EFFECT OF THE METHOD OF GROWING CRYSTALS ON ELECTRICAL CONDUCTIVITY AND THE PROCESS OF FORMATION OF F-CENTERS

M. I. IGNAT'EVA, L. V. GRIGORUK, I. Ya. MEDIK-GAIKAZYAN,
N. S. KRAVCHENKO

In previous works [1, 2] it was shown that the introduction of divalent metallic impurities into alkali-halide crystals causes a change in their optical and electrical properties. The growth of a single crystal with an impurity from a melt [3] or from a solution [4] is accompanied by a complex distribution of the impurity in the crystal. One part of the impurity enters the lattice sites with the formation of substitutional solid solutions; another part either occurs in the form of adsorbed ions of divalent metals on certain planes of the growing crystal [5], or in the form of a metal–halide complex \((M \cdot \mathrm{Hal}_4^{--})\) in the anion sublattice [6]. The defectiveness of a crystal can be changed not only by introducing impurities, but also by using different methods of growth.

The electrical conductivity of alkali-halide single crystals in the temperature range \((20 \div 200)^\circ\mathrm{C}\) depends on that part of the impurity which enters the sites of the crystal lattice with the simultaneous formation of isolated vacancies in the cation sublattice. The density of radiation-created \(F\)-centers in a crystal depends on the conditions of localization of holes, i.e., it also depends on the number of cation vacancies. We studied absorption in the \(F\)-band and in the band with a maximum at \(273\ \mathrm{m}\mu\), as well as the temperature and concentration dependences of the electrical conductivity of NaCl and KCl single crystals with lead impurity, grown by different methods. The crystals were grown from a melt by the Czochralski method and from a supersaturated aqueous solution. NaCl · Pb crystals were grown by evaporation of a solution of \(\mathrm{NaCl} + \mathrm{PbCl}_2 + \mathrm{H}_2\mathrm{O}\) at a temperature of \(30^\circ\mathrm{C}\), and KCl · Pb by lowering the temperature of a solution of \(\mathrm{KCl} + \mathrm{PbCl}_2 + \mathrm{H}_2\mathrm{O}\) from \(50^\circ\mathrm{C}\) to room temperature. The electrical conductivity was measured by the direct-current method at \(E = 400\ \mathrm{V/cm}\). Absorption spectra were measured with an SF-4 spectrophotometer; the crystals were subjected to X-irradiation on a URS-70 installation with a Mo anticathode and a zirconium filter. The impurity concentration in the crystal was determined by the polarographic method.

Figures 1 and 2 present the dependence of the logarithm of the electrical conductivity on the impurity content in “solution-grown” (1) and “melt-grown” (2) KCl · Pb and NaCl · Pb single crystals at a temperature of \(60^\circ\mathrm{C}\). From the data shown in Figs. 1 and 2 it follows that, at low impurity concentrations, the conductivity of the “solution-grown” samples is lower than that of the “melt-grown” ones. This can apparently be explained by the fact that the initial defectiveness in the “solution-grown” single crystal is lower than in the “melt-grown” single crystal.

[7], and the role of the contribution to the value of the electrical conductivity from isolated vacancies introduced by the impurity at these concentrations is insignificant. In the concentration interval AB the electrical conductivity of “solution-grown” single crystals is higher than the conductivity of crystals obtained from the melt.

Fig. 1. Dependence of the logarithm of the conductivity of “melt-grown” (2) and “solution-grown” (1) KCl single crystals on the concentration of PbCl₂ impurity at 60°C.

At temperatures of 20–200°C, charge transport is effected by displacement of the cation of the host lattice through vacancies of the positive ion. The introduction of a divalent metallic ion is accompanied by the appearance of vacancies in the cation sublattice. Consequently,

Fig. 2. Same as in Fig. 1, for NaCl + PbCl₂ crystals.

an increase in electrical conductivity would seem to mean that, at the given concentrations, the fraction of lead ions located at lattice sites is greater in the “solution-grown” crystals than in the “melt-grown” ones. In order to verify

of the correctness of the assumption, we studied the dependence of the absorption band with a maximum at 273 mµ on the impurity content in the single crystal. There is an opinion [8] that lead located at lattice sites is responsible for the absorption at 273 mµ. The dependence \((K_{273}=\varphi(c))\) of the absorption band with a maximum at 273 mµ on the concentration for KCl·Pb crystals is shown in Fig. 3.

Fig. 3. Dependence of the absorption coefficient at the maximum 273 mµ on the lead content in KCl at room temperature.

From the data \(K_{273}=\varphi(c)\) it follows that the number of lead ions in the cation sublattice in a “solution-grown” single crystal is smaller than in a “melt-grown” one at the same impurity content, whereas the dependence of the logarithm of the conductivity on the lead concentration indicates the opposite. The contradiction in the conclusions is removed if one assumes that, when a crystal with an impurity is grown from an aqueous solution, the probability of formation of single cation vacancies is higher than when it is grown from the melt. Works [9, 10] have shown that introduction of a divalent metallic impurity into the lattice is not always accompanied by the appearance of a single cation vacancy. In fact, growth of a crystal from the melt proceeds at a temperature close to 800°C, the mobility of ions is high, and therefore the probability of complex formation is increased. The validity of the proposed picture can be confirmed by studying the dependence of the maximum of the F-band absorption on the impurity concentration in single crystals.

Fig. 4. Dependence of the absorption coefficient at the maximum \((\lambda = 560\ \text{mµ})\) of the F-band on the impurity concentration PbCl₂ in KCl at room temperature; 1 — for “solution-grown,” 2 — for “melt-grown” single crystals.

An increase in the number of single cation vacancies leads to an increase in the number of hole-trapping levels, and this means that the process of recombination of electrons localized by anion vacancies with free holes will be delayed,

as a result of which the concentration of \(F\)-centers increases. Indeed, the number of \(F\)-centers in the “solution-grown” crystal proved to be higher (Fig. 4).

The conclusion that “solution-grown” crystals are enriched to a greater extent with isolated vacancies of the positive ion than are “melt-grown” crystals at the same impurity content in the specimen is also confirmed by the effect of quenching on electrical conductivity. Since high-temperature heating increases complex formation and, consequently, decreases the number of isolated cation vacancies, the electrical conductivity of crystals subjected to quenching should decrease. The “solution-grown” crystals were heated for 15 minutes at a temperature of \(350^\circ\mathrm{C}\), and then cooled in a stream of cold water to room temperature. The conductivity of such crystals decreased to values corresponding to the “melt-grown” crystal (Fig. 5).

Fig. 5. Temperature dependence of the logarithm of the electrical conductivity of a solution-grown single crystal
\(\mathrm{NaCl} + 2.8 \cdot 10^{-3}\ \mathrm{mol.\ \%}\ \mathrm{PbCl}_2\); straight line 1—before quenching; straight line 2—after quenching.

The experimental results obtained make it possible to conclude that, in “solution-grown” crystals, a relatively large fraction of the cation vacancies introduced together with the impurity is in an unbound state, as compared with “melt-grown” crystals.

REFERENCES

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4. I. V. Melikhov, M. S. Merkulova. ZhFKh, 34, 3, 633 (1960).
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