PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
NAMED AFTER S. M. KIROV, HOLDER OF THE ORDER OF THE OCTOBER REVOLUTION
AND THE ORDER OF THE RED BANNER OF LABOR

Vol. 247
1977

EFFECT OF AN IMPURITY ON THE FORMATION OF INTRINSIC RADIATION DEFECTS IN CaF\(_2\) CRYSTALS

E. K. ZAVADOVSKAYA, V. M. LISITSYN, V. A. FEDOROV, A. T. OVCHAROV,
A. I. BARANOV, M. I. KALININ

Under irradiation by protons and electrons in CaF\(_2\) crystals, intrinsic radiation defects are produced; the most numerous among them are complex \(F\)-aggregate color centers, which give rise to an absorption band with a maximum at 560 nm [1, 2, 3]. It has been established that the formation of radiation defects occurs with a greater expenditure of energy than in alkali-halide crystals, which is due to the structural features of the crystal lattice. Similar conclusions were obtained as a result of studying the formation of \(F\)-centers in crystals of fluorides of alkali-halide metals in [4]. The initial defectiveness of the specimen, and especially the presence of an impurity in the crystal, has a large effect on the efficiency of producing new defects by radiation [1–4]. The present work is devoted to studying the effect of yttrium and rare-earth (RE) impurities on the rate of formation of color centers in CaF\(_2\) crystals.

The investigations were carried out on crystals grown from natural purified materials at FIAN and containing impurities of yttrium (less than 0.001%; 0.03%; 1%), holmium (0.003%, 0.03%), neodymium (0.003%, 0.03%, 0.3%), dysprosium (0.001%, 0.04%), and praseodymium (0.3%); crystals grown from synthesized chemically pure salt and free of yttrium and RE impurities were also used. The crystals were irradiated on a cyclotron with protons of energy 4.5 MeV and on an electrostatic generator with electrons of energy 1.2 MeV. During irradiation the temperature of the specimens was kept constant and did not exceed 30°C. The concentration of defects induced by radiation was determined by studying the spectra of optical absorption and the radiation-induced change in the density of the crystals. The optical absorption spectra were measured with an MOM-201 spectrophotometer in the range 196–1000 nm at room temperature. The radiation-induced change in density was measured by the flotation method described in [5].

Experimental results

Irradiation by protons of CaF\(_2\) crystals grown from chemically pure salt and containing no RE impurity, with a dose of \(10^{11}\) rad, leads to the appearance of a poorly resolved optical absorption spectrum with maxima at 230, 325, 410, 440, 525 and shoulders at 560 and 600 nm (Fig. 1).

In CaF$_2$ crystals grown from natural purified fluorite, irradiation with the same doses also leads to the appearance of an absorption spectrum whose bands are poorly resolved and occur at 220, 330, 395, 460, and 550–580 nm. Absorption in the 550–580 nm region is evidently due to overlapping bands with maxima at 560 and 580 nm, which are attributed to $F$-aggregate and yttrium centers, respectively.

Fig. 1. Optical absorption spectra of a CaF$_2$ crystal grown from synthesized chemically pure salt: a—unirradiated; b—irradiated with protons, $D = 1.4 \cdot 10^{11}$ rad.

Irradiation of CaF$_2$ crystals containing yttrium concentrations of 0.007% and 0.003% with electrons and protons leads to the appearance of a typical optical absorption spectrum consisting of bands with maxima at 225, 335, 400, and 580 nm, as in [1]. With increasing irradiation dose, the latter band shifts from 580 to 560 nm and becomes predominant in the entire spectrum. The shift of the band is explained by superposition and by the different growth rates of the absorption band with a maximum at 580 nm, associated with the presence of yttrium impurity, and of the 560 nm band, caused by $F$-aggregate color centers [1]. With increasing yttrium impurity content in the crystals, the shift of the band is observed at higher doses. When the yttrium content in the crystals is about 1%, anomalous growth of the band with maxima at 560–580 nm is not observed.

Analogous results were obtained in studying the optical absorption spectra of CaF$_2$ crystals with different contents of Nd$^{3+}$, Ho$^{3+}$, and Dy$^{3+}$ impurities (Figs. 2, 3, 4), respectively.

At small irradiation doses the crystals have spectra, as is seen from Figs. (2–4), whose structure is determined by the content and type of impurity in the crystal. However, with increasing proton irradiation dose, an optical absorption band with a maximum at 560 nm appears in the spectra of all crystals, and under prolonged irradiation it becomes predominant. At concentrations of neodymium, holmium, dysprosium, and yttrium impurity in CaF$_2$ crystals equal to and above 0.3 wt. %, growth of the absorption band with a maximum at 560 nm is not observed in the proton-irradiation dose interval studied by us (up to $10^{11}$ rad). Growth of the absorption band with a maximum at 560 nm is also absent in CaF$_2$ crystals grown from synthesized chemically pure salt (Fig. 1), and from rare-earth salt, as well as in CaF$_2$ crystals grown at FIAN containing less than 0.001% yttrium impurity. Thus, in crystals containing impurities of yttrium and rare-earth ions, $F$-aggregate color centers are formed, which give rise to the absorption band with a maximum at 560 nm.

