DIELECTRIC LOSSES OF NONEQUILIBRIUM ALKALI HALIDE SOLID SOLUTIONS

E. K. ZAVADOVSKAYA, R. D. DZHAPAROV, D. A. ASANBAEVA

(Presented by the scientific seminar of the laboratory of EnElIS at the Research Institute of Physico-Technical Technologies)

Introduction

Many ionic binary solid solutions, homogeneous and equilibrium at high temperature, decompose into two equilibrium phases at lower temperatures. The decomposition process leads to inhomogeneity of the solid solution and to an increase in the number of defects in its volume. Such a structural change in the solid solution affects its physical properties and, in particular, its dielectric losses.

The purpose of the present study is to clarify the nature of the influence of the decomposition of ionic solid solutions on their dielectric losses. At the same time, X-ray monitoring of the decomposition of the solid solutions was carried out, and the absorption spectrum of \(F\)-centers in these same samples was also measured.

As objects of study, solid solutions NaCl—KCl, NaBr—KBr, where the decomposition mechanism is based on the process of segregation of cations filling the octahedral voids of the closest packing of anions, and KBr—KI, where decomposition occurs by segregation of anions, were selected. In this work only equimolar compositions of the above-mentioned systems were investigated.

Potassium and sodium chlorides form a continuous series of solid solutions with a decomposition region below \(500^\circ\mathrm{C}\) [1]. Potassium and sodium bromides form a continuous series of solid solutions with a critical point of mutual solubility corresponding to a temperature of \(365^\circ\mathrm{C}\) and a composition of \(40\ \mathrm{mol}\%\) KBr [2].

The KBr—KI system, according to Tammann’s data [2], has a critical point of mutual solubility at \(250^\circ\mathrm{C}\) and \(40\ \mathrm{mol}\%\) KI, while according to Popov et al. [3], these solutions do not decompose at room temperature in the absence of moisture.

Experimental Procedure

Crystals of solid solutions were grown by the Kyropoulos method with subsequent quenching. Growth was carried out in an ordinary air atmosphere. Chemically pure (“ch. p.”) reagents served as the starting materials. Quenching of the crystals was performed as follows: the grown single crystal was cooled to a temperature slightly above the boundary of complete mutual solubility, then rapidly cooled in air and placed in dry transformer oil.

From one crystal, three series of samples were prepared. The first series was intended for measuring \(\mathrm{tg}\,\delta\), the second for studying absorption spectra, and the third for X-ray investigation.

Measurement of the dielectric-loss angle and of conductivity was carried out on a complex-measurement setup developed at the Tomsk Polytechnic Institute, using an unbalanced-bridge circuit analogous to the apparatus of B. I. Vorozhtsov [4]. The dielectric losses in crystals of solid solutions were studied in the frequency range from 50 to 5000 Hz and in the temperature interval from 20 to 300°C in vacuum \((10^{-2}—10^{-3}\ \mathrm{mm\ Hg})\). Aquadag was applied to the samples as electrodes.

The absorption spectra of solid solutions of the NaCl—KCl system were measured on an SF-4A spectrophotometer at room temperature. Measurements were made immediately after irradiation in the dark with X rays under reproducible conditions (60 kV, 10 mA) at room temperature for 2 hours (BSV-1 X-ray tube with a tungsten anticathode, URS-70 apparatus). To avoid the influence of moisture during measurement and irradiation, all samples were coated with VM-1 vacuum oil. In the measured wavelength region, VM-1 oil is transparent.

X-ray photographs of the solid solutions were taken in an RKSO camera for stationary samples, also coated with VM-1 oil, in positions close to those reflecting the 200 reflex of the initial solution. Successive rotations of the samples by 1–3° were carried out about the [100] axis of the initial crystals of the solid solution. Mixed radiation from a copper anticathode was used. X-ray photographs of the samples were taken before and after decomposition.

After fixation of the initial state on the Laue pattern (from one sample) and recording of the absorption spectrum on the SF-4A (from another), both samples, together with the one intended for measurements of \(\operatorname{tg}\delta\), were placed in a cell in which annealing was performed with simultaneous measurement of \(\operatorname{tg}\delta\). A vacuum of \(10^{-2} + 10^{-3}\ \mathrm{mm\ Hg}\) was maintained in the cell. Thus the influence of atmospheric moisture was eliminated, and at the same time identical annealing conditions were ensured for all samples.

Experimental results and their discussion

When samples of the equimolar composition of the NaCl—KCl solid solution are heated to 135°C, ohmic losses are observed starting from room temperature (Fig. 1), in contrast to the pure components, in which there is a relaxation part of the losses. The conductivity of the samples increases with increasing temperature (Fig. 1). Probably, the initial samples contain weakly bound ions which, at relatively low temperatures, are capable of participating in conduction.

During annealing at 135°C, at first the dielectric losses decreased over 7 hours by factors of 5 and 20 at frequencies of 50 and 5000 Hz, respectively. Annealing for more than 7 hours produced no substantial changes, and the dielectric losses tended toward saturation.

The conductivity also decreases as a function of annealing time. This is explained by the fact that, as the degree of decomposition increases, the number of ions participating in conduction decreases. With time, the sizes of particles of the precipitating phases increase because of further segregation of cations, and at the same time the number of diffusing ions decreases.

