INTERMEDIATE REACTIONS IN IRRADIATED KClO$_3$

L. V. Sarykov, A. N. Safonov, V. A. Zakharov

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

A number of works carried out on the radiolysis of alkali-metal chlorates [1, 2] confirm the general view that the radiolysis of salts with a complex anion is a complex multistage process. Thus, in particular, it was established that, along with the final products of the radiolysis of chlorates [1, 2], centers are formed that exist only at low temperatures [1, 2, 9]. On the basis of these experimental facts, the mechanism of radiolysis of alkali-metal chlorates should be regarded as consisting of a series of intermediate stages and reactions proceeding with the participation of radicals and centers of electronic nature. In the present work, the thermal stability of the radical ClO$_3$, detected in KClO$_3$ irradiated at −196°C, was studied. The possibility of intermediate radiolysis reactions was also shown, one of which is the reaction of formation of the ozonide ion.

EXPERIMENTAL METHOD

The experimental method did not differ from that described in work [3]. KClO$_3$ single crystals were grown by the method of slow cooling of an aqueous solution of “chemically pure” grade salt, recrystallized three times from bidistillate.

The intervals of the absorbed doses used by the samples varied within the range of 1–1.5 Mrad.

EXPERIMENTAL RESULTS

In preliminary observations it was noted that, in potassium chlorate irradiated at \(-196^\circ\mathrm{C}\) and then heated to higher temperatures, a series of transformations of the EPR spectra occurs. For a more detailed study of the intermediate reactions of the radicals, an experiment was carried out on slow thawing of the irradiated samples. A single crystal was heated to a definite temperature and then refrozen to \(-196^\circ\mathrm{C}\). Thus, all measurements were made at the temperature of liquid nitrogen.

The EPR spectrum of \( \mathrm{KClO_3} \) irradiated at \(-196^\circ\mathrm{C}\) consists of a large number of lines. By rotating the crystal in the magnetic field it is possible to distinguish clearly two intense quartets that differ from one another in the magnitudes of the hyperfine-splitting constants. The central part of the spectrum is a series of lines superimposed on one another. It proved possible, however, to establish the number of paramagnetic centers responsible for them by using their different thermal stabilities. When the crystal is heated to \(-150^\circ\mathrm{C}\), complete loss of the quartet with the HFS constants \((A_{xx}=A_{yy}=114\ \mathrm{G};\ A_{zz}=165\ \mathrm{G})\) is observed. No change in the intensity of the other lines was noted at this temperature. Upon further thawing, beginning in the region \(-100^\circ \div -80^\circ\mathrm{C}\), a singlet appears (Fig. 1a), whose intensity reaches a maximum in the temperature range \(-35^\circ \div 30^\circ\mathrm{C}\) (Fig. 2). During thawing in this temperature region, a change in the color of the single crystal is observed, from violet-blue to yellowish-brown. As a result of further heating, a decrease in the intensity of the singlet that has formed is observed, and the signal disappears completely at room temperature.

When the single crystal is rotated in the magnetic field, the singlet splits into 2 lines (Fig. 1b), thereby revealing a weak anisotropy of the \(g\)-factor. In the position in which the indicated radical forms a singlet, the \(g\)-factor is equal to \(2.0160\).

Fig. 1. ESR spectrum of \(O_3^-\) in irradiated \(KClO_3\).

Fig. 2. Temperature dependence of the formation and decay of \(O_3^-\) in \(KClO_3\).

Fig. 3.
a) Change in the ESR spectrum of a \(KClO_3\) polycrystal as a function of temperature.

1. \(t^\circ\mathrm{C} = -15^\circ\mathrm{C}\).
2. \(t^\circ\mathrm{C} = -70^\circ\mathrm{C}\).
3. \(t^\circ\mathrm{C} = -40^\circ\mathrm{C}\).
4. \(t^\circ\mathrm{C} = 0^\circ\mathrm{C}\).

b) ESR spectrum of \(ClO\) in \(H_2SO_4\) at \(77^\circ\mathrm{K}\).

