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Paramagnetic centers in ultraviolet and X-irradiated potassium sulfate
JOURNAL
OF MAGNETIC
RESONANCE
1, 639-647
ParamagneticCenters in TJltravioletand X-Irradiated Potassium Sulfate N. HARIHARAN Department
of Physics,
Indian
AND J. S~BHANADRI Institute
of Technology,
Madras-36,
kdia
Received January 28, 1969; accepted June 26, 1969 Single crystals of potassium sulfate are grown from aqueous solution and irradiated with ultraviolet and X-radiation. An analysis of the ESR spectra of the induced magnetic centers revealed that in addition to SO; and SO; already identified in y-irradiated potassium sulfate by Morton and Hukuda, several new lines are observed in the present investigation. The six line spectrum has been assigned to an anisotropic paramagnetic center interacting with 39K and H trapped in the crystal. The spel:trum due to 0; is found to be isotropic at room temperature and anisotropic at 77°K. Optical and thermal annealings are done to identify the centers properly. The principal values of the g tensor of SO; and its direction cosines are compared with the values of the previous investigators. INTRODUCTION It is well
known
that
ionizing
radiation,
like X-rays,
gamma
rays or high energy
electrons produces stable paramagnetic centers in a number of compounds. The energy of the incident photon or particle is sufficient to produce many ionized or excited molecules. Electromagnetic radiation in the ultraviolet or visible region, on the other hand, is more selective and each photon excites only one molecule and these excited molecules are distributed homogeneously through the system. Previous investigations on the radiolysis of potassium sulfate has been carried out by using gamma rays at room temperature and 77°K. This paper presents an analysis of the electron spin resonance spectra obtained in X- and ultraviolet-irradiated single crystals of K,SO,. Morton and Gromov (1) and Aiki and Hukuda (2) observed only SO, and SO; centers trapped in irradiated K,S04 crystals, while in the present investigation, some additional centers have been observed. EXPERIMENTAL
Single crystals of K,S04 were grown from aqueous solution at room temperature and were irradiated with X-rays from a copper target (30 kV, 15 mA). No visible coloration could be detected. For ultraviolet irradiation a 100 W mercury discharge tube was used. Electron spin resonance spectra of the induced magnetic centers were recorded at room temperature and at 77°K with an X-band spectrometer (Varian V-4500) operating at 9500 MHz. The magnetic field was calibrated using a proton probe whose frequency was measured with a frequency counter. The g factors were calculated by taking DPPH as standard (g = 2.0036) and are accurate to k 0.0005. The magnetic field was varied in each of the three basic planes and spectra were taken 639
640
HARIHARAN
AND
SOBHANADRI
at every 5”. For studying the effect of optical bleaching with ultraviolet radiation, filters were used, while for examining the effect of thermal bleaching a furnace whose temperature can be raised to 300” was used. Potassium sulfate, which is isomorphous with ammonium, rubidium, and cesium sulfates, belongs to the holosymmetric orthorhombic class with unit cell dimensions a, = 5.713 A, b, = 10.008 A, and c0 = 7.424 A in the space group Vh6 -(Pbnm) (3). There are four molecules in the unit cell. The crystals are prismatic and elongated along the a, axis, developing the faces [021]. There is also a perfect cleavage in the [OOI] plane. The [loo] plane is a reflection plane for the sulfate ion as well as for the whole crystal. The sulfate group is situated such that three of the oxygens are in planes parallel to the [OOI] face and the fourth oxygen and sulphur are such that S-O is along the [OOI] direction. Thus in Fig. 1, On, Om, and Orv are in the ab plane with OIII and OIv along the a direction; while Or, Or,, S, Kr and K,, are in the bc plane with Or and S along the c direction.
a
FIG. 1. Projection diagram of K,SO, onto the three orthogonal planes. Large circles are potassium, small circles are oxygen and dark circles are sulphur atoms. Distances are in angstroms.
RESULTS DESCR~PTIONOFTHE
AND
DISCUSSION
SPECTRA
The radiation damage results in an enriched spectrum consisting of several lines. Figure 2a shows a typical spectrum for magnetic field parallel to [loo] axis. As the
IRRADIATED
A !