The rate of accumulation of $F$-aggregate color centers is largely determined by the impurity content in the crystal (see Fig. 5).

With an increase in the holmium impurity concentration from 0.003 wt. % to 0.03 wt. %, the growth rate of the absorption band with a maximum at 560 nm increases. However, an increase of the holmium impurity in the crystals to

Fig. 2. Optical absorption spectra of a CaF$_2$:NdF$_3$ 0.003% crystal: a—unirradiated; b—irradiated with protons, $D = 2.4 \cdot 10^7$ rad; c—$D = 3.3 \cdot 10^{10}$ rad; d—$D = 1.4 \cdot 10^{11}$ rad.

Fig. 2a. Optical absorption spectra of a CaF$_2$:NdF$_3$ 0.03% crystal: a—unirradiated; b—irradiated with protons, $D = 2.4 \cdot 10^7$ rad; c—$D = 3.3 \cdot 10^{10}$ rad; d—$D = 1.4 \cdot 10^{11}$ rad.

Fig. 3. Optical absorption spectra of a CaF$_2$:HoF$_3$ 0.003% crystal: a—unirradiated; b—irradiated with protons, $D = 2.4 \cdot 10^7$ rad; c—$D = 3.3 \cdot 10^{10}$ rad; d—$D = 1.4 \cdot 10^{11}$ rad.

Fig. 3a. Optical absorption spectra of a CaF₂:HoF₃ 0.03% crystal: a—unirradiated; b—irradiated with protons, \(D = 2.4 \times 10^7\) rad; c—\(D = 3.3 \cdot 10^7\) rad; d—\(D = 1.4 \cdot 10^{11}\) rad.

Fig. 4. Optical absorption spectra of CaF₂:DyF₃ 0.001% crystals: a—unirradiated; b—irradiated with protons, \(D = 2.4 \cdot 10^7\) rad; c—\(D = 3.3 \cdot 10^{10}\) rad; d—\(D = 1.4 \cdot 10^{11}\) rad.

Fig. 4a. Optical absorption spectra of CaF₂:DyF₃, 0.04% crystals:
a — unirradiated;
b — irradiated with protons, \(D = 2.4 \cdot 10^{7}\) rad;
c — \(D = 3.3 \cdot 10^{10}\) rad; d — \(D = 1.4 \cdot 10^{11}\) rad.

Fig. 5. Growth of the optical absorption band with a maximum at 560–570 nm in CaF₂ crystals with RE impurities as a function of the proton-irradiation dose.

0.3 wt.% leads to a decrease in the rate of formation of the \(F\)-aggregate centers responsible for this absorption band. Addition to a CaF\(_2\) crystal of 0.003% neodymium impurity stimulates the accumulation of \(F\)-aggregate centers, whereas a further increase in the neodymium concentration to 0.03% leads to a decrease in the rate of their accumulation, and at neodymium impurity concentrations of 0.3% and 1% no accumulation of \(F\)-aggregate centers is observed. Analogous results were also obtained by us for CaF\(_2\) crystals containing various concentrations of yttrium impurity. Fig. 6 shows kinetic curves for the radiation-induced decrease in density with increasing dose of proton irradiation of CaF\(_2\) crystals.

Fig. 6. Kinetics of the radiation-induced decrease in the density of a CaF\(_2\) crystal with different yttrium-impurity contents as a function of the proton-irradiation dose:
1. 0.003% Y, 2. 1.0% Y, 3. 0.1% Y

The kinetics of the radiation-induced decrease in density has a linear stage caused by the accumulation of \(F\)-aggregate centers responsible for the absorption band with a maximum at 560 nm [5]. The rate of accumulation of \(F\)-aggregate color centers in CaF\(_2\) crystals containing 0.03 mol% YF\(_3\) is much higher than in “pure” crystals. In crystals containing a larger amount of yttrium, however, the rate of accumulation of intrinsic defects is lower than in those containing small impurity concentrations.

Fig. 7. Optical absorption spectra of CaF\(_2\) crystals containing less than 0.001% yttrium impurity:
a — unirradiated; б — irradiated with electrons, \(D = 2.6 \cdot 10^8\) rad; в — \(D = 2.8 \cdot 10^9\) rad; г — \(D = 4.8 \cdot 10^9\) rad; д — \(D = 1.1 \cdot 10^{10}\) rad; е — \(D = 1.5 \cdot 10^{10}\) rad

Similar results in the study of the growth of accumulation of \(F\)-aggregate centers were obtained by us also under electron irradiation of CaF\(_2\) crystals doped with impurities of yttrium and RE.

However, it should be noted that in “pure” CaF$_2$ crystals grown from synthesized salt of special-purity grade, and in crystals grown from natural purified salts containing an yttrium concentration of less than 0.001%, the efficiency of formation of $F$-aggregate color centers is much higher under electron isodose irradiation than under proton irradiation (see Figs. 1, 7, 8).

