INFLUENCE OF SOME MONOVALENT AND DIVALENT IMPURITIES ON THE RADIATION-INDUCED CHANGE IN THE ELECTRICAL CONDUCTIVITY OF ALKALI-HALIDE CRYSTALS

M. I. IGNAT’EVA, L. I. ROSHCHINA

Changes in various properties of crystals depend on the nature of the defects; for example, the number of isolated cation vacancies determines the conductivity of a crystal, while the number of anion vacancies determines, to a significant degree, its colorability.

In the present work we investigated the influence of structural disturbances caused by foreign ions on the electrical conductivity of alkali-halide crystals both before and after irradiation with X-rays. Pure single crystals and single crystals with an impurity were grown by the Kyropoulos method in air. The method used to measure conductivity was the same as in [1]. The impurity content in the crystal was estimated from chemical-analysis data.

It is known that the electrical conductivity of alkali-metal halides in the temperature range \((20 \div 200)^\circ\mathrm{C}\) in weak fields is determined mainly by the motion of the cation through positive-ion vacancies. It has been established experimentally that, at relatively low concentrations, the impurity forms a substitutional solid solution with the host [2–5]. The addition of divalent metallic impurities to a crystal causes the appearance of an additional number of cation vacancies [6], as follows from considerations of the electroneutrality of the crystals. Figure 1 presents our experimental data on the temperature dependence of conductivity for NaCl, KCl, and KBr crystals with various additive contents. The introduction of \(\mathrm{Pb}^{++}\) into KCl and KBr, \(\mathrm{Sr}^{++}\) into KCl, and \(\mathrm{Cu}^{++}\) into NaCl is accompanied by an increase in electrical conductivity. The slopes of the curves for pure and impurity-containing crystals are the same, which indicates that in this temperature interval the current carriers have the same nature both in the pure crystal and in the crystal with impurity. The dependence of electrical conductivity on the additive concentration in the crystal is shown in Fig. 2. At first, at low concentrations (for example, up to \(0.6 \cdot 10^{-3}\) mol.% \(\mathrm{Pb}^{++}\) in KBr; \(2.5 \cdot 10^{-3}\) mol.% \(\mathrm{Pb}^{++}\) in KCl, etc.), the electrical conductivity increases almost proportionally with increasing impurity content. Beginning with a certain concentration, further addition of impurity to the crystal changes the magnitude of the electrical conductivity only slightly.

A comparison of experimental data on changes in electrical conductivity, optical absorption at the maximum of the \(F\)-band, and microhardness as functions of the additive content in the crystal (for example, for KCl·Sr, Fig. 3) indicates that the magnitude of the conductivity is affected by

Fig. 1. Temperature dependence of the electrical conductivity of single crystals.
a) 1 — KCl; 2 — KCl + \(3.43 \cdot 10^{-3}\) mol % PbCl\(_2\); 3 — KCl + \(35.5 \cdot 10^{-3}\) mol % PbCl\(_2\).
b) 1 — KBr; 2 — KBr + \(0.14 \cdot 10^{-3}\) mol % PbBr\(_2\); 3 — KBr + \(1.67 \cdot 10^{-3}\) mol % PbBr\(_2\).
c) 1 — KCl; 2 — KCl + \(3.25 \cdot 10^{-3}\) mol % SrCl\(_2\); 3 — KCl + \(54.4 \cdot 10^{-3}\) mol % SrCl\(_2\).
d) 1 — NaCl; 2 — NaCl + \(0.5 \cdot 10^{-3}\) mol % CuCl\(_2\); 3 — NaCl + \(1.71 \cdot 10^{-3}\) mol % CuCl\(_2\).
e) NaCl + CuCl (\(\bullet\) — pure crystal, \(\times\) — NaCl crystal + \((0.1 \div 2)\cdot 10^{-3}\) mol % CuCl).
f) NaCl + MnCl\(_2\) and NaCl + CdCl\(_2\) (\(\bullet\) — pure crystal, \(\times\) — NaCl + \(0.5 \cdot 10^{-3}\) mol % MnCl\(_2\), \(\triangle\) — NaCl + \(0.5 \cdot 10^{-3}\) mol % CdCl\(_2\)).

