STUDY OF THE ELECTRICAL CONDUCTIVITY OF PARTIALLY DECOMPOSED POTASSIUM PERMANGANATE

A. A. KABANOV

(Presented by the scientific seminar of the Department of Radiation Chemistry)

1. Introduction

The effectiveness of studying electrical conductivity in solving individual questions of the mechanism of thermal decomposition of solid ionic compounds has been confirmed by a number of studies [1—5] carried out in this direction. In the present work, measurement of electrical conductivity was used to estimate the formation of the solid solution \(\mathrm{KMnO_4 + K_2MnO_4}\) during the thermolysis of potassium permanganate.

Potassium permanganate [6] decomposes according to the equation

\[ 2\mathrm{KMnO_4} \to \mathrm{K_2MnO_4} + \mathrm{MnO_2} + \mathrm{O_2}, \]

and, during thermal decomposition, the same products are formed as during radiolysis. For the radiolysis of \(\mathrm{KMnO_4}\) it has been established [7] that \(\mathrm{MnO_2}\) is released immediately as a separate phase, while \(\mathrm{K_2MnO_4}\) at first forms a solid solution with potassium permanganate, which has not yet been proven for thermal decomposition [8]. At the same time, the discussion is continued of the mechanism of electrical conductivity of \(\mathrm{KMnO_4}\), begun by us in works [9, 10], where it was shown that \(\mathrm{KMnO_4}\) should be regarded as an electronic semiconductor.

2. Experimental procedure

In the experiments, potassium permanganate “pure for analysis” was used, additionally purified by double recrystallization. Decomposition of ground \(\mathrm{KMnO_4}\) crystals was carried out at \(220^\circ\mathrm{C}\) in air according to the procedure described in work [11]. Potassium manganate was obtained according to the procedure of work [12], and manganese dioxide was isolated from the products of complete decomposition of potassium permanganate. For measuring electrical conductivity, samples¹ were prepared with dimensions \(25 \times 6 \times 3\ \mathrm{mm^3}\) and \(13 \times 8 \times 2\ \mathrm{mm^3}\) from preparations carefully dried over \(\mathrm{P_2O_5}\), by pressing under a pressure of \(10\ \mathrm{t/cm^2}\). Aquadag electrodes were applied to the \(6 \times 3\ \mathrm{mm^2}\) and \(8 \times 2\ \mathrm{mm^2}\) faces, and the samples were again subjected to drying. Electrical conductivity was measured at a constant voltage of 105 volts with an E6-3 teraohmmeter at a heating rate of \(0.8—1.2\ \mathrm{deg/min}\). Measurements were carried out in air and in a vacuum of \(10^{-3}—10^{-4}\) torr. The total error in measuring electrical conductivity did not exceed \(\pm 15\%\); the results were well reproducible. Preliminary experiments on samples of various shapes revealed that the specific electrical conductivity,

calculated from the formula for bulk conductivity, and the activation energy do not depend substantially on the distance between the electrodes within the range 5–30 mm, on the area of the small electrode within the range 16–80 mm\(^2\), or on the application to the lateral surface of the pellets of a grounded guard ring. These facts make it possible to assume that what we measured was bulk electrical conductivity.

3. Results and Discussion

The results of the experiments are presented in Figs. 1–3 and in the table. The activation energy was calculated from the formula

\[ \sigma = A \cdot \exp\left(-\frac{E}{2kT}\right). \]

The observed monotonic increase in electrical conductivity (up to 80% decomposition) and the corresponding decrease in the activation energy of partially decomposed KMnO\(_4\) (Figs. 1–3) are characteristic only of substitutional solid solutions, especially in semiconductors [13–15], which include both KMnO\(_4\) [9, 10] and the solid products of its decomposition [16, 17]. In impurity semiconductors [15, 18], an increase in the concentration of impurity levels responsible for electrical conductivity leads to an increase in electrical conductivity and a decrease in the activation energy (band-gap width). The change in the activation energy in this case usually ceases after the formation of an impurity band due to overlap of the wave functions of individual impurity atoms (ions),

