SOME DATA ON LATTICE RECOVERY PROCESSES IN ALKALI HALIDE CRYSTALS SUBJECTED TO RADIATION DECOMPOSITION

Yu. F. Kozlov

A sign of partially irreversible decomposition under irradiation may be considered, if the photometric method is used, to be the appearance, at a definite stage of heating, of a colloidal band in the additional-absorption spectrum that is stable up to high temperatures. According to this criterion, among the series of crystals NaCl, KCl, and RbCl, NaCl has the greatest tendency toward decomposition. Among the series of crystals KCl, KBr, KI, KI decomposes most readily, while of the two crystals NaBr and KBr, NaBr is subject to decomposition to a greater degree. In KCl no effects associated with decomposition were observed, even when, all other conditions being equal, the exposure dose exceeded by tens of times the threshold exposure dose for NaBr. It follows from the above that the stability of crystals against the action of reactor irradiation is determined by a more complex regularity than a direct dependence on the lattice energy per ion pair. Radiolysis is not a special consequence of reactor irradiation alone. The advantage of a reactor over other sources in the respect under consideration consists in the ease of obtaining large absorbed doses uniformly throughout the volume of the crystal.

The material presented below concerns elucidation of the general state of some alkali halide crystals after intense reactor irradiation and the development of certain questions touched upon in [1, 2]. The work was carried out on crystals grown from melts of relatively pure salts. Crystals with impurities of interest were also used. Irradiation was carried out at temperatures in the range 15–30°C.

Radiolysis and Impurity Color Centers in Potassium Chloride

In the series under consideration, potassium chloride crystals may be regarded as the most stable against the action of irradiation. Colloidal coloration was not observed in them under any conditions.

In crystals with sulfur (Fig. 1), the appearance of intense absorption in the band with a maximum at 760 nm should be noted. The band forms upon irradiation with integral fluxes above \(3 \cdot 10^{18}\) neutrons/cm\(^2\). In the light it does not change upon heating up to 190°C. At temperatures somewhat above the indicated one, absorption in this region weakens, and two bands with maxima at 720 and 825 nm are resolved. It may be assumed that in the initial state as well the new band represents the sum of the R- and M-bands.

The observed change in the ratio of the concentrations of atomic and complex electron centers does not contradict the established view concerning the action of an excess of divalent anions on the secondary processes accompanying defect generation under irradiation. An increase

Fig. 1. Comparison of the spectra of additional absorption of irradiated potassium chloride crystals with impurities. Dose \(1.6 \cdot 10^{19}\) n/\(\text{cm}^2\). Solid curves are the initial spectra; dashed curves are after bleaching. 1 and \(1'\)—KCl\(_{\text{chem. pure}}\); 2 and \(2'\)—KCl\(_{\text{chem. pure}}\) + 0.1 mol % Na\(_2\)S; 3 and \(3'\)—KCl\(_{\text{chem. pure}}\) + 0.2 mol % SrS\(_2\). 4, 5, 6, and 7 are successive stages of bleaching of KCl\(_{\text{chem. pure}}\) + Na\(_2\)S upon heating to \(250^\circ\)C.

in the total number of electrons localized on single vacancies and vacancy complexes may indicate improved conditions for the removal of atomic halogen during irradiation.

KCl crystals with additions of calcium, strontium, and especially lead are colored only weakly by \(F\)-centers. The character of bleaching in this case is similar to the character of bleaching of crystals after irradiation with accelerated electrons [3]. The electrons released into the light at room temperature are expended equally on the destruction of \(V_2\)-centers and the formation of \(Z_1\)-centers (Fig. 2). Slight heating intensifies the transition \(F \to Z\). The destruction of \(Z_1\)-centers at \(160\text{--}170^\circ\)C is accompanied by a luminescence flash and by the disappearance of the remainder of the \(V_2\)-band. After complete bleaching and annealing at temperatures below \(500^\circ\)C, the absorption spectrum of such crystals can be restored qualitatively and quantitatively by relatively weak \(\gamma\)-irradiation.

Cooling a crystal with \(Z_1\)-centers from temperatures above \(100^\circ\mathrm{C}\) leads to the formation of \(F\)-centers; in the light, at room temperature, \(Z_1\)-centers are again formed. After high-temperature irradiation, regions exist in the crystals without \(V_2\)-centers, in which the transitions just described can be repeated many times. From the nature of the observed transitions it may be assumed that they occur within a single complex consisting of a divalent cation with a cation vacancy and at least one pair of vacancies. The complex is stable in the temperature range characterized by low mobility of anion vacancies. The increase in the concentration of actively acting complexes is a consequence of reactor irradiation.

