ELECTRICAL CONDUCTIVITY OF OXIDES AND HALIDE COMPOUNDS OF GROUP II ELEMENTS

A. A. VOROB′EV and A. P. NAKHODNOVA

Modern electrical insulating materials have become so differentiated in their characteristics and properties that their rational use has become possible only with a thorough study of their physicochemical and technological features. In this connection, the study of the properties of a substance in relation to the composition and position of the elements forming the compounds in the D. I. Mendeleev system acquires not only theoretical interest but also great practical significance.

In view of the fact that the oxides of Group II metals have found wide application in the production of oxide cathodes, ceramic electrical insulating materials, radioporcelain, and high-frequency ceramics, it is of interest to trace the regularities in the changes of electrical properties for the simplest compounds of the elements of this group in the D. I. Mendeleev system.

With respect to the physicochemical properties of compounds of Group II elements, it is known that in the series beryllium, magnesium, calcium, strontium, and barium there occurs a regular change in the properties of the oxides and their hydrate compounds. As the atomic number of the metal ion increases, a decrease in the melting temperature and an increase in the heat of hydration of the oxides are observed, along with an increase in the solubility and dissociation constant of their hydrate compounds. Closely related to the sizes of the metal ions are the density, heat of sublimation, dissociation energy, magnitude of the isoionic potential, and entropy of the oxides of Group II metals. The energy of the crystal lattice depends directly on the radii, valence, and number of particles in the molecule.

The electrical properties of compounds of Group II elements have scarcely been covered in the literature. The available works consist of separate, mutually unrelated studies of the specific electrical conductivity of certain oxides of Group II metals and their mixtures. The absence of systematic measurements under identical conditions does not allow conclusions to be drawn about the dependence of electrical conductivity on the composition of the compound and the energy of liberation of ions in the lattice. In this connection, we undertook measurements of the temperature dependence of the specific electrical conductivity of oxides of beryllium, magnesium, calcium, strontium, barium, and zinc; fluorides and chlorides of calcium, strontium, and barium; and comparisons were made with the energy of the crystal lattice.

The study of specific electrical conductivity was carried out on sintered polycrystalline specimens prepared from “chemically pure” reagents. The specimens had the form of disks 10 mm in diameter and 0.35–0.8 mm thick. The specimens were calcined in a silite furnace for 2 hours in air at the same relative sintering temperature. The ratio of the sintering temperature to the melting temperature was approximately 0.46 for oxides and 0.65 for halide compounds. Barium oxide, which forms a stable peroxide compound in air, was calcined in vacuum. The density of the specimens, relative to the density of single crystals of the substance, was 65–75%. The specimens to be tested were ground, and platinum electrodes were then applied to them by thermal spraying.

The temperature dependence of the electrical conductivity was studied in vacuum at \(P = 10^{-5}\) mm Hg, with direct current, according to the circuit shown in Fig. 1. The specimen was clamped between two plane-parallel electrodes equipped with heating elements and a guard ring. The measured current was determined with a galvanometer whose maximum sensitivity was \(6 \times 10^{-8}\) a/mm. The temperature of the specimen was measured with a thermocouple attached to one of the electrodes. Measurements were carried out at a field strength of \(2.5\)–\(75\) V/cm.

Fig. 1. Electrical circuit for measuring electrical conductivity

When current passed through the specimen, a decrease of current with time was observed (Fig. 2). The electrical conductivity was measured from the steady-state current 6–7 minutes after the voltage was applied. The experimental results showed that, for all the compounds considered, the usual increase of electrical conductivity with temperature, characteristic of ionic conduction, is observed. For a given temperature, the electrical conductivity of the oxides increases in going from beryllium oxide to barium oxide (Fig. 3). A similar character of the change in electrical conductivity was also observed in the group of halide compounds of calcium, strontium, and barium (Figs. 4, 5). As an exception, we note strontium chloride, whose specific electrical conductivity is somewhat greater than that of barium chloride. In this connection it should be noted that the type of crystal lattice and the crystallographic system of strontium chloride differ from those of barium chloride and chlori-

Fig. 2. Dependence of the specific electrical conductivity on time for calcium fluoride

Axis labels and inscriptions in the figure: \(\sigma\), \(\Omega^{-1}\,\mathrm{cm}^{-1}\); time \(t\) in min.; \(T = 560^\circ\mathrm{C}\); \(T = 480^\circ\mathrm{C}\).

