ADSORPTION ON CaF₂ AND SrF₂ STIMULATED BY γ-RADIATION

A. A. Vorob’ev, E. K. Zavadovskaya, N. M. Timoshenko

At the present time a large number of experimental works have been published devoted to the influence of various types of irradiation on the adsorption and catalytic properties of semiconductors and dielectrics. There is no doubt that the scientific interest and the prospects for practical use of these effects require further research in this direction. It is true that the extensive experimental material does not yet provide a unified understanding of the indicated phenomena, the mechanism of which remains undisclosed to this day.

It is known that the surface of a solid possesses a certain amount of energy, which is customarily called “free surface energy.” The tendency of a solid to reduce the magnitude of its free energy determines adsorption processes, as a result of which atoms or molecules of the surrounding medium, being adsorbed by the solid, saturate some part of the unbalanced forces on the surface.

It is appropriate to recall that there are two types of adsorption: physical and chemical, the difference between which is reduced both to the difference in the forces holding the adsorbed molecule on the surface of the solid and to the magnitude of the binding energy between them.

An increase in the free energy of the surface must, obviously, be accompanied by an enhancement of the adsorption and catalytic ability of the solid. One of the ways of increasing the free energy of a solid is irradiation.

The primary act of interaction of radiation with matter is determined by the nature of the radiation: in the case of electromagnetic radiation, ionization processes; in the case of corpuscular radiation, initially elastic collision accompanied by displacement of atoms and ions from the sites of the crystal lattice, and then ionization processes due to the displaced atoms and ions. The primary interaction of radiation causes numerous secondary changes in the substance, of which the most significant are the following: a) nonequilibrium and localized charge carriers (electrons and holes); b) lattice defects of the “vacancy—interstitial” type and their complexes; c) chemical impurities caused by radiolysis of the substance.

All the above-listed defects change both the internal and the surface energy of the substance, which is manifested in changes in the properties of the substance, including the properties of the surface. Such

surface changes in a solid under the action of radiation are manifested, above all, in changes in its adsorption properties.

The complexity of the phenomena of adsorption and catalysis does not at present make it possible to indicate with certainty the defects that arise in a solid under the action of radiation and are responsible for these phenomena. It is true that, in a number of experimental works and review articles [1—3], the authors give preference to electronic concepts in explaining the causes of the observed effects. According to the electronic theory [4], the adsorption and catalytic properties of a solid are due to electrons or holes located on its surface in a free state or localized at surface defects. Ionization processes occurring in a solid under the action of radiation lead to a disturbance of the electronic equilibrium in the crystal, to the appearance of nonequilibrium charge carriers in the bulk and on the surface, which can substantially affect the adsorption and catalytic activity of the specimen.

According to Vol’kenshtein [4], the influence of defects present on the surface on its adsorption properties is exerted in two ways: 1) through the Fermi level, whose position depends on the nature and concentration of defects and on whose position the adsorption activity of the surface depends (the first mechanism), 2) through the direct participation of defects in the adsorption act itself; in this case the defects act as adsorption centers during chemisorption (the second mechanism). It is considered that adsorption under the action of radiation is chemisorption.

In the case of the second mechanism, the chemisorbed particle is a center of localization of an electron or a hole, stripped from electron or hole centers ($F$- and $V$-centers) arising on the surface under the action of radiation. The charged particle becomes associated with an anion or cation vacancy, i.e., is adsorbed by the surface. As a result of such an act, a color center is destroyed. Results of the work of Bauer and Staude [5], who investigated the adsorption of quinone on AgBr crystals, speak in favor of such a mechanism. In uncolored crystals the adsorption capacity was practically zero; after preliminary coloration it increased to a noticeable value. The authors were dealing here with adsorption on $F$-centers. Kon [6], studying color centers in silicate irradiated with X-rays and $\gamma$-rays, observed instantaneous bleaching of the silicate upon adsorption of hydrogen on it, whereas heating to 250°C was not accompanied by bleaching. According to Vol’kenshtein [14], $F$-centers can act as adsorption centers not only in alkali-halide crystals, but also in other crystals.

The catalytic and adsorption properties activated by radiation have been studied in greatest detail in semiconductors such as ZnO, TiO$_2$, CuO, Cu$_2$O. Among dielectrics, various modifications of SiO$_2$ and Al$_2$O$_3$ [7] have been studied most extensively.