Fig. 1. Temperature dependence of the electrical conductivity (\(\sigma\), ohm\(^{-1}\)·cm\(^{-1}\)) of KMnO\(_4\) and products of its decomposition: 1 — K\(_2\)MnO\(_4\) (measurement in air); 2 — KMnO\(_4\), pressed from unground crystals (—×— measurements in air, —●— measurements in vacuum); 3 — KMnO\(_4\), pressed from finely ground crystals (—○— measurements in vacuum, —□— measurements in air); 4 — KMnO\(_4\) + 1 mol % K\(_2\)MnO\(_4\) (unground crystals were pressed, measurements in air); (5, 7–10, and 6) — permanganate that had been preliminarily decomposed by, respectively, 10, 5, 20, 50, 80, and 100%; 11 — manganese dioxide isolated from the decomposition products of KMnO\(_4\) (measurements in air).

Fig. 2. Relative change in electrical conductivity (at 140°C) with the degree (\(i\)) of preliminary decomposition of KMnO\(_4\).

—○— measurements in vacuum,
—△— measurements in air.

which can occur already at a 0.5% impurity content [15]. In the decomposition of solid solutions made from semiconductors, a maximum is usually observed in the functional dependence of electrical conductivity on impurity concentration [13, 14], which was also obtained in our experiments (Fig. 2). The decrease in electrical conductivity after decomposition of the solid solution \( \mathrm{KMnO_4 + K_2MnO_4} \) may be due to the fact that \( \mathrm{K_2MnO_4} \) cannot make a substantial contribution to the overall electrical conductivity of the mixture \( \mathrm{MnO_2} \) with \( \mathrm{K_2MnO_4} \), whereas before decomposition of the solid solution the electrical conductivity increases both because of an increase in the concentration of impurity levels and because of a decrease in the distance between the electrodes owing to the evolution of \( \mathrm{MnO_2} \), which we do not take into account in calculating the specific volume conductivity. With complete decomposition of \( \mathrm{KMnO_4} \), the electrical conductivity of the mixture of decomposition products should be determined by \( \mathrm{MnO_2} \) (Fig. 1), which is confirmed by the corresponding equality of the activation energies of electrical conductivity (Fig. 3).

Fig. 3. Change in the activation energy (1) of electrical conductivity with the degree of preliminary decomposition (—○— measurements in vacuum, —△— measurements in air); 2—mean boundary of the activation energy of electrical conductivity of \( \mathrm{MnO_2} \) separated from the decomposition products of \( \mathrm{KMnO_4} \).

In our experiments we did not detect any specific influence of the atmosphere on the results of measuring the electrical conductivity of potassium permanganate, but a noticeable increase in electrical conductivity was found when \( \mathrm{KMnO_4} \) crystals were ground before pressing to a dispersion of about 200 microns (Fig. 1, curves 2 and 3) and a decrease in the activation energy (Table). This, in our opinion, is associated with partial tribochemical decomposition of \( \mathrm{KMnO_4} \) during grinding of the crystals, which is quite probable [19]. The observed effects from the introduction of \( \mathrm{K_2MnO_4} \) (Table, Fig. 1) are analogous to partial decomposition of potassium permanganate. This confirms the determining influence of potassium manganate on the electrical conductivity of \( \mathrm{KMnO_4} \).

Table

Results of calculating the activation energy \((E)\)

Conclusions

1. During the thermal decomposition of $\mathrm{KMnO_4}$, potassium manganate separates out into a solid solution, which breaks down after approximately 80% decomposition of $\mathrm{KMnO_4}$.

2. The electronic conductivity of potassium permanganate, at least in the interval $40^\circ$–$140^\circ\mathrm{C}$, is due to donor levels formed by $\mathrm{MnO_4^-}$ ions.

3. Grinding $\mathrm{KMnO_4}$ crystals in a mortar may lead to partial tribochemical decomposition of the preparation.

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1. The preparation of specimens for the experiments was carried out by A. N. Oblivantsev, to whom the author expresses his gratitude. ↩