The \(\mathrm{ClO_3}\) Radical in Irradiated \(\mathrm{KClO_4}\)

A. A. Vasil’ev, Yu. A. Sakharov, L. V. Serikov
(Presented by the scientific seminar of the Department of Radiation Chemistry)

The parameters of the electron paramagnetic resonance spectra of the \(\mathrm{ClO_3}\) radical in various solid matrices are known from many works [1–4]. In connection with this, its identification presents no difficulties.

We studied the EPR spectra of \(\mathrm{ClO_3}\) formed in single crystals of pure potassium perchlorate under the action of electrons with an energy of 1.6 MeV at \(T = +20^\circ\mathrm{C}\). The electron source was an ECT-2.5 accelerator; recording of the EPR spectra was carried out using a modernized EPA-2 spectrometer.

Irradiation was performed in the resonator of the spectrometer. The spectra were measured both during irradiation and after its completion. This procedure made it possible to reduce the error in measuring the concentration that could arise owing to the occurrence of post-irradiation reactions.

The dose rates used lay within the range \(0.5 \div 10\) Mrad/min.

The spectrum observed during irradiation is shown in Fig. 1.

The eight-component spectrum (lines \(1'\) and 1) can be confidently assigned to the \(\mathrm{ClO_3}\) radical on the basis of the data of [1–4].

The \(\mathrm{ClO_3}\) radical exists in the \(\mathrm{KClO_4}\) lattice in two orientations relative to the crystallographic axes. When the direction of the magnetic field is changed, each line in the spectrum splits into two (Fig. 1). Splitting is not observed only ...

then, when \(\mathbf H \parallel \mathbf a\), or \(\mathbf H \parallel \mathbf b\) (\(\mathbf a,\mathbf b\), and \(\mathbf c\) are the crystallographic axes of \(KClO_4\)).

For \(ClO_3\) (line I) we obtained the following values of the components of the \(g\)-tensor: \(g_{\parallel}=2.009\); \(g_{\perp}=2.007\); and of the components of the HFS tensor: \(A_{\parallel}=105\) G, \(A_{\perp}=147\) G. Lines II and II′ in the spectra in Fig. 1 belong to the \(ClO_2\) radical, which was studied in detail in [5]. The remaining part of the spectrum apparently cannot be attributed to chlorine-containing centers.

Changes in the concentration of \(ClO_3\) during irradiation at different dose rates are shown in Fig. 2.

Post-radiation changes of \(ClO_2\) and \(ClO_3\) are shown in Fig. 3. In analogous experiments at a temperature of \(-196^\circ C\), the radicals \(ClO_2\) and \(ClO_3\) were not detected, in contrast to the data of work [7].

The accumulation curves have a maximum, the magnitude and position of which depend on the radiation dose rate. This indicates the complexity of the processes of formation and loss of \(ClO_3\).

As a scheme consistent with the results obtained, one may propose the following reactions of formation and loss:

\[ ClO_4^- \leadsto ClO_4 + \bar e;\qquad ClO_4 \to ClO_3 + O \tag{1} \]

\[ 2ClO_3 \rightleftharpoons Cl_2O_6 \tag{2} \]

\[ ClO_3+\bar e \to \text{products} \tag{3} \]

\[ ClO_3 \to ClO_2+O \tag{4} \]

Recombination of \(ClO_4\) with an electron

\[ ClO_4+\bar e \to \text{products} \tag{5} \]

may increase the radiation-chemical yield of \(ClO_3\) through reaction (1).

For constant values of the rate constants of reactions 1–5, the accumulation curve of \(ClO_3\) should not have a maximum. Its existence is apparently associated with an increase during irradiation of the concentration of a product reacting with \(ClO_3\) or with \(ClO_4\).

In the present case these products may be free electrons, recombining with \(\mathrm{ClO_3}\) and \(\mathrm{ClO_4}\) in reactions (3) and (5). The concentration of free electrons may increase when electron traps are filled; such traps may be both structural defects existing in the crystal before irradiation and defects induced by the radiation.

In this case the total concentration of unfilled radiation-induced and preexisting traps should decrease.

A decrease in the number of unfilled traps leads to an increase in the concentration of free electrons and to an increase in the probability of reactions (3) and (5).

The processes of formation, filling, and release of traps after a certain time reach equilibrium, and a stationary concentration of free electrons, and consequently of \(\mathrm{ClO_3}\) radicals, is established.

Partial release of traps after the irradiation is stopped (and, consequently, after the generation of free electrons is stopped) should, upon repetition of the irradiation session, lead to the appearance of yet another maximum on the curve of the dependence of the \(\mathrm{ClO_3}\) concentration on the dose absorbed by \(\mathrm{KClO_4}\).

