SOME OPTICAL PROPERTIES OF NaCl ACTIVATED WITH CERIUM

L. A. CHERVYAKOVA, I. Ya. MEDIK-GAIKAZYAN

It has been shown by many authors [1, 2] that, at high concentrations of Pb, Mn, and Cd, the colorability of alkali-halide crystals decreases. It is undoubtedly of interest to study the behavior in these crystals of trivalent ions possessing strong electron-acceptor properties. The authors of the present work assumed that the influence of trivalent cations on the density of color centers would prove to be quite complex, since the introduction of a trivalent ion into alkali-halide crystals inevitably increases the defectiveness of the cation sublattice, which improves the conditions for localization of holes and, consequently, increases the density of \(F\)-centers.

On the other hand, the presence in the lattice of trivalent ions, which have a high ionization potential and are capable of localizing electrons, will lead to an electron deficit in the conduction band in a crystal excited by radiation, and consequently, at a certain impurity concentration the density of \(F\)-centers will decrease. From this point of view, rare-earth ions, which have a number of special features, are of great interest as activators. The optical electrons of the \(4f\)-shell of trivalent rare-earth ions, having the electronic configuration \(1S^2 \ldots 4f^k 5S^2 5p^6\), are well shielded from external influences by filled shells. The investigation of spectra of rare-earth ions substituting for the cation of the host substance at sites of the crystal lattice is closely connected with the internal structure of the medium in which the ion is located. Therefore rare-earth ions, owing to their ability to luminesce, can serve as sensitive probes introduced into the substance under investigation in the study of physical phenomena in solids [3].

The present work contains preliminary data on the study of certain optical properties of NaCl crystals activated with cerium and prepared by three different methods: 1) growth from the melt [4]; 2) by the method of electrothermodiffusion (ETD) [5]; 3) by the pressing method [6].

Despite the close values of the ionic radii of cerium and sodium (0.99 and 0.98 Å), the solubility of \(\mathrm{CeCl}_3\) in NaCl is small and the formation of solid—

…solid solution by substitution is hindered. Apparently this is connected with the fact that incorporation of one trivalent ion into the lattice requires the formation of two cation vacancies in order to preserve the electroneutrality of the crystals as a whole. Since the solubility of the impurity is a function of the initial defectiveness of the crystal, it was assumed that introducing SrCl₂ into the host, which brings with it an additional number of vacancies, would improve the conditions for formation of the solid solution NaCl·Ce.

All the crystals studied were grown by the experimenter himself in evacuated and sealed quartz ampoules from NaCl salt purified with dithizone, SrCl₂ of chemically pure analytical grade, and CeCl₃. As experiments on growing NaCl·Ce crystals showed, cerium is poorly soluble in the host, and the crystal obtained is translucent.

With equal concentrations of CeCl₃ in the charge of NaCl and NaCl·SrCl₂, in the latter case the crystals grew transparent, which confirmed our assumption concerning improved solubility of CeCl₃ in NaCl upon the simultaneous introduction of SrCl₂ into it.

Fig. 1. Excitation spectra of crystals:
1 — NaCl·Ce, grown from the melt,
2 — NaCl·Sr·Ce, grown from the melt,
3 — NaCl·Sr·Ce, prepared by the ETD method,
4 — NaCl·Ce, obtained by pressing.

The properties of the samples NaCl·Sr·Ce, NaCl·Ce, and NaCl·Sr, respectively, grown under identical conditions, were compared. The content of cerium chloride in the charge was varied up to 0.1 mol%. The properties of NaCl·Sr·Ce crystallophosphors prepared by the ETD method were compared with samples of NaCl·Sr having the same prior history. It was not possible to obtain samples by the method of diffusion from the gas phase [7]. In all crystallophosphors prepared by different methods, the activator can hardly be detected from the absorption spectra at room temperature; however, it manifests itself well in the excitation spectra. In the region from 2.0 to 6.0 eV, NaCl and NaCl·Sr crystals are completely transparent. The fundamental absorption of NaCl and NaCl·Sr begins in the region \(h\nu > 6\) eV; consequently, it may be considered that the spectra shown in Fig. 1 (in the region \(3.5 < h\nu < 5.5\) eV) correspond to direct photoexcitation of impurity centers. Upon excitation in all activator bands, radiation is observed in the blue region, characteristic of cerium. It should be noted that quenching NaCl·Sr·Ce crystals from 450°C leads to a decrease in the intensity in the excitation spectrum, whereas in NaCl·Ce samples with a considerable activator content, after quenching a second maximum appears at \(\lambda = 325\) mμ and the intensity of the first, \(\lambda = 285\) mμ, increases. This is evidently connected with the fact that at considerable activator contents the latter will be located in the host both at regular lattice sites and at its defective sites. As a result—

thermal action, cerium ions diffuse further into the lattice; possibly, additional dissolution of $\mathrm{CeCl_3}$ takes place, the presence of which in $\mathrm{NaCl\cdot Ce}$ crystals was already noted above.