POTASSIUM
CB
SULFATE
641
1DE
I I
2. ESR spectra of K,SO,. Ma;gnetic field parallel to [loo] direction. (A) Irradiated with X-rays, (F%)Irradiated with U.V. light, (C) X-irradiated and bleached with U.V. light (II = ZOOO4000 A), (D) X-irradiated and bleached at 140” for 5 hr, (E) X-irradiated and bleached with visible light. FIG.
43
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HARIHARAN
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direction of the magnetic field is varied, the spectrum reveals large anistropy. For certain orientations, some of the lines split into two lines and the spectrum gets crowded. For all the orientations, however, an isotropic line at g = 2.0025 + 0.0005 is easily identified. In order to reduce the complexity of the spectrum, several methods were tried. These include ultraviolet irradiation, optical and thermal bleaching. For a comparison, the ESR spectra taken with magnetic field parallel to [IOO] axis corresponding to the different processes are presented in Fig. 2. Thus if the crystals are irradiated with ultraviolet radiation, instead of X-rays, the spectrum consists of only six prominent lines at equal intervals (Fig. 2b). The isotropic line at g = 2.0025 is completely absent in this spectrum. After subtracting these lines from Fig. 2a, the spectrum consists of two sets of anisotropic lines and three isotropic lines and the angular variation of these is presented in Fig. 3. When the X-irradiated crystal was bleached with 2960-
2.050-
2.040-
2.030t% 2x)20-
2.010D 2.000 E
t
FIG.
3. Angular variation of the spectral lines in [OlO] plane.
ultraviolet light from a mercury discharge tube the anisotropic lines of Fig. 3 completely disappeared, while the group of six lines as well as the other three isotropic lines remained unaffected (Fig. 2~). A similar effect was observed with the thermal bleaching also. When the crystals were heated for about 5 hr at 120”, the anisotropic lines of Fig. 3 as well as the isotropic line at g = 2.012 were completely bleached. Further bleaching at 140” eliminates the outermost lines of the six line pattern leaving only four lines of equal intensity (Fig. 2d). Ordinary visible light bleaching from a tungsten lamp completely removed all the lines of the ESR spectrum, except a single
IRRADIATED
FIG. 4. Angular variation
POTASSIUM SULFATE
of the hype&e
643
lines in [lOO] plane.
line at g = 1.996 and a weak line at g = 2.002 (Fig. 2e). Ultraviolet radiation of 366 rnp alone when used for bleaching is found to produce no detectable change in the spectrum. In the ultraviolet irradiated crystal the group of six lines showed the same splitting that was observed in the X-irradiated crystal and have identical g values. The angular dependence of these lines in the ESR spectrum for the [loo] plane is shown in Fig. 4. Each of the six lines is composed of two lines which overlap in certain orientations. The intensities of the outermost lines are the same, while the four central lines are nearly three times more intense. THE SIX-LINE SPECTRUM
A comparison with the earlier work on y-irradiated KzS04 by Morton and Hukuda shows that the six line spectrum is obtained only in the present investigations on the U.V. and X-irradiated crystal. The group of six lines centered around the free spin g value is not affected by bleaching in the ultraviolet. However when visible light from a tungsten lamp is used the spectrum completely disappears. When the crystal is heated to 140” for 5 hr the outermost lines disappear leaving only four lines of nearly equal intensity and spacing. This suggests that the two outermost lines of equal intensity form a separate entity, while the central four lines form a different group. But the similarity in the angular variation (Fig. 4) suggests that both the groups have something in common. Further, this center must have two nonequivalent sites in the lattice contributing to anisotropic g-variation but isotropic hyperfine constant. The nuclei with nonzero spin in this crystal are 3gK (93 y0 abundance),
644
HARIHARAN
AND
SOBHANADRI
41K (7 y0 abundance) and 33S (0.75 % abundance) all having spin of 312. The nearly equal intensity of the central four lines suggests that they may arise due to hyperfine structure with a single 3gK nucleus and could be represented by a spin Hamiltonian of the form