Comparison of the results presented makes it possible to draw the following conclusions.

1. The rate of accumulation of intrinsic radiation defects responsible for the absorption band with a maximum at 560 nm in doped crystals depends on the type of ionizing radiation at the same impurity content in the crystal.

Fig. 8. Optical absorption spectra of a CaF$_2$ crystal grown from synthesized special-purity salt containing no yttrium and rare-earth-element impurities: a — unirradiated; b — irradiated with electrons, $D = 1.0 \cdot 10^{10}$ rad; c — $D = 3.3 \cdot 10^{10}$ rad; d — $D = 5.4 \cdot 10^{11}$ rad.

1. The rate of accumulation of these defects for a given type of ionizing radiation is determined by the concentration and type of impurity. There exists a certain optimum impurity concentration at which the rate of defect accumulation is maximal.

Discussion of the Results

In the general case, the presence of impurities in a crystal may lead to the following additional processes during irradiation.

1. Impurity atoms or ions may capture electrons or holes, forming impurity color centers [6].

2. Impurity atoms or ions, being incorporated into sites of the crystal lattice, introduce, for compensation of their charge and of elastic stresses caused by the mismatch between the sizes of the impurity and matrix ions, vacancies and interstitial ions capable of creating color centers [7, 8].

3. By capturing electrons or holes, an impurity may increase the stability of centers of the opposite sign. The stability of the latter may increase through their mutual transformation during the lifetime of the impurity center.

1. Impurity ions can serve as a catalyst for the process of creating intrinsic defects under irradiation. Impurities can store radiation energy and release it at the moment of defect formation.

2. The catalyst of the defect-formation process may be an impurity ion together with a defect of the host substance introduced to compensate its charge (vacancies or interstitial ions) [9].

3. Impurity ions, being effective traps for electrons and holes, can compete with intrinsic defects in the process of electron and hole capture under irradiation.

4. The rate of accumulation of intrinsic radiation defects in calcium fluoride crystals increases upon addition of electron-acceptor impurities \(\mathrm{Y}^{3+}\), \(\mathrm{Ho}^{3+}\), \(\mathrm{Dy}^{3+}\), and the concentration of induced defects may exceed the impurity concentration in the crystal. This indicates that electron-acceptor impurities in calcium fluoride crystals are a catalyst for the process in the generation of intrinsic defects. The presence in the crystal of hole-acceptor impurities (praseodymium) suppresses the rate of accumulation of intrinsic radiation defects.

The following explanation of the observed effect of the influence of rare-earth ion impurities on the creation of intrinsic radiation defects in \(\mathrm{CaF}_2\) seems possible to us.

The introduction of rare-earth impurities leads to the appearance in the crystal of an excess number of interstitial \(\mathrm{F}^{-}\) ions, introduced to compensate the charge of the trivalent impurity. Under the action of radiation, trivalent ions located at an ion of cubic symmetry can be converted to the divalent state, which causes the appearance of characteristic absorption bands in the spectrum of additional optical absorption [10]. Holes can be captured by fluorine ions in interstitial sites. The relative concentration of impurity ions reduced to the divalent state is determined by the kind of impurity, its content in the crystal, and also by the irradiation dose [11]. Thus, the presence of RE ions in \(\mathrm{CaF}_2\) causes the appearance of effective traps for electrons and holes.

Holes localized on a fluorine ion in an interstitial site are low-stability hole centers which, as thermoluminescence studies show, are destroyed at a temperature of \(60\text{--}100^\circ\mathrm{C}\).

With increasing irradiation dose, the hole centers are transformed into more stable ones, for example, fluorine molecules in interstitial sites, which leads to an increase in the stability of the intrinsic radiation defects formed and, consequently, to an increase in the rate of their accumulation. At high impurity concentrations in crystals, interstitial fluorine ions hinder the creation of intrinsic defects by shifting into radiation-produced vacancies.

In addition, impurity ions can serve as catalysts for the process of creating intrinsic defects by accumulating radiation energy and releasing it at the moment of defect birth. It is possible that the catalyst of the defect-formation process is an impurity ion together with the fluorine ion introduced to compensate its charge.

The introduction of a praseodymium impurity into a \(\mathrm{CaF}_2\) crystal leads to a decrease in the rate of defect accumulation, since praseodymium ions are effective traps for electrons and, consequently, competitors of intrinsic electron defects during formation.

References

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6. I. A. Parfianovich, E. I. Shuraleva. “Optics and Spectroscopy,” 6, 189, 1959.

7. G. K. Vale. Proceedings of the IFA, Estonian SSR, 23, 132, 1963.

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10. Ya. E. Kariss, P. P. Feofilov. “Optics and Spectroscopy,” 50, 1531, 1962; 15, 272, 1963.

11. V. A. Arkhangel’skaya, M. I. Kiseleva. FTT, 9, 3523, 1967.