annenkov, V. F. Pichugin, T. S. Frangulyan

(Presented by the scientific seminar of the Electrophysical Laboratory of the Research Institute of Nuclear Physics, TPI)

The thermal stability of color centers depends on the temperature at which they were formed. The stability of \(F\)-centers accumulated at liquid nitrogen temperature is determined by the thermal stability of hole color centers [1, 2]. Therefore, the low-temperature stages of thermal bleaching of solid-solution crystals provide information on the influence of hole relaxation processes on the stability of \(F\)-centers.

In KCl—KBr solid solutions, hole color centers have a molecular structure the same as in KCl and KBr, but in their composition they include anions of different chemical nature, \(\mathrm{Cl}^{-}\), \(\mathrm{Br}^{-}\). Such centers are called heteromolecular [3—5]. Their properties in solid solutions have been studied very little. At the same time, the structural features of heteromolecular centers may lead to a substantial change in a number of their important characteristics. It is already known from the literature [6, 4] that \(V_k\)-centers of the \(\mathrm{ClBr}^{-}\), \(\mathrm{FCl}^{-}\), \(\mathrm{EBr}^{-}\) type have high thermal stability.

In the present work the thermal dissociation of \(F\)- and \(V_1\)-centers in solid solutions of the KCl—KBr system was studied. The color centers were created by X-rays at liquid nitrogen temperature. The investigation was carried out by the methods of thermal bleaching and thermally stimulated luminescence. During linear heating of the irradiated crystal, we simultaneously recorded the change in optical absorption at the maximum of the \(F\)- and \(V_1\)-bands.

The thermal bleaching curves of the \(V_1\)- and \(F\)-bands for KBr crystals and a number of solid solutions are shown in Fig. 1. As can be seen, the destruction of \(V_1\)-centers both in the components and in the solid solutions proceeds according to a single-stage law. The absence of multistage behavior in the course of nonisothermal relaxation of \(V_1\)-absorption in solid solutions indicates the close stability of the different isostructures of heteromolecular \(V_1\) color centers.

It follows from the data of Fig. 1 that in the temperature region of decomposition of the \(V_1\)-centers there also occurs a strong decrease of the \(F\)-band. Detailed studies of the relaxation phenomena occurring upon heating irradiated crystals, performed by I. K. Vitol [7] and V. E. Zirap [8], make it possible to explain the observed transformations in the spectrum as the result of the following reactions:

\[ V_1+\kappa T \to X^{\circ}, \]

\[ X^\circ + F \to R, \]

where \(X^\circ\) is an interstitial halogen atom,
\(R\) is an ideal lattice site.

Thus, when the crystal is heated, interstitial halogen atoms delocalize from \(V_1\)-centers; these atoms destroy \(F\)-centers and restore an ideal site in the lattice.

Fig. 1, a, b, c. Thermal bleaching of the \(F\) = (1) and \(V_1\) = (2) absorption bands in KCl–KBr crystals

By the method of graphical differentiation we determined the temperatures corresponding to the maximum rate of destruction of \(F\)- and \(V_1\)-centers in KCl–KBr crystals of different compositions; these are listed in the table.

The temperature obtained for delocalization of \(V_1\)-centers in KBr (\(-160^\circ\)C) agrees well with the known literature data [1, 8]. For KCl, according to the literature data, the temperature of destruction of \(V_1\)-centers is \(-140^\circ\)C. In our experiments it was equal to \(-123^\circ\)C. For all crystals, good agreement is observed in the temperature region corresponding to the maximum rate of destruction of \(V_1\)- and \(F\)-centers.

As can be seen from the data presented in the table, the annealing temperature of \(V_1\)- and \(F\)-centers, depending on the chemical composition of the solid solution, changes according to a law close to linear.

The annealing temperatures of \(V_1\)-centers in solid-solution crystals estimated by the method of thermal bleaching are certain averaged characteristics over all isostructural \(V_1\)-centers, whose stability does not differ very greatly. We attempted, using the more sensitive method of thermally stimulated luminescence, to determine the destruction temperatures of heteromolecular \(V_1\)-centers of coloration.