only that portion of the impurity which enters regular sites of the crystal lattice, bringing with it a cation vacancy [1, 7]. However, experiments show that some divalent metallic impurities entering regular sites of the crystal lattice do not produce the expected increase in conductivity (Fig. 1f). By the EPR method it has been established that Mn\(^{++}\) ions enter lattice sites of NaCl and create an excess number of cation vacancies [4], which are bound to a high degree with the impurity ion (0.4 eV). Similarly, for NaCl·Cd crystals it has been established that Cd\(^{++}\) has a large binding energy (0.38 eV) [8] with the cation vacancy it introduces. It follows from this that the change in conductivity in the temperature range \((20 \div 200)^\circ\)C is affected by that divalent metallic impu-

impurity, which enters the sites of the crystal lattice, creating isolated cation vacancies. For additional confirmation of this proposition we studied the influence of a monovalent copper impurity, entering the sites of the crystal lattice [9, 10] without changing the defectiveness of the cation sublattice, on the conductivity of NaCl. Addition of CuCl in the interval

Fig. 2. Dependence of the logarithm of the electrical conductivity of single crystals on the content of divalent impurity, at a temperature of 60°C:
(a) for KCl·Pb; (b) KBr·Pb; (c) KCl·Sr.
Curve 1 is for unirradiated single crystals; curve 2 is for single crystals irradiated with x rays; curve 3 is the change in the logarithm of the conductivity \((\Delta \log \sigma = \log \sigma_{\text{unirr.}} - \log \sigma_{\text{irr.}})\) as a function of impurity content.

Fig. 3. Dependence of
(a) the logarithm of the conductivity of an unirradiated single crystal,
(b) the absorption coefficient at the maximum of the \(F\)-band,
(c) the microhardness of KCl crystals on the strontium content.

of concentrations from \(0.1 \cdot 10^{-3}\) to \(2 \cdot 10^{-3}\) mole% gives no change in conductivity (Fig. 1, e). Analyzing the data on the change in conductivity with impurity content, presented in Fig. 2, one may conclude that introducing the same amount of different divalent impurities into a crystal leads to different changes in conductivity. Thus, for example, \(0.6 \cdot 10^{-3}\) mole% \(Pb^{++}\) in KCl increases the conductivity by 38%, while the same amount of \(Sr^{++}\) in KCl increases it by 17.5%. This probably indicates that some fraction of the strontium ions that have entered the lattice is associated with a cation vacancy.

If, indeed, the conductivity of crystals with an impurity is determined by the presence of isolated cation vacancies, then the different surrounding

the motion of impurity ions in the crystal should also exert a different influence on the radiation-induced change in electrical conductivity. To determine the influence of impurities on the radiation-induced change in resistance, the crystals were subjected to isodose irradiation of the order of \(10^4\) roentgens, under which the principal effect of the radiation is the formation of electron and hole color centers. Localization of holes on those same cation vacancies which, before irradiation, were responsible for ionic conductivity should lead to a decrease in the number of current carriers and, consequently, to a decrease in conductivity. This is confirmed by our experiments. Figure 4 shows the temperature dependence

Fig. 4. Temperature dependence of the electrical conductivity of unirradiated (straight line 1) and X-ray-irradiated (straight line 2) single crystals: \(a\)—NaCl, \(b\)—KCl, \(c\)—KBr.

of the conductivity of unirradiated (curves 1) and irradiated (curves 2) “pure” crystals of NaCl, KCl, and KBr by X-rays at room temperature. The dependence of the electrical conductivity of an unirradiated crystal has the usual rectilinear increase with temperature. The straight-line dependence of electrical conductivity on temperature for irradiated crystals in the range \(\sim(20 \div 200)^\circ\mathrm{C}\) has two breaks at \(\sim(60 \div 80)^\circ\mathrm{C}\) and \(\sim(120 \div 150)^\circ\mathrm{C}\). In the temperature region \(\sim(20 \div 130)^\circ\mathrm{C}\), a decrease in conductivity compared with the unirradiated crystal is observed for all crystals. Thus, for example, at a temperature of \(60^\circ\mathrm{C}\), the electrical conductivity decreased by a factor of 12.5 for NaCl, by a factor of 6.8 for KCl, and by a factor of 4 for KBr. There are two points of view on the mechanism of the decrease in conductivity as a result of the action of ionizing radiation. The first point of view, proposed by Seitz [13], is that, during irradiation, holes become localized on current carriers. The fact that conductivity decreases at \(20^\circ\mathrm{C}\) as a result of irradiation at room temperature by X-rays (for example, NaCl) indicates that, upon irradiation at room temperature, the number of current carriers is indeed reduced, which agrees with Seitz’s point of view. The second point of view, proposed by Smoluchowski [14], is that irradiation creates only traps for current carriers, which begin to act only at temperatures above room temperature. The result we observed—an increase in the difference between the conductivity values of crystals before and after irradiation upon heating the crystal (Fig. 4, region \(20 \div 60^\circ\mathrm{C}\))—is proof-