1. Irradiated at \(-196^\circ\mathrm{C}\) (\(D\) grade).
2. \(-140^\circ\mathrm{C}\).
3. \(-100^\circ\mathrm{C}\).
4. \(-40^\circ\mathrm{C}\).
5. Room temperature.
6. Background of an unirradiated crystal.

Fig. 4. Low-temperature optical spectra of irradiated \(KClO_3\).

The appearance of a narrow line in the EPR spectrum can also be observed upon thawing polycrystalline samples irradiated at \(-196^\circ\mathrm{C}\) (Fig. 3a). It should also be noted that at temperatures of \(-30^\circ\) to \(-20^\circ\mathrm{C}\), along with the nitrogen hyperfine structure in the central part of the spectrum, a decrease in the intensity of several more lines is observed. This fact indicates the presence of yet another center, characterized by a narrow EPR signal consisting of several lines. The disappearance of the lines indicated in the EPR spectrum also occurs in the temperature range \(-30^\circ\) to \(0^\circ\mathrm{C}\).

The optical spectra of potassium chlorate irradiated at \(-196^\circ\mathrm{C}\) are shown in Fig. 4. As in the case of the EPR spectra, recording of the spectrum after warming the single crystal to a given temperature was carried out at \(-196^\circ\mathrm{C}\). It is evident from the figure that, along with the progressive thermal bleaching of all spectral bands, heating the single crystal to \(-40^\circ\mathrm{C}\) produces an increase in the intensity of the absorption maximum at 450 nm. Further heating to room temperature causes a decrease in the optical density of this band simultaneously with a decrease in the intensity of the band with a maximum at 270 nm. It should be noted that a similar effect is absent for irradiated \(\mathrm{NaClO_3}\) single crystals, both in the EPR spectra and in the optical spectra. For sodium chlorate, a progressive decrease in the intensities of all absorption bands up to complete bleaching of the centers is observed.

DISCUSSION OF RESULTS

The quartet with appreciable splitting in the EPR spectrum of potassium chlorate irradiated at \(-196^\circ\mathrm{C}\) is, in its parameters and temperature stability, similar to the center previously found in single crystals [2] and frozen aqueous solutions of \(\mathrm{NaClO_3}\) [3] and assigned to the radical \(\cdot\mathrm{ClO_3}\).

The formation of the radical \( \mathrm{ClO_3} \) is naturally to be regarded as occurring upon ionization of the anion:

\[ \mathrm{ClO_3^-} \leadsto \mathrm{ClO_3} + e . \tag{1} \]

The lower limiting temperature of the disappearance of the EPR line responsible for \( \mathrm{ClO_3} \) is striking. There are experimental results [4] in which it is shown that the \( \mathrm{ClO_3} \) radical in sodium and ammonium perchlorates is stable at higher temperatures. In this connection it is difficult to suppose that the disappearance of the quartet responsible for \( \mathrm{ClO_3} \) occurs as a result of its thermal decomposition. This is also indicated by the fact that, when the intensity of the EPR signal due to the \( \mathrm{ClO_3} \) radical decreases, the formation of chloroxy radicals is not observed. Therefore, we believe that, in the temperature region in which the EPR signal of the \( \mathrm{ClO_3} \) radical disappears, an electron localized in shallow traps is released and enters into the recombination reaction

\[ \mathrm{ClO_3} + \bar e \longrightarrow \mathrm{ClO_3^-}. \tag{2} \]

Since in the optical spectra at these temperatures no increase is observed in the optical densities of the absorption bands belonging to chlorate anions, the recombination reaction apparently is not dissociative in character.

A hole transfer of an electron, or migration of the radical itself, may lead to the dimerization reaction

\[ \mathrm{ClO_3} + \mathrm{ClO_3} \longrightarrow \mathrm{Cl_2O_6}. \tag{3} \]

The different thermal stability of the electron centers is, apparently, the reason for the different stability of the \( \mathrm{ClO_3} \) radical in chlorate and perchlorate matrices. \( \mathrm{ClO_3} \) in a \( \mathrm{KClO_4} \) matrix is formed at room temperature, being the product of a secondary reaction. The stability of the electron centers at room temperature in this case determines the thermostability and existence of \( \mathrm{ClO_3} \) in perchlorates.