Fig. 2. Interaction of \(F\)-, \(V_2\)-, and \(Z_1\)-centers in KCl contaminated with calcium under light. 1 — spectrum of crystals after reactor irradiation and spectrum of the same crystals after annealing below \(500^\circ\mathrm{C}\) and additional \(\gamma\)-excitation. 2 — \(Z_1\)-band. Points between 1 and 2 — position of the absorption maximum after equal doses of light irradiation. 3 — spectrum of a control specimen after \(\gamma\)-excitation.

Usually \(Z\)-centers are observed and studied in additively colored KCl crystals quenched from \(400^\circ\mathrm{C}\). Apparently, a necessary condition for the formation of \(Z\)-centers, in addition to the presence of a dissociated complex—divalent cation–cation vacancy—and of a free electron, is a certain supersaturation of the crystal with vacancies. Up to a certain limit, the excess vacancies turn out to be bound to the cation complex.

The action of reactor irradiation on crystals with divalent impurity cations, in the aspect under consideration, amounts to creating all the necessary conditions for the formation of \(Z\)-centers.

Optical properties of irradiated potassium and sodium bromide crystals

In the initial state, the absorption in irradiated crystals is due to the presence of electron centers; an insignificant fraction of which—

... centers are destroyed under the action of light. The fraction of electronic centers stable to light up to 200°C increases with increasing irradiation dose in the reactor. At low doses, the subsequent bleaching differs little from that described for KCl.

In NaBr crystals irradiated with an integrated flux above \(2.3 \cdot 10^{18}\) neutrons/cm\(^2\), destruction of the electronic bands upon heating to 200°C is accompanied by the formation of a new electronic band with a maximum at 600 nm. Upon heating to 380°C, the absorption in this part of the spectrum increases, and the absorption maximum shifts continuously to 580 nm. At temperatures above 380°C, the absorption maximum begins to shift into the long-wavelength region of the spectrum. The shift of the absorption into the short-wavelength region of the spectrum is accompanied by substantial changes in absorption in the regions of 300 and 400 nm. Analogous transformations in the spectrum of KBr (Fig. 3) are observed after irradiation with integrated fluxes above \(1 \cdot 10^{19}\) neutrons/cm\(^2\). From comparison of spectra 3 and 4

Fig. 3. Colloidal and impurity absorption bands in potassium bromide. Dose \(1.6 \cdot 10^{19}\) neutrons/cm\(^2\). 1 — initial spectrum, 2 — after heating to 220°C, 3 — 380°C, 4 — after one hour at 380°C, 5 — after one hour at 420°C, 6 — 450°C.

it follows that the thermal destruction of the centers responsible for absorption in the 570 nm band is accompanied by the formation of intrinsic electronic centers, which upon further heating coagulate into colloidal particles.

Related absorption bands at 570 nm in KBr and 580 nm in NaBr arise at a certain stage of heating in all sufficiently strongly irradiated crystals, including crystals with impurity sulfur anions. The introduction of calcium and strontium ultimately weakens the colloidal bands formed in these crystals at temperatures above 380°C; absorption is also weakened at the preceding stages of heating. An intense absorption band with a maximum

570 nm is observed in KBr crystals with an addition of 0.5 mol.% Na₂S upon heating after irradiation with a flux of \(3 \cdot 10^{19}\) neutrons/cm². In this case, the centers giving the 570-nm band, in the temperature interval 200—380°C, are in dynamic equilibrium with the \(F\)-centers of thermal origin. Heating the crystal above the indicated limit leads to an irreversible transformation of the new centers into \(F\)-centers.

In KBr crystals with a strontium addition, a broad band arises with a maximum at 570 nm; on its short-wavelength slope, when the temperature of photometry is lowered, one more band is resolved. In this case the maximum of the principal absorption band does not change position as the temperature is lowered, i.e., it behaves like a colloidal absorption band. The change in the spectrum caused by the introduction of an excess of divalent cations makes it possible to suppose that the absorption bands at 570 and 580 nm in KBr and NaBr are due to the formation of color centers on impurity cations.