&f = P[s,H,S,4g,H,S,+g,H,s,l+I.A.S, with S = l/2 and I =3/2, where g,, g, and g, are given by 2.0068, 1.9979, and 2.0016 and A by 25& 1 G. As the lines are weak in intensity the possible hyperfine structure due to the 7% abundant 41K isotope could not be detected. TO account for the two extreme lines, separated by about 120 G, it is suggested that, as the crystal is grown from aqueous solution, small amounts of nascent hydrogen might have been trapped in the host crystal which can interact with the unpaired electron of the radical giving rise to a doublet of such separation. However this hypothesis has to be confirmed by growing the crystal from D,O, but due to lack of facilities, this could not be immediately tested. Nevertheless, the bleaching of the two outermost lines of equal intensity, with temperature suggest that they must correspond to hyperfine splitting with a nucleus of spin l/2. It is difficult to identify the actual center containing the unpaired electron from g values alone. Since it has two nonequivalent sites, the center may be associated with the sulfate radical of the crystal, but no such anisotropic center has been identified so far with an average g value corresponding to the free spin g value. Further investigations are needed to assign the center to any particular radical. THE ISOTROPIC
SPECTRA
From Fig. 3 it can be seen that there are three kinds of centers giving rise to isotropic spectra. Radical D has an isotropic g value of 2.0025. In many sulfur oxy compounds (4) the radical SO; has been identified as having a g value of 2.0036 t- 0.0007. A comparison with the earlier investigations by Gromov and Morton (1) on y-irradiated K,S04 suggests that this radical is SO;. The radical C has isotropic distribution with g = 2.0125 & 0.0005 and is very similar to the resonance line observed in Na,SO, and attributed to 0; (5). However 0; must have three different g values in the absence of vibratory or rotational motion as observed by Symons et al. in KClO, at 77°K (6). Also in Na,SO, and I.,ii,S04*H20, low temperature results revealed three different g-values for the ozonide ion (5, 7). When K,S04 is cooled to 77°K the corresponding spectrum is marked with shoulders at high and low fields resembling the spectrum to be expected of a single radical with no magnetic nuclei but three different principal g values. The three values obtained are 2.0139, 2.0127 and 2.0102, with the average of 2.0123 agreeing well with the earlier results of 0;. Further, heating the crystal to 120” completely removes this line. Andersen pointed out (8) that 0; is stable below 100-125” depending on the crystal preparation technique but decomposes in a few hours at 130-150”. Optical bleaching in the visible region is also found to eliminate this line and this is similar to the bleaching of 0; observed in NaClO, (9). All these measurements give support to the identification of radical C as 0;. It should be pointed out that 0; identified in other crystals has a characteristic optical absorption band in the 420-470 rnp region. This could not be observed in the
IRRADIATED
POTASSIUM
645
SULFATE
case of KzS04, where there was no visible coloration after irradiation. The assignment of radical C to 0, is hence not supported by optical absorption results. However, optical and thermal annealing results together with the ESR results suggest that radical C is probably 0;. It may be added that no optical absorption band has been observed for SO; also in this crystal, whereas in other investigations a band at 240 rnp was observed for this ra.dical (4,lO). The line E with g value 1.996 is present only in the X-irradiated crystal, but not in the U.V.-irradiated crystal. Optical bleaching or thermal bleaching is found to have no effect on this center. Thi.s may be due to an electron type center and its origin is not fully understood. THE ANISOTROPIC
SPECTRA
The principal g values and their direction cosines with respect to the crystal axes are presented in Table 1. The principal g values of A and B centers are comparable and both centers disappear at the same temperature when the crystal is heated. A TABLE PRINCIPAL
Center
A B
C
D E Mean of hyperbe lines
Tensor 2.0513 -0.0505 -0.0633 2.0116 1-0.0133 +0.0116 Isotropic Isotropic Isotropic 2.0017 +0.0014 +0.0019
g VALUES
ANJJ DIRECTION COSINES OF THE RADICALS X-IRRADIATED K&SO,
in abc axis system -0.0505 2.0230 $0.0068 +0.0133 2.0195 +0.0145 g value g value g value +0.0014 1.9984 +0.0011
1
-0.0633 to.0068 2.0’159 SO.011 16 +0.01145 2.01352
+0.0019 +0.0011 2.0062
Principal g-values 2.0572 2.0246 2.0091 2.0027 2.0512
Principal
(+0.9210 (+0.3603 (+0.0233 (-0.8429 (+0.5329
0.00;“‘0711 2.0135 2 0125 +f 0.0005 2:0025 1.997 rt. 0.001 (+0.2626 2.0068 1.9979 (+0.2075 2.0016 (+0.9424
TRAPPED
IN
directions axis system
in abc
+0.2976 -0.7058 1-0.6604 +0.3915 $0.5174 to.7609
$0.2511) -0.6099) -0.7505) +0.3692) +0.6691) -0.6450)
+0.2472 -0.9584 +0.1424
+0.9327) +0.1958) -0.3026)