Fig. 2. Excitation spectra of $\mathrm{NaCl\cdot Ce}$:
1 — unannealed; 2 — after annealing at $450^\circ\mathrm{C}$;
3 — after irradiation with X-rays;
a) $\mathrm{NaCl}$; UFS-3 filter.

It is possible that the long-wavelength band $\lambda = 325\,m\mu$ in the excitation spectrum is associated with an electronic transition in an impurity center located at a regular lattice site, while the short-wavelength band $\lambda = 285\,m\mu$ may be assigned to centers of a second type, which may be represented as a complex consisting of an activator ion and cation vacancies.

Under the action of X-rays the positions of the bands in the excitation spectrum do not change; however, their intensity decreases greatly and the spectrum becomes rather diffuse (Fig. 2). On the short-wavelength slope of the $F$ band (in the region from $260\,m\mu$ to $340\,m\mu$), where there is a slope in “pure” $\mathrm{NaCl\cdot Sr}$ crystals, a band is observed in samples activated with cerium, but at room temperature it cannot be isolated. It should be noted that $\mathrm{NaCl\cdot Sr\cdot Ce}$ and $\mathrm{NaCl\cdot Ce}$ crystals are readily colored by irradiation with X-rays. A strong $F$ band is formed, whose half-width is considerably greater than in $\mathrm{NaCl\cdot Sr}$ and $\mathrm{NaCl}$ ($0.83\,ev$ and $0.54\,ev$, respectively). It is possible that in our phosphor crystals this broadening is caused by the superposition on the $F$ band of another band associated with activator centers produced under the action of X-rays; however, it is not possible to separate these bands analytically. I. A. Parfianovich and co-workers [8], studying the luminescent properties of $\mathrm{NaCl\cdot Eu}$ and $\mathrm{KCl\cdot Eu}$ crystals, expressed a similar supposition concerning the complex character of the $F$ band. After the action of $F$ light the activator band appears more distinctly and has a complex structure (Fig. 3). If, from the excitation spectra, a relative comparison can be made of the cerium content in the crystals, then from a comparison of the coloration of $\mathrm{NaCl\cdot Ce}$ and $\mathrm{NaCl}$ crystals it seems possible to conclude that the main role in the localization of electrons is played by activator centers. At equal Sr concentration in the crystal, in the case of a higher cerium content the crystal is colored more weakly than a sample with a lower

concentration of cerium, which once again confirms the strong manifestation of the electron-acceptor properties of the activator.

Fig. 3. Absorption spectra of crystals irradiated with X-rays:
1 — NaCl·Sr·Ce, 2 — NaCl·Ce, 3 — long-wavelength part of the \(F\)-band after exposure to \(F\)-light:
a — immediately after irradiation, b — after exposure to \(F\)-light for 10 min, c — after exposure to \(F\)-light for 30 min.

In the experiments carried out, we did not observe an increase in the colorability of NaCl·Sr·Ce and NaCl·Ce crystals. Obviously, further studies are necessary in order to verify this assumption.

REFERENCES

1. I. Ya. Melik-Gaikazyan. Izvestiya AN SSSR, seriya fizich., 1965 (in press).
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3. M. A. El’yashevich. Spectra of the Rare Earths, 1953.
4. G. G. Bakli. Crystal Growth, 1954.
5. A. S. Artsybychev. Proceedings of the Physical Institute of the USSR Academy of Sciences, vol. I, no. 3, 3–70, 1938.
6. E. K. Zavadovskaya, S. L. Kashchuk. The Action of Radiation on the Properties of Materials, Novosibirsk, 1963.
7. Ch. B. Lushchik, N. E. Lushchik. Proceedings of the Institute of Physics and Astronomy, Academy of Sciences of the Estonian SSR, 11, 1960.
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