Dependence of Some Physical Properties of Group II Metal Fluorides on Their Chemical Composition

E. K. Zavadovskaya, N. M. Timoshenko, V. A. Popov, V. A. Chernyshev,
L. A. Lisitsyna

Studies carried out at the Tomsk Polytechnic Institute have established dependences of the physicochemical properties of alkali-halide crystals on their chemical composition and on the binding energy of ions in the lattice. Among alkali-halide crystals, compounds with a higher lattice energy have greater mechanical, thermal, and chemical stability and higher electrical strength [1]. Their radiation resistance also depends on chemical composition [2]. Thus, for example, crystals with a higher lattice energy have a higher density of \(F\)-centers [3].

Extending these investigations to the group of ionic compounds that have high ion-binding energies in the lattice is of theoretical and practical interest. Such compounds include the fluorides of alkaline-earth metals. The latter differ from alkali-halide crystals in the structure of the crystal lattice, in the higher binding energy of ions in the lattice, in higher melting temperatures, in chemical stability, etc. We are studying the dependence of certain properties of alkaline-earth metal fluorides on their chemical composition and the influence of irradiation on these properties. This communication presents preliminary results of the investigations. The investigations were carried out on crystals grown in vacuum by the Stockbarger method [4]. Growth was performed in crucibles made of electrical graphite. Artificially synthesized salts, as well as reagents of “especially pure” grade, served as the starting materials for growing the crystals.

In order of decreasing lattice energy, the compounds are arranged as follows: \(\mathrm{CaF_2}\), with lattice energy \(27.15\) eV per ion pair; \(\mathrm{SrF_2}\), respectively \(25.6\) eV per ion pair; and \(\mathrm{BaF_2}\)—\(24.07\) eV per ion pair. With a change in the lattice energy of the fluorides, as in the case of alkali-halide crystals, changes are observed in some of their physicochemical properties. Thus, as the lattice energy of a compound decreases, its boiling temperature and chemical stability decrease, the heat of formation and heat of dissolution decrease, and the density, heat capacity, entropy, solubility, etc. increase. Calculation of the relative free volume and electron density showed that in passing from \(\mathrm{CaF_2}\) to \(\mathrm{SrF_2}\) and to \(\mathrm{BaF_2}\) there is an incre—

an increase in the relative free volume, i.e., a decrease in packing density, as well as an increase in electron density.

Experimental Results

1. Microhardness. Investigation of the microhardness of fluoride crystals made it possible to establish its dependence on the lattice energy. The greatest microhardness is exhibited by CaF$_2$—the material with the highest lattice energy (H = 150 kg/mm$^2$), followed by SrF$_2$ (H = 144 kg/mm$^2$); BaF$_2$ has the lowest microhardness (H = 82 kg/mm$^2$). It should be noted that the microhardness of fluorides is considerably higher than the microhardness of alkali-halide crystals. After irradiation with X-rays (dose $1.8 \cdot 10^5$ roentgen), the microhardness in CaF$_2$ and SrF$_2$ increases (in CaF$_2$ by 2%, in SrF$_2$ by 1.5%).

2. Electrical conductivity. On the basis of the results of studies of electrical conductivity in the temperature range from 20°C to 400°C, it may be concluded that crystals with a higher lattice energy have a higher electrical conductivity; i.e., the dependence of electrical conductivity on lattice energy for fluorides is analogous to that for alkali-halide crystals.

Fig. 2. Dependence of tgδ on temperature for CaF$_2$, SrF$_2$, and BaF$_2$ crystals before and after irradiation with X-rays (dose $1.2 \cdot 10^5$ r).

It was observed that preliminary irradiation with X-rays at an absorbed dose of $1.2 \cdot 10^5$ roentgen leads to a decrease in electrical conductivity in CaF$_2$ and SrF$_2$ crystals (Fig. 1), and the decrease in electrical conductivity in CaF$_2$ is considerably stronger than in SrF$_2$ (in CaF$_2$ by 13%, in SrF$_2$ by 7.5%). In BaF$_2$, irradiation increases the electrical conductivity.