similar feature was observed by Aiki and Hukuda (2) in y-irradiated K,SO, where the two centers were attributed to SO,. Morton et al. from their ESR spectra of y-irradiated K,SO, examined at 77°K identified only a single paramagnetic species, namely SO;. The projection diagram of K,SO, (Fig. 1) reveals that there are two nonequivalent sites for the sulfate ion. The two lines observed for the anisotropic spectra suggest that the center responsible for the spectrum must be associated with the sulfate group. The principal values for A and B centers are comparable with the earlier results suggesting that both the centers are due to SO;. A comparison of the g-values of Morton et al. and Aiki and Hukuda reveals that for one center the values agree well within 0.001 (2.048, 2.009, and 2.004) but the present results (2.051, 2.013, and 2.003) for the B center show a slightly larger
646
HARIHARAN
AND
SOBHANADRI
deviation. For the A center, the agreement between the present results and Hukuda’s results is good for 2.057 but there is deviation for the other two values. Nevertheless, the annealing properties and the presence of two nonequivalent sites suggest that the centers may be attributed to SO;. It can be mentioned here that SO, identified in Na$O, also reveal similar annealing properties getting completely bleached with ultraviolet irradiation. From Table 1 it is seen that one of the principal g values for the A center (g = 2.0091) and one of those for the B center (g = 2.0135) lie in the bc plane along the same direction. From crystal structure data it can be calculated that the vector Or-O,, has direction cosines (I) = (0.0, 0.5790, and 0.8153) referred to the abc axis system. This suggests that one of the principal values of the two centers A and B is along this direction, while the other two should be in a plane perpendicular to Or-On. It may be calculated that the vector passing through S and OnI (m) and the perpendicular from O,, to Or-Ott (n) in the (0, On Or,,) plane form an orthogonal system with the Or-On direction. The direction cosines referred to the abc axis system are m = (0.8179, 0.4692, 0.3333) and n = (0.5753, 0.6668, 0.4732) and by a comparison with Table 1, these agree with the direction cosines for the other two principal values within the combined error of the present investigation and crystal structure data. Since OrOII is common for the two ellipsoids the other two principal values form an ellipse in a plane perpendicular to the Or-O,, direction and the two ellipses are inclined to each other. It can also be seen that this plane contains the crystallographic a axis. The principal directions obtained by Morton (II) and Hukuda (2) are very much different and differ with the values in the present investigation. Aiki and Hukudu observe that the A center in their investigation is oriented such that the defect occurs on the mirror plane containing S, namely the be plane, while the other center is in a different orientation as in the case of Morton’s investigation. In the present investigation, it is observed that both the centers corresponding to SO, are oriented such that the defect occurs in the same plane, namely the be plane. CONCLUSION
Electron spin resonance investigations on ultraviolet and X-irradiated single crystals of K,S04 indicated the presence of several paramagnetic centers. Optical and thermal bleaching experiments have been done to identify the different paramagnetic species in the crystal. (i) The six line spectrum is found to be due to hyperfine splitting of the unpaired electron with a potassium nucleus of spin 312 and a proton of spin l/2. This hypothesis is tested by thermal bleaching experiments, in which the two lines due to proton splitting disappeared at 140” leaving only four lines of equal intensity. (ii) The isotropic line at g = 2.0125 attributed to 0; on the basis of a comparison of its g value by earlier investigators, is replaced at 77°K by an anisotropic spectrum having principal values 2.0139, 2.0127, and 2.0102. The assignment is supported by bleaching experiments, particularly the thermal bleaching at 120” that is characteristic of 0;. (iii) The isotropic line at g = 2.0025 is similar to the one observed in the y-irradiated crystal by Morton and Hukuda and is attributed to SO;. No optical
IRRADIATED
POTASSIUM
SULFATE
647
absorption band characteristic of SO; or 0; is observed in the present investigation. (iv) The isotropic line at g = 1.996 could not be assigned to any specific radical. However, it is thought to be an electron type center unaffected either by optical or thermal bleaching. (v) The two paramagnetic centers giving highly anisotropic spectra are attributed to SO; and its g values are compared with the corresponding values in the y-irradiated crystal. Both the centers are found to disappear with either thermal bleaching or optical bleaching. ACKNOWLEDGMENT The authors are grateful to Prof. C. Ramasastry for his interest in the work and to Prof. Putcha Venkateswarlu, I.I.T. Kanpur, for facilities extended in the use of their ESR spectrometer.