The same picture was also observed at a temperature of 160°C. N. P. Bogoroditskii et al. [5], when measuring \(\operatorname{tg}\delta\) at a frequency of 1 MHz at a temperature of 200°C on polycrystalline quenched samples of the same composition, observed that the dielectric losses pass through a maximum with time. In samples obtained by slow cooling, no changes in dielectric losses were observed. The authors

they believed that dielectric losses caused by ion relaxation prove to be especially noticeable in the state of their decomposition.

A. M. Elistratov and R. A. Zvinchuk [6] investigated the process of nucleation and development of inhomogeneities (during isothermal decomposition) in single crystals of nonequilibrium (supersaturated) solid solutions of this system. The authors showed that, for intermediate compositions of potassium and sodium chlorides, the process of rapid breakup of single crystals of a supersaturated solution into elastically coupled microregions, corresponding in composition and parameters to the equilibrium phases, is the first stage of decomposition.

Fig. 1. Dielectric losses and conductivity of an equimolar-composition NaCl—KCl solid solution (—●—$\sigma$, —·—50 Hz, —△—100 Hz, —○—500 Hz, —□—1000 Hz, —×—5000 Hz): a) as a function of temperature; b) as a function of annealing time at 135°C

In the first stage of decomposition the crystal is in a stressed state. The second stage is recrystallization of the equilibrium phases, the final establishment of two-phase equilibrium. Growth of some microvolumes occurs at the expense of others of the same composition. At the same time, the stresses in the solid solution are gradually relieved.

It should be noted that, in the first stage of decomposition, the dielectric losses must pass through a maximum because of the increased stress in the crystal. In the second stage, however, because of the reduction of stresses, the dielectric losses must decrease.

The decrease in dielectric losses during isothermal annealing in our cases corresponds to the second stage of decomposition. On the X-ray diffraction patterns of the initial specimens, apart from sharply expressed Laue spots corresponding to the solid solution, two systems of weak uniform Debye rings are visible, which indicates a small degree of decomposition. Therefore, in our case the expected maximum was not observed, because the first stage had already occurred during quenching.

On the X-ray diffraction patterns of the specimens after decomposition, two systems of intense Debye rings with local intensifications are observed. Traces of Laue spots corresponding to the initial state are also present. The formation of two systems of lines with local intensifications indicates that the specimens have decomposed to a significant extent into two equilibrium phases. At the same time, a strictly mutually parallel texture of both precipitating phases is observed.

The equimolar composition of the NaBr—KBr solid solution qualitatively repeats the behavior of the NaCl—KCl solid solution at 157°C, but the values of \(\operatorname{tg}\delta\) decrease by only a factor of 2–3 over 10 hours.

In the initial specimens obtained from a single crystal of the equimolar composition of the NaCl—KCl solid solution, measurement of the absorption spectrum in the absorption region of \(F\)-centers showed that there is a broad band with a maximum at 530 nm, corresponding probably to the \(F\)-centers of the initial composition. The position of the \(F\)-band does not obey the law of additivity. Along with the \(F\)-band of the initial composition, there is a low-intensity band with a maximum at 480 nm. The broadening of the \(F\)-band and the appearance of the band at 480 nm are explained by the decomposition of the quenched crystal to a small extent, which is confirmed by the X-ray diffraction pattern of the initial specimen, where traces of decomposition are present. Therefore the band with a maximum at 480 nm corresponds to the \(F\)-band of the precipitated phase (Fig. 2).

Fig. 2. Absorption spectra of the equimolar composition of the NaCl—KCl solid solution: a) before decomposition; b) after decomposition

In specimens annealed at a temperature of 135°C for 10 hours in vacuum, three absorption bands were found, with maxima at 510 nm, 480 nm, and 590 nm. This indicates a clear influence of decomposition, which is confirmed by X-ray diffraction investigation. The intense bands at 480 nm and 590 nm are the bands of the phases that precipitated during decomposition of the solid solution. The X-ray diffraction patterns of these specimens also show that, in addition to the decomposition products, traces of the initial solid solution are present. Therefore the band at 510 nm corresponds to the \(F\)-band of the initial solid solution, which had not yet had time to decompose completely under this annealing regime.

Specimens of the equimolar composition of the KBr—KJ system, after isothermal annealing at a temperature of 150°C for 80 hours, showed no change in \(\operatorname{tg}\delta\) as a function of time. On the X-ray diffraction patterns of the initial specimen and after annealing, clear Laue spots characteristic of a solid solution are observed. Since in this system the anions, which constitute the closest packing in the lattice, are subject to segregation,

of the solid solution, their motion is more difficult in comparison with the motion of cations in the NaCl—KCl system. Therefore, decomposition in the solid solution of the KBr—KJ system proceeds considerably more slowly, especially in vacuum, where the influence of air moisture is excluded.

The temperature dependence of $\operatorname{tg}\delta$ for the equimolar composition KBr—KJ (from 20 to 360°C) has a relaxation maximum (Fig. 3). With an increase

Fig. 3. Dependence of $\operatorname{tg}\delta$ of the equimolar composition KBr—KJ on temperature ($-\bullet-$ 50 cps, $-\triangle-$ 100 cps, $-\circ-$ 500 cps, $-\square-$ 1000 cps, $-\times-$ 5000 cps)

in the frequency of the applied voltage, the maximum of $\operatorname{tg}\delta$ shifts toward higher temperatures. This maximum is probably associated with the transition from the region of the two-phase state to the region of the single-phase state [2].

REFERENCES

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