ON THE MICROHARDNESS OF SOLID SOLUTIONS OF ALKALI HALIDES

R. D. DZHAPAROV

(Presented by the scientific seminar of the Laboratory of Dielectrics and Semiconductors)

Introduction

Many investigators [1—3] have obtained, for solid solutions that are thermodynamically stable at room temperature, such as KCl—KBr, KBr—KJ, and NaCl—NaBr, a dependence of microhardness on composition in the form of a curve with a maximum, which corresponds to compositions rich in the component with the higher lattice energy; whereas for solid solutions that are thermodynamically unstable at room temperature, such as KCl—KJ and NaCl—KCl [4], dependences of microhardness on composition have been obtained in the form of a complex curve. For the KCl—KJ system a dependence of microhardness on composition was obtained in the form with two maxima, which correspond to compositions of low concentrations of one component in the other, and a minimum corresponding to intermediate compositions. For the NaCl—KCl system the curve of the dependence of microhardness on composition is the same as for the KCl—KJ system; the microhardness for compositions rich in the KCl component was not determined.

From the results of earlier works it is impossible to arrive at a general conclusion concerning the dependence of microhardness on the composition of solid solutions of alkali halides. In this connection we set ourselves the task of determining the dependence of microhardness on composition for solid solutions of the KCl—KBr and NaCl—KCl systems, as thermodynamically stable and unstable at room temperature.

It is of interest to consider the influence of complete decomposition of a solid solution on the magnitude of microhardness. For this purpose we chose the equimolar composition of the NaCl—KCl solid solution, which is more unstable in comparison with the other compositions, with the exception of those that are in the vicinity of the critical point of mutual solubility of the components of this system in the solid state.

Measurement procedure

Single crystals of solid solutions were grown from the melt by the Kyropoulos method, followed by quenching in air from 520—530°C to room temperature. The starting reagents were salts NaCl and KCl of “KhCh” grade and KBr of “ChDA” grade.

To avoid concentration inhomogeneity, the total weight of the charge did not exceed 200—250 grams. The composition of the solid solutions was determined by flame photometry using a UM-2 monochromator and a photoelectronic multiplier with an accuracy of ±2%.

Measurement of microhardness was carried out on a PMT-3 instrument on fresh cleavages. A load of 20 g was used.

The average microhardness value of individual specimens was determined by measuring 10 impressions from each composition of the solid solution; to determine the microhardness, 3–4 specimens were taken, and by secondary averaging of the mean microhardness values of these specimens the average microhardness value of the given solid solution was determined.

To clarify the influence of decomposition on the magnitude of the microhardness of the equimolar composition of the NaCl—KCl system, specimens of this composition were annealed for 1 hour at a temperature of 250°C. The microhardness of one and the same specimen was measured before and after annealing.

Results of microhardness measurements and their discussion

For solid solutions of the KCl—KBr system we obtained the same dependence of microhardness on their composition as in works [1—3], differing only in numerical values; we explain this difference by the thermodynamic states of our crystals.

The dependence of microhardness on the composition of solid solutions of the NaCl—KCl system, obtained by us, is presented in Fig. 1, a, where it is shown that the dependence of microhardness on the composition of the solid solution of the NaCl—KCl system is characterized by a curve with a maximum. With increasing concentration, the microhardness increases both in compositions rich in the NaCl component and in compositions rich in the KCl component. But the rate of increase of the microhardness of solid solutions with increasing concentration in compositions rich in the NaCl component, with the larger energy, is greater than in compositions rich in the KCl component, with the smaller lattice energy. For example, the microhardness value of the composition 10 mol % KCl in NaCl is equal to the microhardness value of the composition 30 mol % NaCl in KCl. The microhardness of the equimolar composition is greater than for the other compositions. But this is not the maximum value of the microhardness, since from the course of the curve one can find the approximate position of its maximum. The position of the maximum is shifted from the equimolar composition toward compositions rich in the component with the larger lattice energy, and lies in the composition interval 60—70 mol % NaCl. The general form of the curve has the same shape as the diagram of mutual solubility of the components of the NaCl—KCl system in the solid state [5].