1.
2. 3. 4. 5.
6. 7. 8. 9.
IO. 11.
REFERENCES V. V. GROMOV AND J. R. MORTON, Can. J. Chem. 44,521(1966). KUNIO AIKI AND KENZI HUKUDA, .I. Phys. Sot. Japan 22, 663 (1967). R. W. G. WYCKOFF, “The Structure of Crystals,” 2nd Ed., p. 291. Chemical Catalog Co. Inc., New York (1931). G. W. CHANTRY, A. HORSFIELD, J. R. MORTON, J. R. ROWLANDS AND D. H. WHLFFEN, Mol. Phys. 5, 233 (1962). N. HARIHARAN AND J. SOBHANADRI, Mol. Phys. 1968 (in press). P. W. ATKINS, J. A. BRNATI, N. KEEN, M. C. R. SYMONS AND P. A. TREVALION, J. Chem. Sot. 4785 (1962). N. HARIHARAN AND J. SOBHANADRI, J. Phys. Chem. Solids 30, 778 (1969). T. ANDERSON, H. E. LUNDAGER MADSEN AND K. OLESEN, Trans. Faraday Sot. 62,2409 (1966). C. RAMASASTRY AND S. B. S. SASTRY, Solid State Communications 5,199 (1967). N. HARIHARAN AND J. SOBHANADE?I, Znd. J. Pure and Appl. Phys. 6, 73 (1968). J. R. MORTON, DAVID M. BISHOP .AND M. RANDIC, J. Chem. Phys. 45,1885 (1966).
OF MAGNETIC
RESONANCE
1, 639-647
ParamagneticCenters in TJltravioletand X-Irradiated Potassium Sulfate N. HARIHARAN Department
of Physics,
Indian
AND J. S~BHANADRI Institute
of Technology,
Madras-36,
kdia
Received January 28, 1969; accepted June 26, 1969 Single crystals of potassium sulfate are grown from aqueous solution and irradiated with ultraviolet and X-radiation. An analysis of the ESR spectra of the induced magnetic centers revealed that in addition to SO; and SO; already identified in y-irradiated potassium sulfate by Morton and Hukuda, several new lines are observed in the present investigation. The six line spectrum has been assigned to an anisotropic paramagnetic center interacting with 39K and H trapped in the crystal. The spel:trum due to 0; is found to be isotropic at room temperature and anisotropic at 77°K. Optical and thermal annealings are done to identify the centers properly. The principal values of the g tensor of SO; and its direction cosines are compared with the values of the previous investigators. INTRODUCTION It is well
known
that
ionizing
radiation,
like X-rays,
gamma
rays or high energy
electrons produces stable paramagnetic centers in a number of compounds. The energy of the incident photon or particle is sufficient to produce many ionized or excited molecules. Electromagnetic radiation in the ultraviolet or visible region, on the other hand, is more selective and each photon excites only one molecule and these excited molecules are distributed homogeneously through the system. Previous investigations on the radiolysis of potassium sulfate has been carried out by using gamma rays at room temperature and 77°K. This paper presents an analysis of the electron spin resonance spectra obtained in X- and ultraviolet-irradiated single crystals of K,SO,. Morton and Gromov (1) and Aiki and Hukuda (2) observed only SO, and SO; centers trapped in irradiated K,S04 crystals, while in the present investigation, some additional centers have been observed. EXPERIMENTAL
Single crystals of K,S04 were grown from aqueous solution at room temperature and were irradiated with X-rays from a copper target (30 kV, 15 mA). No visible coloration could be detected. For ultraviolet irradiation a 100 W mercury discharge tube was used. Electron spin resonance spectra of the induced magnetic centers were recorded at room temperature and at 77°K with an X-band spectrometer (Varian V-4500) operating at 9500 MHz. The magnetic field was calibrated using a proton probe whose frequency was measured with a frequency counter. The g factors were calculated by taking DPPH as standard (g = 2.0036) and are accurate to k 0.0005. The magnetic field was varied in each of the three basic planes and spectra were taken 639