is evidence that the Smoluchowski mechanism also occurs under the action of radiation at room temperature.

Thus, upon irradiation at room temperature by X-rays, evidently both localization of holes on current carriers and the formation of traps for current carriers take place. When the crystal is heated, the current carrier becomes fixed at a trap, causing the effect of a further decrease in the conductivity of the previously irradiated crystal. The traps mentioned may be excess anion vacancies generated in the crystal by the Varley mechanism.

We have shown experimentally above that an excess number of single cation vacancies leads to an increase in conductivity. A supersaturation content of positive-ion vacancies as a result of the action of radiation should cause a more significant decrease in conductivity than in a pure crystal. This is confirmed, for example, for KCl and KBr crystals activated with lead, $\mathrm{Pb}^{++}$ (comparison of Figs. 4c and 5ab).

In Fig. 2, a, c (curve 3) the dependence is given of the change $\Delta \lg \sigma$ (the difference between $\log \sigma_{\mathrm{unirr.}} - \log \sigma_{\mathrm{irr.}}$) on the lead content in KCl and KBr. From a comparison of curves 1, 2, 3 in Fig. 2, a, c it follows that only that part of the impurity which is incorporated into the sites of the crystal lattice, causing the appearance of single cation vacancies, has an influence on the increase in conductivity of the unirradiated crystal and on the decrease in electrical conductivity of the irradiated crystal. It should be noted that a radiation-induced decrease in conductivity greater than that for a pure crystal is observed at any lead content in the crystal.

The works of Parfianovich [10], Lushchik [11], and Vale [12] established that an impurity in a crystal, depending on its environment, can localize electrons or holes under the action of ionizing radiation. Different environments of an impurity in the lattice exert different influences on the change in conductivity of both unirradiated (Fig. 1) and irradiated crystals (Fig. 5). Since the impurities $\mathrm{Mn}^{++}$, $\mathrm{Cd}^{++}$, and $\mathrm{Cu}^{+}$ introduced into the lattice do not cause the appearance of single cation vacancies (Fig. 1, e, f), as a result of the action of ionizing radiation we should not observe a change in the conductivity of doped crystals in comparison with a pure crystal. This assumption is confirmed experimentally (Fig. 5, e, f).

If conditions are created in the crystal for the localization of holes not only at single cation vacancies, which cause a decrease in conductivity, but also at defects that do not take part in charge transport (for example, a strontium impurity in KCl and copper in NaCl), the effect of the radiation-induced change in electrical conductivity should decrease in comparison with a pure crystal. Figure 5 gives the temperature dependence of the electrical conductivity of $\mathrm{NaCl}\cdot\mathrm{CuCl}_2$ crystals (Fig. 5, d); $\mathrm{KCl}\cdot\mathrm{SrCl}_2$ (Fig. 5, c) (compare Fig. 4, a with Fig. 5, d and Fig. 4, c with Fig. 5, c). At a temperature of $60^\circ\mathrm{C}$ the decrease in conductivity is, for the crystals, $\mathrm{NaCl}$—12.5-fold, $\mathrm{NaCl}\cdot\mathrm{CuCl}_2$—6.0-fold, $\mathrm{KCl}$—6.8-fold, $\mathrm{KCl}\cdot\mathrm{SrCl}_2$—2.8-fold. The change $\Delta \log \sigma$ of KCl as a function of the strontium concentration is given in Fig. 2, c, curve 3. It follows from the figure that the more strontium there is in the crystal, i.e., the more competitors to cation vacancies with respect to hole localization, the less the conductivity of the irradiated single crystal decreases. For $\mathrm{KCl}\cdot\mathrm{Sr}$ crystals we found a hole band with $\lambda_{\max} = 335$ m$\mu$, responsibility for which is attributed to Hayes–Nichols centers [15]. The nature of this band was established by us upon optical destruction by light from the $F$ band, and also by comparing the absorption spectra of photochemically and additively colored-

