GENERATION OF $F$-CENTERS IN ALKALI-HALIDE CRYSTALS DOPED WITH RARE-EARTH AND ALKALINE-EARTH METALS

I. Ya. Melik-Gaikazyan, L. A. Chervyakova, V. M. Lisitsyn

Despite intensive study of the problem of creating point defects in alkali-halide crystals under the action of ionizing radiation, at present there is still no unified opinion concerning the mechanism by which such defects are produced. Among the proposed mechanisms, the formation of anion vacancies by displacement of halide ions from normal lattice sites appears to be a quite probable process and is confirmed by a number of experimental works [1]. Displacement of halide ions occurs as a result of double ionization of an ion or single ionization of two neighboring ions. The Frenkel defect thereby formed at liquid-helium temperature can be transformed into an $F$-center and an $H$-center. Hayes and Nichols [2] showed that at sufficiently high temperatures, approximately $-100^\circ\mathrm{C}$ for KCl, halide atoms are displaced into existing cation vacancies. In this case anion vacancies are formed which, by localizing an electron on themselves, can become $F$-centers. Hole centers of the type $X_2^{-}$ are formed in parallel (a molecular halide ion oriented along the (100) axis). It is obvious that displacement of halide atoms into cation vacancies is energetically more favorable than into interstices. Therefore, in crystals containing a large number of cation vacancies, the rate of formation of anion vacancies is higher. In the spectra of irradiated alkali-halide crystals at room temperature, mainly $F$-, $M$-, $V_2$-, and $V_3$-bands have been detected. According to the currently accepted models of color centers, $V_2$- and $V_3$-centers are molecular halide ions of the type $X_3^{-}$ and $X_3^{--}$, respectively, with one of the halides located in a cation vacancy. Consequently, it may be assumed that even at room temperature displacement of a halide into a cation vacancy takes place (similarly to the displacement of a hole in the molecular halide ion $X_2^{-}$—according to Hayes [2]), leading to the formation of $V_2$- and $V_3$-centers.

The present work is devoted to an experimental verification of this assumption.

It is known that the number of cation vacancies in alkali-halide crystals can be regulated by introducing into the crystal cation-substituting ions of various metals. Indeed, replacing monovalent cations in regular lattice sites, divalent and trivalent impurity ions, from the condition of maintaining electroneutrality,

introduce cation vacancies into the lattice, i.e., we assume that the excess positive charge is compensated by the appearance of additional cation vacancies.

Thus, by changing the number of cation vacancies in the crystal, one can change the conditions for generation of anion vacancies as well. The rate of generation of anion vacancies can be calculated from the experimental accumulation curves of \(F\)-centers. For this purpose, in the present work we investigated the kinetics of accumulation of \(F\)-centers in pure crystals and crystals doped with divalent alkaline-earth metals and triply charged rare-earth ions.

The investigations were carried out on the systems NaCl, NaBr, KCl, KCl·Sr, KBr·Sr, NaCl·Ca, NaBr·Ca, NaCl·Nd, NaCl·Ce, KCl·Nd. A cyclotron accelerating protons to energies of 4.5 MeV was chosen as the source of ionizing radiation. Irradiation and measurement of the absorption spectra on an SF-4 spectrophotometer were carried out at room temperature. The concentration of the alkaline-earth-metal impurity in the crystal was determined by a complexometric method using an FEK-56 nephelometer colorimeter.

The accumulation curves of \(F\)-centers in a KBr crystal with an admixture of \(\mathrm{Sr}^{2+}\) are shown in Fig. 1. The final linear segment of the curve characterizes the accumulation of \(F\)-centers formed at vacancies created under irradiation and is determined by the relation [3].

\[ a^{*}=\frac{ac}{c+\beta}, \]

where \(a^{*}\) is the rate of accumulation of centers, determined by the angle of inclination of the final linear segment of the curve;

\(a\) is the rate of generation of anion vacancies;

\(c\) is the rate of capture of electrons by anion vacancies created under irradiation;

\(\beta\) is the rate of radiation destruction of centers.

As is seen from the experimental results presented in Fig. 1, the rate of accumulation of \(F\)-centers created at vacancies generated by irradiation is greater in doped crystals than in pure ones. This is possible when the rate of generation of anion vacancies is increased, or when the rate of radiation destruction of \(F\)-centers is decreased. The rate of electron capture by vacancies is almost independent of the content of alkaline-earth metals in the crystal. By calculating, by the iteration method, the values of the parameters of the Mitchell expression describing the kinetic curve of \(F\)-center accumulation, we were able to show that in doped crystals the rate of generation of anion vacancies is higher. For example, in a KBr \(+\) 0.023 mole % \(\mathrm{SrBr}_{2}\) crystal the rate of generation of anion vacancies is 4.75 times greater than in pure KBr at an irradiation intensity of \(4.08\cdot 10^{7}\) erg/cm\(^2\) min, and 3.37 times greater at an irradiation intensity of \(2.04\cdot 10^{7}\) erg/cm\(^2\) min [4]. Thus, the increase in the rate of accumulation of \(F\)-centers on the linear segment of the accumulation curve in doped crystals can be explained mainly by an increase in the rate of generation of anion vacancies. An increase in the rate of formation of anion vacancies was also found in \(\mathrm{Sr}^{2+}\)-doped KCl crystals.