640
HARIHARAN
AND
SOBHANADRI
at every 5”. For studying the effect of optical bleaching with ultraviolet radiation, filters were used, while for examining the effect of thermal bleaching a furnace whose temperature can be raised to 300” was used. Potassium sulfate, which is isomorphous with ammonium, rubidium, and cesium sulfates, belongs to the holosymmetric orthorhombic class with unit cell dimensions a, = 5.713 A, b, = 10.008 A, and c0 = 7.424 A in the space group Vh6 -(Pbnm) (3). There are four molecules in the unit cell. The crystals are prismatic and elongated along the a, axis, developing the faces [021]. There is also a perfect cleavage in the [OOI] plane. The [loo] plane is a reflection plane for the sulfate ion as well as for the whole crystal. The sulfate group is situated such that three of the oxygens are in planes parallel to the [OOI] face and the fourth oxygen and sulphur are such that S-O is along the [OOI] direction. Thus in Fig. 1, On, Om, and Orv are in the ab plane with OIII and OIv along the a direction; while Or, Or,, S, Kr and K,, are in the bc plane with Or and S along the c direction.
a
FIG. 1. Projection diagram of K,SO, onto the three orthogonal planes. Large circles are potassium, small circles are oxygen and dark circles are sulphur atoms. Distances are in angstroms.
RESULTS DESCR~PTIONOFTHE
AND
DISCUSSION
SPECTRA
The radiation damage results in an enriched spectrum consisting of several lines. Figure 2a shows a typical spectrum for magnetic field parallel to [loo] axis. As the
IRRADIATED
A !
POTASSIUM
CB
SULFATE
641
1DE
I I
2. ESR spectra of K,SO,. Ma;gnetic field parallel to [loo] direction. (A) Irradiated with X-rays, (F%)Irradiated with U.V. light, (C) X-irradiated and bleached with U.V. light (II = ZOOO4000 A), (D) X-irradiated and bleached at 140” for 5 hr, (E) X-irradiated and bleached with visible light. FIG.
43
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HARIHARAN
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direction of the magnetic field is varied, the spectrum reveals large anistropy. For certain orientations, some of the lines split into two lines and the spectrum gets crowded. For all the orientations, however, an isotropic line at g = 2.0025 + 0.0005 is easily identified. In order to reduce the complexity of the spectrum, several methods were tried. These include ultraviolet irradiation, optical and thermal bleaching. For a comparison, the ESR spectra taken with magnetic field parallel to [IOO] axis corresponding to the different processes are presented in Fig. 2. Thus if the crystals are irradiated with ultraviolet radiation, instead of X-rays, the spectrum consists of only six prominent lines at equal intervals (Fig. 2b). The isotropic line at g = 2.0025 is completely absent in this spectrum. After subtracting these lines from Fig. 2a, the spectrum consists of two sets of anisotropic lines and three isotropic lines and the angular variation of these is presented in Fig. 3. When the X-irradiated crystal was bleached with 2960-
2.050-
2.040-
2.030t% 2x)20-
2.010D 2.000 E
t
FIG.