Curves of this kind were indeed observed by us. Intermittent irradiation of samples, with intervals between switching off the electron beam and subsequently switching it on, produces a series of successive maxima of the \(\mathrm{ClO_3}\) concentration.

Finally, the effect observed in (6) on the rate of accumulation of \(\mathrm{ClO_3}\) of \(\mathrm{Ag^+}\) ions introduced into the \(\mathrm{KClO_4}\) lattice, which are electron acceptors, also agrees with the assumption made concerning the important role of recombination reactions of \(\mathrm{ClO_3}\) and \(\mathrm{ClO_4}\) with free thermalized electrons.

The proposed reaction scheme is described by the following system of differential equations:

\[ \frac{dc_1}{dt}=A-k_1c_1-k_5c_1c_3 \]

\[ \frac{dc_2}{dt}=k_1c_1-k_3c_2c_3-k_2c_2^2-k_4c_2+2k_7c_5 \]

\[ \frac{dc_3}{dt}=H-k_3c_2c_3-k_5c_1c_3-k_6c_3c_4 \]

\[ \frac{dc_4}{dt}=B-k_6c_3c_4 \]

\[ \frac{dc_5}{dt}=\frac{1}{2}k_2c_2^2-k_7c_5 \]

Fig. 1. EPR spectrum of KClO₄ irradiated with a dose of 1 Mrad, \(T = +20^\circ\mathrm{C}\).

Fig. 2. Accumulation of \(\mathrm{ClO}_3\) at dose rates of 1 Mrad/min (1), 5 Mrad/min (2), 8 Mrad/min (3).

Fig. 3. Changes in \([\mathrm{ClO}_3]\) (1) and \([\mathrm{ClO}_2]\) (2) after irradiation.

Fig. 4. Dependence of the positions of the lines in the EPR spectrum of \(\mathrm{Cl}^{35}\mathrm{O}_3\) (1) and \(\mathrm{Cl}^{37}\mathrm{O}_3\) (2) on the orientation of the crystal in the magnetic field.

Here \(A\) is the rate of formation of \(\mathrm{ClO_4}\) radicals and free electrons as a result of ionization of perchlorate anions; \(t\) is the time over which the process proceeds; \(C_1, C_2, C_3^*, C_4, C_5\) are the concentrations of \(\mathrm{ClO_4}\), \(\mathrm{ClO_3}\), \(e^-\), electron traps, and \(\mathrm{Cl_2O_6}\), respectively.

\(K_1, K_2, K_3, K_4, K_5\) are the rate constants of reactions [1–5], respectively; \(K_r\) is the rate of the reaction \(\mathrm{ClO_4^0 - 2ClO_3}\); \(B\) is the rate of formation of traps; \(K_6\) is the rate constant for recombination of electrons with traps.

Modeling by means of an analog computer made it possible to assess the applicability of this scheme.

It turned out that the \(\mathrm{ClO_3}\) accumulation curves calculated using this model can have a maximum only when the initial concentration of electron traps \(C_4^0\) is not equal to zero, while \(B < A\) and \(K_6 \ne 0\).

The position of the maximum along the time axis, over a wide range of values of \(K_1\)–\(K_6\), can be estimated by the relation:

\[ t_{\max}=\frac{C_4^0}{A-B} \]

Using this formula, one can find an upper estimate of the value of \(C_4^0\), using the experimental value of \(t_{\max}\) at a known radiation dose rate.

This value for \(\mathrm{KClO_4}\) proved to be equal to \(\le 5 \cdot 10^{18}\ \frac{\text{traps}}{\text{gram}}\).

LITERATURE

1. C. Cellier, T. Cole, Chem. Eng. News, 39, 38, 1961.

2. R. W. Atkins, J. A. Beivati, H. Keen, M. C. R. Symons, J. Chem. Soc., 1962, 4785.

3. T. Cole, J. Chem. Physics, 35, 1169, 1961.

4. L. V. Serikov, A. A. Vasil’ev, Yu. A. Zakharov, Izv. TPI, 199, 76, 1969.

5. J. R. Byberg, S. J. K. Jensen, L. T. Morns, J. Chem. Physics, 46 (1), 131, 1967.

6. A. A. Vasil’ev, Yu. P. Safonov, Yu. A. Zakharov, L. V. Serikov, present collection, p.

7. A. V. B. Dubovitskii, G. B. Manelis, Kinetics and Catalysis, 6, 828, 1965.