1. Dielectric losses. Dielectric losses were measured in the temperature range from 20°C to 220°C. Fig. 2 presents the temperature dependences of dielectric losses in CaF$_2$, SrF$_2$, and BaF$_2$ crystals, unirradiated and irradiated with X-rays at a dose of $1.2 \cdot 10^5$ roentgen. As can be seen from the figure, in the temperature course of dielectric losses there are two regions—the region of relaxation losses and the region of ohmic losses. With decreasing lattice energy, the region

— — — irradiated with X-rays, \(1.2 \cdot 10^5\) roentgens
——— unirradiated

Fig. 1. Dependence of \(\lg \delta\) on \(1/T\) for crystals of \(\mathrm{CaF_2}\), \(\mathrm{SrF_2}\), and \(\mathrm{BaF_2}\) before and after irradiation with X-rays (dose \(1.2 \cdot 10^5\) roentgens).

Fig. 3. Absorption spectra in \(\mathrm{CaF_2}\) crystals irradiated with protons and X-rays.

the relaxation-loss region shifts toward high temperatures. In CaF₂ it was not possible to measure the region of relaxation losses, since the latter lies in the low-temperature region. In the region of relaxation losses, crystals with a higher lattice energy have lower dielectric losses; in the region of ohmic losses, the dielectric losses are in agreement with the electrical conductivity and the lattice energy. Irradiation with X-rays leads to a decrease in the dielectric losses in CaF₂ and SrF₂ and to an increase in them in BaF₂, i.e., the effect is analogous to that observed in electrical conductivity.

1. Optical properties. The study of optical properties was carried out on CaF₂ crystals grown from natural material, under irradiation with X-rays (on the URS-70 and RUP-200 installations) and with protons at room temperature. The absorption spectra of CaF₂ crystals are given in Fig. 3. The positions of the maxima correspond to the data of a number of authors [5, 6, 7].

As is seen from the graphs, the absorption spectra in the crystals under X-ray irradiation are similar to the absorption spectra under proton irradiation. There are four clearly expressed bands with maxima at 230 mμ, 335 mμ, 400 mμ, and 565 mμ. The half-widths of the bands with maxima at 335 mμ, 400 mμ, and 565 mμ are respectively 0.530 eV, 0.533 eV, and 0.43 eV. The position of the band maxima and the magnitude of the half-width do not depend on the type of irradiation. Studies of the absorption spectra of CaF₂ crystals under irradiation with different doses showed that, with increasing exposure dose, the absorption coefficient at the band maxima increases.

Conclusion

1. In the series CaF₂, SrF₂, BaF₂, on going from a compound with higher lattice energy to a compound with lower lattice energy, a decrease in microhardness and electrical conductivity is observed. Dielectric losses in the relaxation region are smaller, and in the conductivity region larger, in crystals with higher lattice energy.

2. Irradiation with X-rays at a dose of \(1.2 \cdot 10^5\) roentgens leads to a decrease in electrical conductivity and dielectric losses in CaF₂ and SrF₂ crystals and to an increase in the latter in BaF₂.

3. The absorption spectra in CaF₂ crystals under irradiation with X-rays and protons are similar.

4. The results on microhardness, electrical conductivity, and dielectric losses before and after irradiation agree with the results for alkali-halide crystals.

LITERATURE

1. A. A. Vorob’ev. Physical Properties of Ionic Crystals. Tomsk, TSU Publishing House, 1960.

2. B. V. Budylin, A. A. Vorob’ev. Effect of Radiation on Ionic Structures. 1962.

3. E. K. Zavadovskaya, I. Ya. Melik-Gaikazyan, V. G. Elizarov. Effect of radiation on the properties of materials. Proceedings of the Conference, Novosibirsk, 1963.

4. I. V. Stepanov, P. P. Feofilov. Growth of Crystals, vol. 1, 1957.

5. A. Smakula. Phys. Rev. 77, 408, 1950.

6. S. Beril. J. Chem. Phys. 20, 297, 1952.

7. A. Smakula. Phys. Rev. 91, 1570, 1953.