3. Angular variation of the spectral lines in [OlO] plane.
ultraviolet light from a mercury discharge tube the anisotropic lines of Fig. 3 completely disappeared, while the group of six lines as well as the other three isotropic lines remained unaffected (Fig. 2~). A similar effect was observed with the thermal bleaching also. When the crystals were heated for about 5 hr at 120”, the anisotropic lines of Fig. 3 as well as the isotropic line at g = 2.012 were completely bleached. Further bleaching at 140” eliminates the outermost lines of the six line pattern leaving only four lines of equal intensity (Fig. 2d). Ordinary visible light bleaching from a tungsten lamp completely removed all the lines of the ESR spectrum, except a single
IRRADIATED
FIG. 4. Angular variation
POTASSIUM SULFATE
of the hype&e
643
lines in [lOO] plane.
line at g = 1.996 and a weak line at g = 2.002 (Fig. 2e). Ultraviolet radiation of 366 rnp alone when used for bleaching is found to produce no detectable change in the spectrum. In the ultraviolet irradiated crystal the group of six lines showed the same splitting that was observed in the X-irradiated crystal and have identical g values. The angular dependence of these lines in the ESR spectrum for the [loo] plane is shown in Fig. 4. Each of the six lines is composed of two lines which overlap in certain orientations. The intensities of the outermost lines are the same, while the four central lines are nearly three times more intense. THE SIX-LINE SPECTRUM
A comparison with the earlier work on y-irradiated KzS04 by Morton and Hukuda shows that the six line spectrum is obtained only in the present investigations on the U.V. and X-irradiated crystal. The group of six lines centered around the free spin g value is not affected by bleaching in the ultraviolet. However when visible light from a tungsten lamp is used the spectrum completely disappears. When the crystal is heated to 140” for 5 hr the outermost lines disappear leaving only four lines of nearly equal intensity and spacing. This suggests that the two outermost lines of equal intensity form a separate entity, while the central four lines form a different group. But the similarity in the angular variation (Fig. 4) suggests that both the groups have something in common. Further, this center must have two nonequivalent sites in the lattice contributing to anisotropic g-variation but isotropic hyperfine constant. The nuclei with nonzero spin in this crystal are 3gK (93 y0 abundance),
644
HARIHARAN
AND
SOBHANADRI
41K (7 y0 abundance) and 33S (0.75 % abundance) all having spin of 312. The nearly equal intensity of the central four lines suggests that they may arise due to hyperfine structure with a single 3gK nucleus and could be represented by a spin Hamiltonian of the form
&f = P[s,H,S,4g,H,S,+g,H,s,l+I.A.S, with S = l/2 and I =3/2, where g,, g, and g, are given by 2.0068, 1.9979, and 2.0016 and A by 25& 1 G. As the lines are weak in intensity the possible hyperfine structure due to the 7% abundant 41K isotope could not be detected. TO account for the two extreme lines, separated by about 120 G, it is suggested that, as the crystal is grown from aqueous solution, small amounts of nascent hydrogen might have been trapped in the host crystal which can interact with the unpaired electron of the radical giving rise to a doublet of such separation. However this hypothesis has to be confirmed by growing the crystal from D,O, but due to lack of facilities, this could not be immediately tested. Nevertheless, the bleaching of the two outermost lines of equal intensity, with temperature suggest that they must correspond to hyperfine splitting with a nucleus of spin l/2. It is difficult to identify the actual center containing the unpaired electron from g values alone. Since it has two nonequivalent sites, the center may be associated with the sulfate radical of the crystal, but no such anisotropic center has been identified so far with an average g value corresponding to the free spin g value. Further investigations are needed to assign the center to any particular radical. THE ISOTROPIC
SPECTRA