Thus, we can arrive at the following general conclusion: the microhardness of solid solutions of alkali halides, both thermodynamically stable and unstable at room temperature, changes as a function of composition along an asymmetric curve with a maximum, which is shifted toward compositions rich in the component with the larger lattice energy. Apparently, the microhardness of solid solutions of the NaCl—KCl system is connected with the degree of mutual solubility of its components in the solid state (Fig. 1б). The greater the solubility of one component in the other, the lower the microhardness for these compositions, and vice versa.

Barrett and Wallace [6], on the basis of the change in the lattice parameter with composition, calculated that the concentration of Schottky defects as a function of composition (Fig. 1, в) also changes along an asymmetric curve, with the maximum corresponding to a composition of 70 mol % NaCl. Thus, the maxima of microhardness, the concentration of Schottky defects, and the diagram of mutual solubility lie in the interval 60—70 mol % NaCl. With increasing concentration of Schottky defects, the internal stress of the crystal increases.

The internal stress of the crystal is greater in compositions rich in NaCl, since the solubility of KCl in NaCl is less than the solubility

NaCl in KCl. The weaker the mutual solubility of the components, the greater the defectiveness and inhomogeneity of the crystal and the greater the overall internal stress of the crystal; consequently, the greater the microhardness. Treshchina [7], studying the microhardness of solid solutions of the KCl—KBr and NaCl—NaBr systems, came to the conclusion that the microhardness of crystals of solid solutions is determined mainly by the number of Schottky defects.

With an increase in the concentration of one component in the other, the scatter of microhardness values increases. In our case the maximum scatter corresponds to the equimolar composition. This indicates that, with increasing concentration, the inhomogeneity and defectiveness of the crystal of solid solutions of alkali halides increase.

Fig. 1. Dependence of microhardness (a), concentration of Schottky defects on composition, according to the data of Barrett and Wallace (v), and the diagram of mutual solubility of the NaCl—KCl system in the solid state, according to Nacken (b).

In work [8] it was shown by X-ray investigation that heating crystals of solid solutions of the NaCl—KCl system to temperatures lying below the curve of mutual solubility of the components in the solid state leads to the decomposition of these solutions; it was shown that, when a sample of the equimolar NaCl—KCl composition is heated from room temperature to 220°C, complete decomposition occurs within 10 minutes.

Consequently, the holding time chosen by us (1 hour) at a temperature of 250°C was sufficient for complete decomposition of the equimolar NaCl—KCl composition.

After annealing a sample of the equimolar NaCl—KCl composition, bulk clouding occurs, which is the final stage of decomposition [8, 9]; in this case the sample has a white enamel-like color. After annealing, the microhardness value of the equimolar NaCl—KCl composition decreases by 17% in comparison with its value before annealing. The decrease in microhardness after annealing is explained by the fact that in this process the solid solution passes from a state corresponding to the quenching temperature to a state corresponding to the annealing temperature, which is accompanied by its decomposition and a decrease in internal mechanical stress.

The microhardness of the equimolar NaCl—KCl composition, thermodynamically unstable at room temperature, is greater than the relative

the microhardness value of the NaCl component by 72%. The microhardness of the equimolar composition KCl—KBr, thermodynamically stable at room temperature, is 44% greater than the microhardness value of the KCl component.

According to Hovi [10], the heat of formation of the equimolar composition of the KCl—KBr system is 219 cal/mol, while for the equimolar composition NaCl—KCl it is \(1046 \pm 6\) cal/mol [6]. Consequently, one may come to the conclusion that the greater the heat of formation, the greater the microhardness of the given composition of the solid solution.

Conclusions

1. The change in microhardness of the composition of solid solutions of alkali halides, both thermodynamically stable and unstable at room temperature, has the form of a curve with a maximum shifted from the equimolar composition toward compositions rich in the component with the greater lattice energy.

2. The microhardness, the diagram of mutual solubility of the components in the solid state, and the concentration of Schottky defects have the same character of variation with composition. It follows from this that microhardness is connected with the mutual solubility of the components in the solid state and with the defectiveness of single crystals.

3. The decomposition of solid solutions of alkali halides that are thermodynamically unstable at room temperature leads to a decrease in microhardness.

4. Comparison of the microhardness values of equimolar solid solutions of the KCl—KBr and NaCl—KCl systems and correlation of their relative magnitude with the heat of formation of these compositions shows that the microhardness of solid solutions of alkali halides is the greater, the greater the magnitude of their heat of formation.

References

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