Fig. 5. Temperature dependence of the electrical conductivity of unirradiated (line 1) and X-ray-irradiated (line 2) single crystals:
$a$ — KCl + $3.43 \cdot 10^{-3}$ mole % PbCl$_2$;
$b$ — KBr + $23 \cdot 10^{-3}$ mole % PbBr$_2$;
$c$ — KCl + $54.4 \cdot 10^{-3}$ mole % SrCl$_2$;
$d$ — NaCl + $0.5 \cdot 10^{-3}$ mole % CuCl$_2$;
$e$ — NaCl + $0.3 \cdot 10^{-3}$ mole % CuCl;
$f$ — NaCl + $5 \cdot 10^{-3}$ mole % MnCl$_2$.

Fig. 6. Absorption spectrum of a KCl + $123 \cdot 10^{-3}$ mole % SrCl$_2$ single crystal at room temperature.
1 — absorption spectrum after X-ray irradiation with a dose of $4 \cdot 10^{8}$ roentgens;
2 — absorption spectrum after bleaching with light from the $F$ band;
3 — absorption spectrum of an additively colored crystal.

crystals (Fig. 6). Using the EPR method [15], it was found that the Hays—Nichols center is a localized molecular ion $\mathrm{Cl}_2^-$ on the $\mathrm{Sr}^{++}$ complex ($\mathrm{Sr}^{++}$ plus a cation vacancy).

In the phosphor $\mathrm{NaCl}:\mathrm{CuCl}_2$, localization of holes occurs not only at cation vacancies, but also at the monovalent copper ion. It is known [9] that, when $\mathrm{CuCl}_2$ is added to the melt, most of the copper enters the crystal in the form of monovalent ions, which apparently explains only the slight increase in the conductivity of NaCl with increasing additive concentration (Fig. 1, d). With small additions of $\mathrm{CuCl}_2$, localization of holes on monovalent copper ions is experimentally confirmed by studies of the absorption spectra of irradiated crystals: in the absorption spectrum (Fig. 7) a band appears with $\lambda_{\max}=220$ m$\mu$, which is due to divalent copper ions [9, 16].

Thus, the addition to the crystal of impurities that promote localization of holes on defects which do not participate in charge transport causes a decrease in the radiation-induced change in conductivity.

Fig. 7. Absorption spectrum of a $\mathrm{NaCl}+0.5\cdot10^{-3}$ mol % $\mathrm{CuCl}_2$ single crystal at room temperature before irradiation (curve 1) and after irradiation with X-rays (curve 2).

Conclusions

1. The change in the conductivity of an unirradiated and irradiated alkali-halide single crystal is affected by that part of the impurity which enters regular sites of the crystal lattice, creating isolated cation vacancies.

2. Depending on its environment in the lattice, the impurity contributes:

a) to an enhancement of the radiation-induced change in electrical conductivity, if its introduction into the lattice is accompanied by the appearance of isolated cation vacancies (for example, $\mathrm{Pb}^{++}$ in KCl, KBr);

b) to its weakening, if the impurity creates additional impurity levels for the capture of holes (for example, $\mathrm{SrCl}_2$ in KCl, $\mathrm{CuCl}_2$ in NaCl);

The impurity does not change the conductivity in comparison with an irradiated pure crystal if it does not create impurity centers for the capture of holes and does not cause the appearance of isolated cation vacancies (for example, $\mathrm{MnCl}_2$, $\mathrm{CdCl}_2$, $\mathrm{CuCl}$ in NaCl).

1. Under the action at room temperature of X-radiation in doses whose main effect is the formation of electron–hole color centers, localization of ho-

a role of current carriers (the Seitz mechanism [13]), as well as the formation of traps according to Smoluchowski [14].

In conclusion, the authors express their deep gratitude to Professor, Doctor E. K. Zavadovskaya for supervising the work, and to Docent I. Ya. Melik-Gaikazyan for useful advice and constant interest in the work.

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