We carried out studies of the adsorption properties of the dielectrics CaF$_2$, SrF$_2$, and BaF$_2$ under the action of $\gamma$-radiation. The increased interest in the above compounds, as stable systems (chemically, thermally, etc.), requires a detailed study of their properties, especially under the action of radiation.

The adsorption properties of CaF$_2$ and SrF$_2$ were noticed by us in studying radiolysis in these compounds under the action of $\gamma$-radiation.

The investigations were carried out on powders of CaF$_2$ and SrF$_2$ of “especially pure” grade, in order to exclude the influence of impurities of extraneous substances on the behavior of the materials. A weighed portion of powder was placed in a glass ampoule; the latter was sealed to an LT-2 manometric lamp. Such

The design made it possible to study the dynamics of gas absorption and gas evolution during irradiation and annealing. The LT-2 lamp was soldered to the vacuum apparatus; the powder was degassed at a vacuum of about \(10^{-5}\) mm Hg with heating to \(400\)—\(500^\circ\mathrm{C}\) for many hours (“conditioning”). After sealing off, the lamp containing the powder was held for several days until a constant pressure was established, and was then irradiated with \(\gamma\)-rays from \(\mathrm{Co}^{60}\).

After sealing off the LT-2 lamp with the conditioned powder, the vacuum in it improves, and this occurs within several minutes. Repeated heating and cooling of the conditioned powder is accompanied by a deterioration of the vacuum upon heating and a return to the initial value upon cooling. The rapid improvement of the vacuum after sealing off suggests physical adsorption on the powder. Prolonged storage of the conditioned powder is accompanied by a slight improvement of the vacuum (Fig. 2).

Irradiation of \(\mathrm{CaF_2}\) and \(\mathrm{SrF_2}\) powders with \(\gamma\)-rays is accompanied by a considerable improvement of the vacuum in the lamps. Figure 1 shows the adsorption capacity of \(\mathrm{CaF_2}\) in the radiation field. Irradiation with \(\gamma\)-rays to a dose of \(1.45 \cdot 10^8\) roentgen is accompanied by gas absorption. When the irradiated powder is heated, gas evolution occurs; moreover, the pressure in the lamp falls to a value appreciably lower than during heating before irradiation. This is probably due to the liberation, upon heating, of radiolysis products (gas-forming ones), whose amount increases with increasing exposure dose.

It should be noted that the effect of radiation-stimulated adsorption is reproducible for one and the same specimen: heating of the irradiated specimen is accompanied by desorption, and subsequent irradiation again stimulates adsorption; in other words, “poisoning” of the powder is not observed.

At the beginning of irradiation of \(\mathrm{CaF_2}\) powder, intensive sorption of gas is observed, the rate of which decreases with irradiation time, despite the increase in exposure dose (Fig. 3). The most intensive sorption is observed upon irradiation to a dose of up to \(2 \cdot 10^7\) roentgen; saturation then sets in. This character of the dependence is probably determined by the fact that an equilibrium concentration of \(F\)-centers and an equilibrium surface density of adsorbed gas have been reached. In the indicated dose region, saturation of the maxima of the optical absorption bands is observed (according to data of L. A. Lisitsyna), as well as a tendency toward saturation of the energy stored during irradiation.

Fig. 1. Gas adsorption on \(\mathrm{CaF_2}\) during irradiation and desorption upon heating

All of the above concerning \(\mathrm{CaF_2}\) also applies to \(\mathrm{SrF_2}\), whose behavior in the \(\gamma\)-radiation field is shown in Fig. 2. Of particular interest for \(\mathrm{SrF_2}\) is the decrease in pressure in the lamp containing the powder by two orders of magnitude, which occurs after four hours of irradiation (dose rate 300 roentgen/sec).

It is interesting to note that \(\mathrm{BaF_2}\) powder, under the action of \(\gamma\)-radiation, evolves gas, which is probably due to radiolysis. The character of the dependence presented in Fig. 3 is typical of a two-stage accumulation process. Intensive gas evolution at doses up to \(4 \cdot 10^7\) roentgen is replaced by a tendency toward saturation. Radiation-

chemical yield for BaF$_2$ is equal to $1.6 \cdot 10^{-4}$ gas molecules per 100 eV of absorbed energy.