The kinetics of accumulation of \(F\)-centers in NaCl·Ca (Fig. 2) and NaBr·Ca crystals has a somewhat different form. The slope of the linear segment of the curve is almost the same for pure and doped crystals. This makes it possible to conclude that the generation rate in crystals containing \(\mathrm{Ca}^{2+}\) and in pure crystals is approximately the same. Thus, the introduction of alkaline-earth-metal impurities into KCl and KBr crystals increases the rate of generation of anion vacancies, whereas in NaCl crystals

and NaBr with different impurity contents this rate remains constant. An explanation of this phenomenon can be obtained from consideration of the geometrical parameters of the crystal. As shown by Rabin and

Fig. 1. Kinetics of accumulation of \(F\)-centers in crystals:
1—KBr, 2—KBr \(+ 0.0026\%\) mol. SrBr\(_2\), 3—KBr \(+ 0.0094\%\) mol. SrBr\(_2\), 4—KBr \(+ 0.017\%\) mol. SrBr\(_2\), 5—KBr \(+ 0.028\%\) mol. SrBr\(_2\).

Click [5], the efficiency of formation of \(F\)-centers at the temperature of liquid helium is determined to a considerable extent by the ratio of the sizes of the halide ion and the interstitial site, since in this case displacement of

Fig. 2. Kinetics of accumulation of \(F\)-centers in crystals:
1—NaCl, 2—NaCl \(+ 0.007\%\) mol. CaCl\(_2\), 3—NaCl \(+ 0.016\%\) mol. CaCl\(_2\), 4—NaCl \(+ 0.030\%\) mol. CaCl\(_2\), 5—NaCl \(+ 0.046\%\) mol. CaCl\(_2\).

the halide into an interstitial site occurs. It is possible that in KCl and KBr crystals, during irradiation, displacement of halide atoms into cation vacancies occurs, whereas in NaCl crystals this process is hindered, since the efficiency

such a process depends on the ratio of the radii of the anions and cations. As is seen from Table 1 [6], the ratio of the anion and cation radii is much greater for NaCl and NaBr than for KCl and KBr; i.e., the conditions for displacement of a halide into a metal vacancy in NaCl and NaBr are less favorable, which also explains the insignificant effect of large concentrations of cation vacancies on the generation of anion vacancies in these crystals. In studying the kinetics of accumulation of \(F\)-centers in NaCl crystals doped with \(\mathrm{Nd}^{3+}\) and \(\mathrm{Ce}^{3+}\), a significant change was observed

Table 1

Ratio between the radii of anions and cations in alkali-halide crystals

in the rate of accumulation of \(F\)-centers as compared with pure crystals. From the results presented in Figs. 3a, 3b it is seen that the rate of accumulation of \(F\)-centers is higher in crystals containing small amounts of rare-earth-metal impurity, and lower in crystals with large additions of rare earths, as compared with pure NaCl.

Indeed, if the geometrical factors of the lattice play an important role in the process of generation of anion vacancies, the displacement of a halide into a cation vacancy in a NaCl crystal doped with rare-earth

Fig. 3a. Kinetics of accumulation of \(F\)-centers in crystals:
1 — NdCl, 2 — NdCl + 0.01 mol.% \(\mathrm{NdCl}_3\), 3 — NdCl + 0.5 mol.% \(\mathrm{NdCl}_3\)

metals is more probable than in NaCl doped with alkaline-earth metals, since trivalent rare-earth ions introduce with them at once pairs of cation vacancies. The stresses arising when a halide is displaced into a cation vacancy are partly compensated by the neighboring vacancy of a positive ion. It should be noted that the rate of electron capture by anion vacancies created by irradiation-

vacancies \(c\) cannot be taken as a constant quantity, as in the case of NaCl·Ca, since rare-earth ions are good traps

Fig. 3b. Kinetics of the accumulation of \(F\)-centers in crystals: 1—NaCl, 2—NaCl + 0.1 mol % CeCl\(_3\), 3—NaCl + 0.01 mol % CeCl\(_3\).

for electrons; the magnitude of the capture rate can only decrease more or less appreciably as a result of the acceptor properties of the dopant, depending on the amount of activator contained in the crystal. Consequently, a change in \(a^*\), which depends on several radiation parameters, is possible both as a result of a change in the rate of accumulation of anion vacancies \(a\), and of an increase in the radiation stability of \(F\)-centers, i.e., a decrease in \(\beta\), since recombination of electrons and holes, competing with localization of electrons at anion vacancies, will be hindered because of the predominance of the process of localization of electrons on impurity ions (at a high activator content, \(\sim 0.5\) mol %). Similar results were also obtained for KCl·Nd crystals (Fig. 3c). The concentration of rare-earth elements is indicated for the melt.

Fig. 3c. Kinetics of the accumulation of \(F\)-centers in crystals: 1—KCl, 2—KCl + 0.3 mol % NdCl\(_3\), 3—KCl + 0.1 mol % NdCl\(_3\), 4—KCl + 0.05 mol % NdCl\(_3\).

Thus, the presence of a large amount of metallic heterovalent impurities in the crystal is by no means a sufficient condition for increasing the generation of anion vacancies.

References

1. V. Ritz, Phys. Rev. 133, 5A, 1964.
2. W. Hayes, Y. M. Nichols, Phys. Rev. 117, 993, 1960.
3. P. V. Mitchell, D. A. Wiegand, R. Smoluchowski, Phys. Rev. 121, 484, 1961.
4. V. M. Lisitsyn, I. Ya. Melik-Gaikazyan. Radiation generation of vacancies in alkali-halide crystals doped with strontium. Izv. vuzov (in press).
5. H. Rabin, C. C. Klick, Phys. Rev. 117, 1005, 1960.
6. K. Tharmalingam. J. Chem. Phys. Solids, 25, 2, 1964.