The formation of intense coloring in potassium chlorate irradiated at \(-196^\circ\mathrm{C}\) occurs upon heating the single crystal from \(-30^\circ\mathrm{C}\) to \(+35^\circ\mathrm{C}\). At these same temperatures in the spec-

In optical spectra, an increase in the intensity of the absorption maximum at 450 nm is observed. The profiles of the temperature dependences of the singlet intensity in the EPR spectra and of the line intensity at 450 nm in the optical spectrum, realized upon heating crystals irradiated at \(-196^\circ\mathrm{C}\), coincide qualitatively: the maximum intensity is observed in the region of \(-35^\circ\mathrm{C}\). With a further increase in temperature, complete disappearance of the singlet in the EPR spectra is observed, whereas the intensity of the line in the optical spectra decreases only to a certain value and then does not change further. The residual intensity of the band at 450 nm in the warmed crystals coincides with the intensity of this band upon isodose irradiation of the crystal at room temperature. These observations show that the band with absorption in the region of 450 nm at temperatures \(-90^\circ\) to \(0^\circ\mathrm{C}\) actually consists of two bands. One of them in \(\mathrm{KClO_3}\) is due to a paramagnetic center. The latter is confirmed by the following.

Upon heating \(\mathrm{NaClO_3}\) irradiated at \(-196^\circ\mathrm{C}\), where formation of the corresponding singlet is not observed, the EPR spectra also do not show the increase in intensity of the optical band at 450 nm that is characteristic of irradiated \(\mathrm{KClO_3}\). The small width of the line of the observed singlet (\(3\) G), the \(g\)-factor equal to 2.0160, and the weak anisotropy of the \(g\)-factor make it possible to consider that the paramagnetic center responsible for this signal and for the thermally unstable component of the band at 450 nm is the ozonide ion. In addition, it is known [4] that the maximum of optical absorption of the ozonide ion lies in the region of 450 nm. The parameters of the EPR signals of pure alkali-metal ozonides have been well studied and are contained in [5, 6]. The second, thermally stable component of the line at 450 nm, as proposed in [7], is attributed to an electron trapped in a trap. The formation of the ozonide ion can be represented as migration of an oxygen atom with capture of an electron stabilized in the lattice, the presence of which is assumed in the low-temperature optical spectra of irradiated \(\mathrm{KClO_3}\) [7].

Oxygen atoms, having an electron affinity of 2.2 eV, should serve as effective electron traps.

\[ \mathrm{O_2} + \mathrm{O} + \bar{e} \longrightarrow \mathrm{O_3^-}. \tag{4} \]

Upon further heating of the crystal, beginning at a temperature of \(-30^\circ\mathrm{C}\), intense destruction of \(\mathrm{O_3^-}\) occurs, apparently explained by the thermal instability of ozonides at elevated temperatures [6]:

\[ \mathrm{O_3^-} \longrightarrow \mathrm{O_2^-} + \mathrm{O}. \tag{5} \]

Comparison of the central part of the EPR spectrum of a KClO\(_3\) single crystal irradiated at \(-196^\circ\mathrm{C}\) (Fig. 3a) with the EPR spectrum of the ClO radical [6] (Fig. 3b) gives grounds to consider that one of the products of the low-temperature radiolysis of KClO\(_3\) is chlorine monoxide. Thermally unstable chlorine monoxide decomposes in the temperature range \(-30^\circ\)–\(0^\circ\mathrm{C}\). This fact is confirmed by a decrease in the intensity of the EPR signal and of the optical absorption band at 270 nm, corresponding to the absorption maximum of ClO.

The thermal destruction of chlorine monoxide may be represented by the reaction:

\[ \mathrm{ClO} \longrightarrow \mathrm{Cl} + \mathrm{O}. \tag{6} \]

In view of the fact that the destruction of the ozonide ion and of chlorine monoxide occurs simultaneously, the possibility of the reaction

\[ \mathrm{O_3^-} + \mathrm{ClO} \longrightarrow \mathrm{ClO_2^-} + \mathrm{O}. \tag{7} \]

also cannot be excluded.

The authors thank L. T. Butenko for comments made during the reading of the manuscript of the article.

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