Fig. 3. Dependence of the logarithm of the specific electrical conductivity of oxides of Group II metals on \(1/T\)

Axis labels and inscriptions in the figure: \(\lg \sigma\), \(\Omega^{-1}\,\mathrm{cm}^{-1}\); \(10^3/T\); BaO; SrO; ZnO; CaO; MgO; BeO.

Dependence of the logarithm of the electrical conductivity on \(1/T\) for fluoride compounds of the alkaline-earth metals.

Upper curve — calcium fluoride.
Middle curve — strontium fluoride.
Lower curve — barium fluoride.

Fig. 5. Dependence of the logarithm of the electrical conductivity on \(1/T\) for chloride compounds of the alkaline-earth metals.
I. Calcium chloride. II. Strontium chloride. III. Barium chloride.

of pure calcium. Apparently, this factor accounts for the observed deviation of the value of the specific electrical conductivity for strontium chloride.

The linear dependence of the logarithm of electrical conductivity on reciprocal temperature has a break. The slope of the straight lines of the logarithm of electrical conductivity as a function of reciprocal temperature is different at low and high temperatures, which indicates the presence of current carriers with different activation energies. From the slope of the straight lines, the values of the activation energy of the current carriers were calculated. In the high-temperature region the activation energy changes in accordance with the energy of the crystal lattice of the substance. The highest value of the activation energy in the high-temperature region is found for beryllium oxide (2.14 eV), and the lowest for barium oxide (1.38 eV). In the low-temperature region the activation energy for all the compounds is considerably smaller than in the high-temperature region, and for the oxides it varies within the limits 0.15–0.48 eV. No regularity is observed in the change in the magnitude of the activation energy of the current carriers for this region. This is apparently connected with the fact that, in the low-temperature region, the electrical conductivity is determined mainly by the presence of impurities in the sample.

The dependence of the electrical conductivity on the field strength was investigated. The experiments showed that in the region of weak fields (2.5–75 V/cm) the electrical conductivity of the compounds under consideration increases with increasing field strength. Measurements were carried out in fields where relatively small deviations from Ohm’s law were observed.

Since the measurements of electrical conductivity were carried out on porous polycrystalline samples, we studied the influence of the density of the samples on the magnitude of the specific electrical conductivity. For this purpose samples were prepared whose relative density varied within the limits 30–70%. The results of the experiments showed that, with an increase in the density of the sample, the magnitude of the specific electrical conductivity increases (Fig. 6). The curves of the logarithm of electrical conductivity as a function of reciprocal temperature for samples of different density are arranged almost parallel to one another. The values of the activation energy calculated from the slope of the curves for samples of different density practically do not change.

Fig. 6. Dependence of the logarithm of electrical conductivity on \(1/T\) for the oxides of beryllium, magnesium, and zinc on the forward (▲) and reverse (○) run.

Conclusions

1. The electrical conductivity of sintered samples of oxides and halide compounds of metals of the second group increases according to an exponential law characteristic of ionic conductivity.

1. Under identical experimental conditions, the electrical conductivity of compounds increases with decreasing crystal-lattice energy of the substance.

2. The values of the activation energy of the current carriers decrease regularly with decreasing crystal-lattice energy of the substance.

3. In the low-temperature region, the electrical conductivity of the simplest compounds of Group II elements and the activation energy do not have a clearly expressed regular behavior and, apparently, depend on the presence of impurities in the sample under study.

4. Electrical conductivity and activation energy are connected with the basic chemical characteristics of the substance and are determined by the crystal-lattice energy both for compounds having a common anion and for compounds in which one metalloid ion is successively replaced by another of larger radius.

Table 1

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Errata