ON COLOR CENTERS IN SOLID SOLUTIONS OF THE NaCl—AgCl SYSTEM

D. A. ASANBAEVA

(Presented by the scientific seminar of the Faculty of Physics and Technology)

Introduction

Owing to their good isomorphism, the salts NaCl and AgCl form a continuous series of substitutional solid solutions up to high concentrations of the activator.

The various properties of alkali-halide crystals activated with silver have been studied by many investigators [1—9, 11—14], but the absorption spectra of solid solutions of the NaCl—AgCl system have been studied insufficiently; moreover, the information on the nature and structure of the centers responsible for the observed absorption bands is contradictory.

Smakula [1], who was the first to study the absorption spectra of alkali-halide crystals with silver, found in the spectrum of NaCl—AgCl one activator band with a maximum at 210 mμ and showed that the absorption coefficients at the band maximum are proportional to the concentration of silver in the crystal. This indicates that silver ions enter the NaCl lattice as ions substituting for Na ions.

According to Forro [2], the band at 210 mμ in the spectrum of NaCl—AgCl proved to consist of small bands with maxima approximately at 208 and 218 mμ; at low temperature one more weak band with a maximum near 230 mμ was also found.

Kato’s data [3] on the investigation of the absorption spectra of NaCl—AgCl agree with Smakula’s data.

Etzel, Schulman, and others [4] analyzed three absorption bands in the spectrum of NaCl—AgCl (2095 Å, 2170 Å, 2295 Å). They assert that the absorption at 2295 Å is due to the presence of a pair of silver ions, whereas the absorption at 2170 Å is clearly caused not by pairs but by individual silver ions.

Thus, in alkali-halide crystals activated with silver, atomic centers, usually observed in colored crystals, can arise without x-irradiation.

In alkali-halide crystals activated with silver, a halogen vacancy and the silver ion located next to it act as a single electron trap. The ionization energy of silver atoms (7.54 eV) is greater than that of alkali-metal atoms (Na—5.12 eV); therefore the binding energy of the electron in an atomic center will be greater than in an ordinary F-center. The ratio of atomic radii to ionic radii for silver is considerably smaller than for alkali metals; therefore

one may say that the electron in an atomic center will be more tightly bound to the silver ion than to each alkali-metal ion separately; however, the atomic center cannot be identified with a silver atom, since in the atomic center the binding energy of the electron is determined by its interaction not only with the silver ion, but also with the vacant site of the halide ion and with the five adjacent alkali-metal cations [8].

Fig. 1. Absorption spectra of NaCl—AgCl.
1 — 0.5 mol% AgCl in NaCl, 1′ — 0.5 mol% AgCl in NaCl after X-irradiation; 2 — 3 mol% AgCl in NaCl, 2′ — 3 mol% AgCl in NaCl after X-irradiation; 3 — 7 mol% AgCl in NaCl, 3′ — 7 mol% AgCl in NaCl after X-irradiation; 4 — 15 mol% AgCl in NaCl, 4′ — 15 mol% AgCl in NaCl after X-irradiation.

In all studies of the absorption spectra of the NaCl—AgCl system known in the literature, small concentrations of silver were used. We set ourselves the task of investigating the absorption spectra of solid solutions of the NaCl—AgCl system at high silver concentrations before and after irradiation with X-rays.

Discussion of the Experimental Results

We grew a concentration series of NaCl—AgCl single crystals with silver contents of 0.5; 1; 3; 5; 7; 10; 13; and 15 mol% in the melt, by the Kyropoulos method with subsequent quenching. For the crystals of this concentration series the absorption spectra were measured on an SF-4 spectrophotometer. The same samples, immediately after the absorption spectrum had been recorded, were irradiated with X-rays under identical conditions (56 kV, 8 mA) at room temperature for 4 hours, using an X-ray tube with a tungsten anticathode.

Smakula [5], Schulman [6], and others in their early works asserted that the addition of impurity ions of heavy metals to alkali-halide crystals leads to an increase in the concentration of \(F\)-centers in all cases, assuming that the appearance of coloration characteristic of \(F\)-centers is generally due to the presence of concentrations of foreign impurities.

Kanzig [7, 8] established that the concentration of ordinary \(F\)-centers arising under the action of X-rays in alkali-halide crystals activated with silver decreases with increasing activator concentration. This view is confirmed in work [9].

The reasons for the decrease in the limiting concentration of \(F\)-centers in alkali-halide crystals at high impurity contents are explained by B. V. Budylin and A. A. Vorob’ev [10] as follows: impurity ions, being located on the interblock surfaces of the crystal, can localize electrons regardless of their valence; at low impurity concentrations, however, their amount on the surfaces of the interblocks is comparatively small, while the percentage of impurity entering regularly into vacant sites of the crystal lattice is much larger, and these play a greater role in the creation of stable trapping centers.

