INVESTIGATION OF THE NATURE OF ABSORPTION BANDS IN THE SPECTRUM OF COLOR CENTERS IN CaF₂ CRYSTALS

E. K. ZAVADOVSKAYA, L. A. LISITSYNA, V. M. LISITSYN

Great progress in the study of radiation defects in ionic alkali-halide crystals, achieved by the present time, makes it possible to proceed to the investigation of ionic crystals with a more complex structure. In recent years a number of works have appeared devoted to the study of radiation defects in crystals with a fluorite-type lattice. However, these crystals have a number of features that hinder a broad investigation of their properties. In particular, because of the high lattice energy, the fluorides of alkaline-earth metals have the edge of intrinsic absorption in the deep ultraviolet region of the spectrum, which does not allow the complete absorption spectra of irradiated crystals to be studied with ordinary instruments. This makes interpretation of the results obtained difficult. By the present time it has been possible to study in considerable detail the absorption spectrum of irradiated CaF₂ crystals in the interval 210–900 mµ. The identity of the spectra of additively and photochemically colored crystals unambiguously determines the electronic nature of the bands that arise in this wavelength region. However, there is no single opinion in the literature regarding the structure of the centers responsible for the absorption bands in CaF₂. Moreover, the question has not yet been resolved as to whether the bands are due to different centers or to centers of one type, in particular to an yttrium impurity.

In the present work results are presented on the investigation of absorption spectra in the range 130–1000 mµ of CaF₂ crystals of different prior history. The investigations were carried out on natural transparent CaF₂ crystals and on crystals grown in the laboratory of our institute (CaF₂ TPI) and in the State Optical Institute (CaF₂ GOI). The samples were irradiated at room temperature with X-rays from RUP = 200 (180 kV, 10 mA), with γ-rays from Co⁶⁰, and with protons of energy 4.5 MeV. The absorption spectra were measured on a vacuum spectrophotometer of the Institute of Physics and Astronomy of the Academy of Sciences of the Estonian SSR and on an SFD-2 spectrophotometer.

Absorption spectra of CaF₂ crystals

Upon irradiation with X-rays, γ-rays, and protons, four additional bands appear in the absorption spectra of the crystal, with maxima at 225, 335, 400, and 580 mµ (Fig. 1, curve 2). During prolonged irradiation of the crystals with γ-rays (560 hours) we did not observe any shift with irradiation dose of any of the bands, which does not agree-

with Przibram’s assumption [1] on the shift of bands with maxima at 400 and 580 mµ to positions of the maxima at 380 and 540 mµ as a result of a strong disturbance of the crystal lattice. Upon irradiation of the crystals with protons, a shift of the band with a maximum at

Fig. 1. Absorption spectra of CaF₂ crystals (GOI) irradiated with protons:
1—unirradiated crystal, 2—irradiation dose \(5 \cdot 10^{14}\) protons/cm², 3—irradiation dose \(1.8 \cdot 10^{16}\) protons/cm², 4—irradiation dose \(0.6 \cdot 10^{17}\) protons/cm², 5—irradiation dose \(1.1 \cdot 10^{17}\) protons/cm².

580 mµ to 560 mµ was observed. At higher doses of proton irradiation (beginning with \(10^{15}\) protons/cm²), an additional band appears in the spectra of the irradiated crystals, with a maximum at 700 mµ, which shifts into the long-wavelength region of the spectrum with irradiation dose (Fig. 1, curves 3, 4, 5).

Fig. 2. Absorption spectra in the far ultraviolet region of CaF₂ crystals (TPI) irradiated with protons:
1—unirradiated crystal; 2—irradiation dose \(7.5 \cdot 10^{14}\) protons/cm², 3—irradiation dose \(6.8 \cdot 10^{15}\) protons/cm², 4—irradiation dose \(1.3 \cdot 10^{16}\) protons/cm².

The rate of growth of this band depends on the prior history of the specimen. At identical absorbed doses, the band in GOI CaF₂ crystals is considerably larger than in natural CaF₂.

In the far ultraviolet region of the spectrum, two new bands with maxima at 186 and 194 mµ were found (Fig. 2). Near the edge of the intrinsic absorption of the crystal, with increasing irradiation dose, a considerable change in the absorption spectrum is observed; however, the structure of the additional absorption is not resolved.

Growth curves of absorption bands with irradiation dose

When crystals are irradiated with X-rays and γ-rays, the bands with maxima at 225, 335, 400, and 580 mµ saturate with irradiation dose, while the ratios between the bands remain constant. These results agree with the data of Smakula [2] and Berilli [3] for X-irradiated crystals. Saturation of all four bands with irradiation dose, and constancy of the spectrum upon intro-

in the crystal of an yttrium impurity served for a number of authors as the basis for assigning all four bands to a center of one type, namely: an electron localized on divalent yttrium ions. However, in studying the growth curves of the absorption bands with the dose of proton irradiation, we found anomalous behavior of the band with a maximum at 580 mµ [4].