Figure 2 presents curves of thermally stimulated luminescence of KBr crystals and of several compositions of solid solutions. The low-temperature luminescence peak (\(-160^\circ\)C) of KBr falls in the temperature region of destruction of \(V_1\)-centers (Fig. 2).

In the solid solution 10KCl—90KBr (Fig. 2), the maximum at \(-155^\circ\)C is not elementary. On the high-temperature side, another weak peak appears at \(-145^\circ\)C, closely overlapping the main one. This result can be explained by the following ob—

distinct. In the solid solution 10KCl—90KBr, when the concentration of Cl⁻ ions is not very large, \(V_1\)-centers consisting exclusively of Br ions are formed predominantly. A smaller fraction is made up of \(V_1\)-centers that include Cl anions. Therefore, the main luminescence peak at \(-155^\circ\)C in the 10KCl—90KBr crystal is probably associated with the destruction of \(V_1\)-centers consisting of bromine anions, since its position agrees satisfactorily with the low-temperature luminescence peak of KBr. The temperature \(-145^\circ\)C, in all likelihood, corresponds to the delocalization temperature of heteromolecular \(V_1\)-centers. This is also indicated by the thermoluminescence curves of crystals of other solid-solution compositions. With increasing content of Cl⁻ ions in the KBr lattice, the probability of formation of heteromolecular \(V_1\)-centers increases. For this reason, the \(-160^\circ\)C peak in a 25KCl—75KBr crystal is suppressed, and the luminescence in the region of \(-145^\circ\)C increases significantly. For a crystal of a solid solution of equimolar composition, in which the probability of formation of heteromolecular \(V_1\)-centers is greatest, the broad luminescence maximum at \(-145^\circ\)C becomes the principal one (Fig. 2).

Fig. 2. Curves of nonisothermal relaxation of optical absorption at the maximum of the \(V_1\)-band (1), and curves of thermally stimulated luminescence (2) for crystals of KBr, 10 KCl—90 KBr, and 50 KCl—50 KBr.

The considerable broadening of this maximum in comparison with the luminescence peak at \(-160^\circ\)C in KBr is probably connected with the influence of different isostructures of dihole \(V_1\) color centers.

Similarly, when Br⁻ ions are introduced into KCl, there is a decrease in the luminescence at \(-123^\circ\)C, caused by the destruction of \(V_1\)-centers consisting only of Cl anions, and an increase in the peak in the temperature region of \(-140^\circ\)C, associated, as we suppose, with the delocalization of heteromolecular \(V_1\)-centers.

Thus, the thermal stability of heteromolecular \(V_1\)-centers is somewhat higher than the stability of \(V_1\)-centers consisting of Br⁻ anions, but lower than the stability of \(V_1\)-centers consisting of Cl⁻ anions. This is easy to understand. Indeed, an interstitial impurity atom Cl°, having a small radius in the KBr lattice, should be more stable than a Br° atom in an interstitial site of the host substance. Conversely, an interstitial impurity atom Br° in the KCl lattice, because of its large size, should be less stable than an interstitial atom of the host substance.

It follows from all of the foregoing that, in terms of stability, the \(V_1\)-centers, and likewise the \(F_1\)-centers, at low temperatures in solid solutions occupy an intermediate position between the stability of the corresponding centers in the components KCl and KBr.

Table

Destruction temperatures of \(F\)- and \(V_1\)-centers
in KCl—KBr solid-solution crystals
according to thermo-bleaching data

REFERENCES

1. M. L. Kats. Luminescence and electron-hole processes in photochemically colored crystals of alkali-halide compounds. Saratov, 1960.

2. I. H. Schulman, W. D. Compton. Color Centers in Solids, Pergamon Press, Oxford (1964).

3. A. Smakula, N. C. Maynard, A. Repucci. Phys. Rev., 130, 113 (1963).

4. D. Schoemaker, C. Delbec, P. H. Yuster. Bull. Am. Phys. Soc., 9, 629 (1964).

5. D. Schoemaker. Phys. Rev., 149, 2 (1966).

6. A. A. Pung, Yu. Yu. Khaldre. Izv. AN SSSR. “Fizika,” 30, 1433, 1966.

7. I. K. Vitol. Abstract of Candidate’s dissertation. Riga, 1969.

8. V. E. Zirat. Candidate’s dissertation. Riga, 1968.