STORED ENERGY IN ALKALI HALIDE CRYSTALS WITH AN IMPURITY

A. P. Aryanov

(Presented by the scientific seminar of the Laboratory of Energetics and Electronics of Ionic Structures)

At present, the effect of radiation on alkali halide crystals at low absorbed energies has been well studied.

Because of a number of technical difficulties, measurement of optical-absorption spectra in heavily irradiated alkali halide crystals is difficult. We believe that, in this field of radiation damage, the application of the stored-energy method to the study of defects in solids is very promising.

According to current concepts [1, 2, 3], the main contribution to the magnitude of the stored energy is made by electron-hole centers. Many works [4, 5, 6, 7, 8, 9, 10] have been devoted to studying the influence of impurities on the kinetics of accumulation of color centers at relatively small irradiation doses. It has been shown that the introduction into crystals of divalent cation-substitutional impurities causes the appearance of an additional number of cation vacancies [9] in order to preserve the electroneutrality of the crystal. The addition of Sr\(^{++}\) and Ca\(^{++}\) impurities to alkali halide crystals creates additional levels for hole localization and causes an increase in the rate of accumulation of \(F\)-centers not only at “pre-radiation” vacancies, but also at vacancies generated by irradiation [11]. Therefore, it is of definite interest to investigate the influence of impurities on the magnitude and character of the accumulation of stored energy in alkali halide crystals.

As shown in [5], the introduction into crystals of a divalent anion-substitutional impurity S\(^{--}\) increases the number of anion vacancies, which, according to the law of mass action, decreases the concentration of single cation vacancies. This is evidenced by a decrease in the electrical conductivity of KCl·S\(^{-}\) crystals in comparison with pure KCl. An increase in the number of anion vacancies leads to a decrease in radiation stability and in the rate of accumulation of \(F\)-centers at pre-radiation defects in KCl·S\(^{--}\) crystals.

We know of only one paper, by Trik [12], in which the author investigated the influence of Ca\(^{++}\) impurity on the magnitude of the stored energy in NaCl crystals after X-ray irradiation. Unfortunately, these investigations are purely qualitative in character.

In the present work, the cyclotron of the Research Institute of Nuclear Physics at TPI was used as the irradiation source, with a proton energy of 4.5 MeV. Irradiation

the specimens was carried out in air. To prevent bleaching, the specimens were wrapped in aluminum foil 7 microns thick. The stored energy was measured by the dissolution method in a differential Calvet-type calorimeter. The kinetics of accumulation of stored energy in crystals of KCl, KCl·OH$^-$, and KCl·S$^{--}$ were investigated (Fig. 1, curves 1, 2, 3, respectively). It is seen from the figure that the stored energy of KCl·S$^{--}$ and KCl·OH$^-$ crystals is greater than that for pure KCl.

KCl·S$^{--}$ crystals (0.1 mol% Na$_2$S), as mentioned above, have an excess number of anion vacancies. At high irradiation doses, apparently, the predominant process is the generation of radiation defects by the Varley mechanism [13], according to which the halide is displaced from regular lattice sites into interstices. But alkali-halide crystals are materials with close packing. Therefore, displacement of the halide from regular sites of the anion sublattice will be considerably facilitated if conditions are created for the diffusion of chlorine into defective regions of the crystal (dislocations, mosaic blocks). In this case the formation of $F$-centers at vacancies created by irradiation should increase. As a result, the stored energy in KCl·S$^{--}$ crystals should be higher than in pure KCl, as is seen from the figure (curves 1, 2). For KCl crystals with a KOH impurity (0.35 mol%) the stored energy is also greater than for pure KCl. This may be due to the following.

Fig. 1. Kinetic curves of accumulation of stored energy in KCl, KCl·OH$^-$, and KCl·S$^{--}$ crystals (curves 1, 2, 3, respectively)

As shown in [14], OH$^-$ ions are incorporated into regular sites of the crystal lattice, replacing Cl$^-$ ions. Upon cooling of the grown crystal, OH$^-$ ions form with Ca$^{++}$ ions, which are always present in crystals as contamination, a Ca$^{++}$(OH$^-$)$_2$ complex occupying one anion lattice site. The resulting excess number of anion vacancies changes the sign of the dislocation charge to the opposite one. This fact may serve as evidence that in our KCl·OH$^-$ crystals there is an excess number of anion vacancies, which facilitate the diffusion of chlorine displaced from a lattice site by the Varley mechanism. This, apparently, also accounts for the increase in stored energy at high irradiation doses in KCl·OH$^-$ crystals in comparison with pure KCl.

The stored energy in KCl crystals is released along a curve with a maximum. Moreover, for “pure” crystals the maximum is shifted toward lower values, and for KCl·S$^{--}$ toward higher absorbed energies. The maximum value of the stored energy in KCl is 1.7 J/g; for KCl·OH$^-$, 2 J/g; for KCl·S$^{--}$, 2.4 J/g.

Conclusions

1. The stored energy in KCl crystals is released along curves with a maximum.

2. The introduction of anion-substituting impurities increases the stored energy in comparison with “pure” crystals.

References

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