Fig. 3. Growth curves of absorption bands with maxima at 225 mµ (1), 335 mµ (2), 400 mµ (3), and 580 mµ (4) in CaF₂ (State Optical Institute) crystals as a function of the dose of proton irradiation. Intensity \(I = 0.2\ \mu\text{A}\).

It must be noted that the maximum dose of proton irradiation used by us was four orders of magnitude greater than, for example, in [2]. As can be seen from the results presented in Fig. 3, the bands with maxima at 225, 335, and 400 mµ tend toward saturation, whereas the growth of the band at 580 mµ proceeds in three stages: a stage of rapid growth, a stage of saturation, and then further slow growth. The growth rate in the third portion is determined by the radiation intensity. Of the three types of CaF₂ crystals studied by us, the point of inflection on the growth curve (the moment when generation begins) occurs earliest for natural CaF₂ crystals and latest of all in CaF₂ TPI.

Fig. 4. Curves of the ratios of the absorption coefficients at the maxima of the bands at 400, 335, and 225 mµ in CaF₂ crystals.

As can be seen from Fig. 3, the form of the growth curves of the bands with maxima at 225, 335, and 400 mµ is the same. We studied the relationships between these bands (Fig. 4). It was found that these relationships do not depend on the pre-

history of the samples, on the intensity and dose of irradiation. Points taken from spectra in the works of various authors also fall well on these straight lines. The curves start from the origin, which indicates that the centers responsible for the bands at 225, 335, and 400 mμ arise simultaneously under the action of irradiation. The absence of dependence of the ratio between the bands on the prehistory of the sample and on the regimes and type of irradiation allows one to suppose the following: 1) either these bands are due to electron transitions on a single defect; such a defect may be an impurity ion; 2) or the same defect is a constituent part of the centers responsible for these bands.

Absorption spectra of CaF₂ crystals

The yttrium impurity was introduced in the form of YF₃; the yttrium concentration was determined by the method of spectral analysis. The yttrium impurity enters the crystal in the form of Y⁺³; compensation of the excess positive charge is carried out by means of an interstitial F⁻ ion [5, 6].

The spectrum of CaF₂–Y crystals irradiated with protons, as well as the spectrum of “pure” CaF₂ crystals, consists of four bands with maxima at 225, 335, 400, and 580 mμ. Despite the fact that the yttrium concentration in the crystal was very high (0.5 mol %), we did not observe an increase of the band at 400 mμ relative to all the other bands of the spectra, as follows from Smakula’s work [7]. Moreover, we found that the ratios between the bands at 225, 335, and 400 mμ in impurity-containing and pure crystals are the same. However, these bands are much larger in impurity-containing crystals under equal-dose irradiation.

It was found that the first stage of growth with irradiation dose of the band at 580 mμ is suppressed. Apparently, the growth of this band is hindered either by the large amount of yttrium impurity in the crystal, or by the accompanying impurity defects (in particular, the interstitial fluorine ion).

Thus, the anomalous behavior of the band with a maximum at 580 mμ upon irradiation of both pure and impurity-containing crystals speaks in favor of the assumption of a different nature of the defects responsible for this band in comparison with the other three bands of the spectrum.

Thermal annealing of absorption bands in CaF₂–Y crystals

Thermal annealing of the crystals was carried out in vacuum in order to avoid the penetration of oxygen into the crystal during heating. The heating rate was 1° per min. The selected irradiation doses of the CaF₂ crystals corresponded to points on the first, second, and third stages of the growth curve of the band at 580 mμ under proton irradiation. Thermal annealing proceeds according to the following law (Fig. 5): up to a certain temperature the centers in the crystal are stable; above the critical temperature, destruction of the bands begins, which leads to complete bleaching of the crystal. It was found that, independently of the irradiation dose of the crystal, the bands with maxima at 335, 400, and 580 mμ are destroyed at the same temperature and at the same rate; however, the temperature of band destruction varies from crystal to crystal: it is lowest for CaF₂ crystals grown at the State Optical Institute, and highest for CaF₂–Y crystals.

As noted above, our experiments unambiguously showed that the nature of the band at 580 mμ differs from the nature of the bands at 335 and 400 mμ. Consequently, to explain the results obtained on annealing, one may suppose that the destruction of all the investigated

bands is due to one and the same process; for example, the destruction of these electron bands is caused by the destruction of one type of hole center.

Fig. 5. Thermal annealing curves of absorption bands in proton-irradiated CaF₂ crystals (GOI).

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