BaF$_2$ crystals, in the series of fluoride compounds under consideration, have the smallest lattice energy, the largest free volume, and, as a consequence, lower radiation stability. Therefore, upon irradiation of BaF$_2$, its chemical decomposition occurs.

It is quite possible that, upon irradiation of all three compounds, gaseous radiolysis products are released. However, under the action of radiation in CaF$_2$ and SrF$_2$, considerably more active adsorption centers are created than radiolysis products are released. This increased concentration of adsorption centers is responsible not only for the adsorption

Fig. 2. Gas adsorption on SrF$_2$ under irradiation and desorption upon heating

Fig. 3. Dependence of gas absorption on CaF$_2$ and gas evolution in BaF$_2$ on the dose of gamma radiation

of the released radiolysis products, but also, additionally, of gas molecules from the surrounding medium. The fact that the concentration of defects induced and preserved under the action of radiation in CaF$_2$ and SrF$_2$ exceeds the concentration of defects in BaF$_2$ is confirmed by stored-energy studies [8]. Upon irradiation of BaF$_2$, part of the radiolysis products is adsorbed on the color centers that arise. In the process of heating irradiated BaF$_2$ powder, annealing of the color centers takes place, accompanied by additional gas evolution.

Of particular interest is the study of the thermal stability of the adsorption effect. In the process of heating irradiated CaF$_2$ powder, an interesting phenomenon is observed: an induction period up to a temperature of 100°C is replaced by intense gas evolution in the region of 100–120°C; further heating again leads to restoration of the vacuum in the temperature region 200–220°C, and, finally, to intense gas evolution at temperatures above 300°C (Fig. 4). Similar behavior is observed

Fig. 4. Gas evolution and gas absorption in irradiated CaF$_2$ and SrF$_2$ upon heating

is also observed for irradiated SrF$_2$ powder, the only difference being that intense gas evolution (greater than for CaF$_2$) occurs at a lower temperature—about 270° [9].

It is still difficult to give a reliable explanation of the observed phenomena. However, if color centers located in the surface layer are responsible for adsorption in dielectrics, then the destruction of the latter should be accompanied by gas evolution. At the same time, the destruction of color centers is accompanied by changes in the physical properties of the substance.

Evidence that, in the compounds studied by us, defects created by radiation are responsible for adsorption is provided by the coincidence of the temperatures of the maxima of the stored-energy release peaks with the temperatures at which gas evolution and gas absorption occur in irradiated powders during their heating. As for the nature of the defects responsible for adsorption, we are inclined to regard them as electron–hole centers. In support of the latter are studies of the thermoluminescence of irradiated samples. As is known, the destruction of electron–hole centers is accompanied by luminescence of the substance. During the heating of irradiated CaF$_2$, intense luminescence caused by the destruction of electron–hole centers is observed in the same temperature regions as gas evolution and gas absorption during heating of the irradiated powders (Figs. 4, 5).

Fig. 5. Thermoluminescence spectra of CaF$_2$ irradiated with gamma rays.

Conclusions

1. The above results of the investigations are of scientific and practical interest. First of all, the investigations resulted in an expansion of the range of dielectrics possessing radiation-stimulated adsorption.

2. The ability of CaF$_2$ and SrF$_2$ to improve the vacuum under the action of $\gamma$ radiation makes it possible to propose using these materials as dielectrics operating in vacuum systems located in radiation fields.

3. Since CaF$_2$ and SrF$_2$ are chemically and thermally stable systems, the combination of these properties with radiation-stimulated adsorption makes the use of these materials for gas separation highly promising.

4. The adsorption properties of CaF$_2$ and SrF$_2$ under the action of $\gamma$ radiation are, in all probability, due to electron–hole centers generated by the radiation.

5. The activation energy calculated from the thermoluminescence peaks corresponding to the temperatures of gas evolution is of the order of 1 eV; i.e., adsorption stimulated by $\gamma$ radiation is chemisorption.

6. The amount of gaseous products released from irradiated powders upon heating increases in the series of alkaline-earth metal fluorides as the lattice energy of the compound decreases, i.e., in going from CaF$_2$ to BaF$_2$.

References

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