In the crystals considered, passage through the temperature boundary 380—400°C is characterized by a relative increase in the mobility of anion vacancies, intensification of diffusion processes and, as a consequence, a decrease in the supersaturation of the crystal with vacancies. One may arrive at a conclusion analogous to that reached in considering \(Z\)-centers in potassium chloride: the centers are formed and exist in the presence of a certain supersaturation of the crystal with vacancies. The validity of this conclusion is confirmed by the effect of boundary bleaching in bromides, observed in the optimum temperature interval for the existence of \(Z\)-centers. The regularity of the displacement of the bleaching boundary into the depth of the crystal coincides with the regularity of displacement of the surface of equal supersaturation with vacancies during isothermal annealing [4].

Only part of the light scattering observed at temperatures above 380°C can be explained by scattering on growing colloidal metal particles. Electrons participating, at a certain stage of crystal heating, in the formation of the metallic phase can be removed by the action of a constant electric field. When a crystal bleached in this way is heated, the scattering intensity continues to increase. At temperatures above 380°C, colloidal coloration is also removed under the action of a constant field. By alternating isothermal high-temperature annealing in a field and without a field in one and the same crystal, one can obtain a series of zones differing in scattering intensity. The scattering capacity of each zone is the greater, the longer, during annealing, it was in the colored state. The facts presented establish the existence of interaction between absorbing and scattering centers and the possibility of their separation. Light scattering occurs on microscopic pores, which after prolonged annealing become visible at low magnification.

Thus, for the crystals considered, the temperature region above 380°C is the region of formation and interaction of the metallic and void phases. In the central region of the annealed crystal, supersaturation with vacancies can be maintained through dissolution of pores and metallic particles whose sizes are smaller than critical. Near the crystal boundary, which is an unlimited-volume sink for vacancies, the equilibrium shifts toward a decrease in the sizes of particles and pores and toward their complete dissolution. This constitutes the general explanation of the effect of boundary bleaching observed during annealing of crystals subjected to radiation decomposition.

Generalization

The sharp difference in the resistance of the crystals being compared to the action of reactor irradiation is explained by the difference in the mobility of the decomposition products. Since the final state of the crystal is associated with the diffusional removal of the decomposition products, one might have expected a noticeable influence of the crystal boundaries on the irradiation results. Massive samples irradiated at room temperature prove to have undergone decomposition uniformly throughout the entire volume. Raising the irradiation temperature leads to the appearance of a nonuniform distribution of single and aggregated defects associated with the boundaries of the sample. In this case phenomena are observed that are, in the main, similar to the phenomena accompanying the evaporation of zinc from brass [4]. In the region of the maximum supersaturation by vacancies, located near the boundary of the sample, in the general case, including the case of potassium chloride, a series of densely colored zones is observed. After thermal bleaching, accompanied by the transformations described above, the region of the maximum scatters light intensely.

During the processes of lattice recovery of irradiated crystals upon annealing, three stages are distinguished. In the first stage, optically undetectable traps are filled with electrons of the disintegrating electron centers. The concentration of traps effectively acting at temperatures below 200°C is either constant or changes only slightly with increasing irradiation dose.

The second stage is characterized by the capture, by divalent cations, of the electrons remaining after the traps have been filled in the first stage. In some respects this process may be represented as similar to the process of thermal polarization.

The beginning of the third stage is similar to additive coloration. The newly formed \(F\)-centers participate in the formation of the metallic phase. Recovery at this stage is characterized by the interaction of an ensemble of pores and metallic particles. The theoretical treatment of the recovery processes at this stage may be similar to that given in [4].

In conclusion, it may be noted that alkali-halide crystals with impurities irradiated in a reactor constitute an interesting material for the study of impurity centers. The transition \(F \leftrightarrows Z\) in the case of potassium chloride is possible in the presence of an anion vacancy bound to a divalent cation. The bond is preserved throughout the entire temperature interval of low mobility of anion vacancies. Depending on external conditions, the electron belonging to the complex may be localized either on the divalent cation or on the anion vacancy bound to the complex. Apparently, the \(Z\)-center cannot be regarded as an isolated divalent impurity cation with an electron localized on it. In the example of bromide, the dynamic character of the constancy of the concentration of anion vacancies bound to divalent cations is apparent. A decrease in the overall supersaturation by vacancies leads to the destruction of impurity centers.

LITERATURE

1. B. V. Budylin, A. A. Vorob’ev. Action of Radiations on Ionic Structures, Gosatomizdat, 1962.
2. B. V. Budylin, Yu. F. Kozlov. FTT, 6, 6, 1573, 1964.
3. P. Camagni, G. Chiarotti, F. G. Fumi, L. Gialotto. Phil. Mag., 45, 361, 225, 1954.
4. Ya. E. Geguzin, Macroscopic Defects in Metals, Metallurgizdat, Moscow, 1962.