Fig. 2. Effect of annealing on the absorption spectra of NaCl--AgCl.
\(1\) — annealed 10 mol% AgCl in NaCl; \(1'\) — annealed 10 mol% AgCl in NaCl after X-ray irradiation; \(2\) — quenched 10 mol% AgCl in NaCl.

The decomposition of \(F\)-centers occurs most rapidly in the presence of an activator than in a pure and heated crystal [11].

Khalilov and Mamedov [12] found that the \(F\)-band is considerably weakened at 0.05 mol% AgCl in NaCl, and at 0.5 mol% it disappears completely. Since the NaCl--AgCl single crystals grown by us begin with 0.5 mol% AgCl in NaCl, in the absorption spectra obtained by us (Fig. 1) there is no \(F\)-band, nor are there other electron centers (\(F'\), \(M\), etc.) present in the spectrum of colored crystals of pure NaCl both before irradiation and after irradiation with X-rays in all compositions. At 0.5 mol% AgCl in NaCl (Fig. 1, curve \(1\)), bands appeared with maxima at 210, 217 \(m\mu\), and a barely noticeable band at 229 \(m\mu\). With increasing silver concentration, the bands at 217 and 229 \(m\mu\) grow more rapidly in comparison with the band at 210 \(m\mu\). The band at 229 \(m\mu\) has an especially high rate of growth. At relatively low concentrations the band with a maximum at 210 \(m\mu\) is the most intense, but as the AgCl concentration increases the other bands overlap it. For samples with a silver concentration greater than 3 mol%, the shape of the absorption bands is strongly distorted and the position of the short-wavelength absorption bands becomes difficult to determine; at the same time, the band at 229 \(m\mu\) becomes the principal band at relatively high silver concentrations. Judging from the behavior of this band, it may be assumed that at high silver concentrations colloidal particles of silver atoms are responsible for this band. The following experimental data lead to this assumption:

a) the rate of growth of this band with increasing silver in NaCl is the highest in comparison with the other bands. This is probably connected

in that, with increasing silver concentration, the number of silver ions substituting for Na\(^+\) ions in the regular sites of the crystal lattice (which are responsible for the band at 217 \(m\mu\)) increases more slowly than the amount of silver localized on interblock surfaces and dislocations;

b) at low concentrations of silver in NaCl, pairs of silver ions [4, 13] are responsible for this band; consequently, it may be assumed that at high concentrations they coarsen, turning into colloidal particles;

c) the position of the maximum of this band in the spectrum of NaCl—AgCl with different silver concentrations shifts from 229 \(m\mu\) to 240 \(m\mu\), which may be due to the difference in the sizes of the colloidal particles;

d) Geck [14] observed that this band is bleached only at the melting point of the crystal (800°C), which is characteristic of colloidal silver particles, since colloidal sodium particles would disappear at a temperature of 455°C.

It is known that, upon x-irradiation of NaCl—AgCl crystals with a low silver concentration, additional bands appear in the absorption spectrum (A band at 235 \(m\mu\), B — 275, C — 305, D — 330, E — 410 \(m\mu\)), absent from the spectrum of induced absorption of NaCl crystals, which are evidently associated with trapping centers in whose formation silver atoms and ions play an essential role.

At high silver concentrations, however, after x-irradiation of NaCl—AgCl crystals under the conditions described above, only one very broad C band appeared at 305 \(m\mu\), which grows with increasing silver concentration and shifts toward longer wavelengths. All the bands that were present in the spectrum of NaCl and AgCl before irradiation grow only slightly after irradiation, while the width of the bands changes almost not at all (Fig. 1, curves \(1'\), \(2'\), \(3'\), \(4'\)).

An annealed single crystal with a composition of 10 mol % AgCl in NaCl has, in the absorption spectrum, a band with a very large absorption density extending beyond the measurement limits of the SF-4 spectrophotometer (Fig. 2, curve 1), whereas a quenched crystal of the same composition has a finite absorption density (Fig. 2, curve 2). Upon x-irradiation of the annealed sample of this composition, one C band also appeared at 305 \(m\mu\) (Fig. 2, curve \(1'\)).

As is known, annealing of crystals causes the accumulation of impurities on the surfaces of the block structure and at dislocations, which leads to enhancement of activator absorption and to broadening of the band width. Apparently, during annealing, segregation of silver atoms occurs at defective sites of the lattice of NaCl—AgCl crystals.

Conclusions

1. On the basis of previously known and our own data, it may be assumed that colloidal silver particles are responsible for the band at 229 \(m\mu\) in the spectrum of NaCl—AgCl at high silver concentrations.

2. In solid solutions of the NaCl—AgCl system, at high silver concentrations, electron centers such as \(F\)-, \(M\)-, and others, which are present in a colored crystal of pure NaCl, do not appear after x-irradiation.

LITERATURE

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2. Forto. Zs. Phys. 56, 534, 1927.
3. M. Kato. Phys. Chem. Rev. Tokyo. 41, 113, 1943; 41, 1135, 1943; 44, 35, 1944.

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