From Fig. 3 it can be seen that there are three kinds of centers giving rise to isotropic spectra. Radical D has an isotropic g value of 2.0025. In many sulfur oxy compounds (4) the radical SO; has been identified as having a g value of 2.0036 t- 0.0007. A comparison with the earlier investigations by Gromov and Morton (1) on y-irradiated K,S04 suggests that this radical is SO;. The radical C has isotropic distribution with g = 2.0125 & 0.0005 and is very similar to the resonance line observed in Na,SO, and attributed to 0; (5). However 0; must have three different g values in the absence of vibratory or rotational motion as observed by Symons et al. in KClO, at 77°K (6). Also in Na,SO, and I.,ii,S04*H20, low temperature results revealed three different g-values for the ozonide ion (5, 7). When K,S04 is cooled to 77°K the corresponding spectrum is marked with shoulders at high and low fields resembling the spectrum to be expected of a single radical with no magnetic nuclei but three different principal g values. The three values obtained are 2.0139, 2.0127 and 2.0102, with the average of 2.0123 agreeing well with the earlier results of 0;. Further, heating the crystal to 120” completely removes this line. Andersen pointed out (8) that 0; is stable below 100-125” depending on the crystal preparation technique but decomposes in a few hours at 130-150”. Optical bleaching in the visible region is also found to eliminate this line and this is similar to the bleaching of 0; observed in NaClO, (9). All these measurements give support to the identification of radical C as 0;. It should be pointed out that 0; identified in other crystals has a characteristic optical absorption band in the 420-470 rnp region. This could not be observed in the
IRRADIATED
POTASSIUM
645
SULFATE
case of KzS04, where there was no visible coloration after irradiation. The assignment of radical C to 0, is hence not supported by optical absorption results. However, optical and thermal annealing results together with the ESR results suggest that radical C is probably 0;. It may be added that no optical absorption band has been observed for SO; also in this crystal, whereas in other investigations a band at 240 rnp was observed for this ra.dical (4,lO). The line E with g value 1.996 is present only in the X-irradiated crystal, but not in the U.V.-irradiated crystal. Optical bleaching or thermal bleaching is found to have no effect on this center. Thi.s may be due to an electron type center and its origin is not fully understood. THE ANISOTROPIC
SPECTRA
The principal g values and their direction cosines with respect to the crystal axes are presented in Table 1. The principal g values of A and B centers are comparable and both centers disappear at the same temperature when the crystal is heated. A TABLE PRINCIPAL
Center
A B
C
D E Mean of hyperbe lines
Tensor 2.0513 -0.0505 -0.0633 2.0116 1-0.0133 +0.0116 Isotropic Isotropic Isotropic 2.0017 +0.0014 +0.0019
g VALUES
ANJJ DIRECTION COSINES OF THE RADICALS X-IRRADIATED K&SO,
in abc axis system -0.0505 2.0230 $0.0068 +0.0133 2.0195 +0.0145 g value g value g value +0.0014 1.9984 +0.0011
1
-0.0633 to.0068 2.0’159 SO.011 16 +0.01145 2.01352
+0.0019 +0.0011 2.0062
Principal g-values 2.0572 2.0246 2.0091 2.0027 2.0512
Principal
(+0.9210 (+0.3603 (+0.0233 (-0.8429 (+0.5329
0.00;“‘0711 2.0135 2 0125 +f 0.0005 2:0025 1.997 rt. 0.001 (+0.2626 2.0068 1.9979 (+0.2075 2.0016 (+0.9424
TRAPPED
IN
directions axis system
in abc
+0.2976 -0.7058 1-0.6604 +0.3915 $0.5174 to.7609
$0.2511) -0.6099) -0.7505) +0.3692) +0.6691) -0.6450)
+0.2472 -0.9584 +0.1424
+0.9327) +0.1958) -0.3026)
similar feature was observed by Aiki and Hukuda (2) in y-irradiated K,SO, where the two centers were attributed to SO,. Morton et al. from their ESR spectra of y-irradiated K,SO, examined at 77°K identified only a single paramagnetic species, namely SO;. The projection diagram of K,SO, (Fig. 1) reveals that there are two nonequivalent sites for the sulfate ion. The two lines observed for the anisotropic spectra suggest that the center responsible for the spectrum must be associated with the sulfate group. The principal values for A and B centers are comparable with the earlier results suggesting that both the centers are due to SO;. A comparison of the g-values of Morton et al. and Aiki and Hukuda reveals that for one center the values agree well within 0.001 (2.048, 2.009, and 2.004) but the present results (2.051, 2.013, and 2.003) for the B center show a slightly larger
646
HARIHARAN
AND
SOBHANADRI
deviation. For the A center, the agreement between the present results and Hukuda’s results is good for 2.057 but there is deviation for the other two values. Nevertheless, the annealing properties and the presence of two nonequivalent sites suggest that the centers may be attributed to SO;. It can be mentioned here that SO, identified in Na$O, also reveal similar annealing properties getting completely bleached with ultraviolet irradiation. From Table 1 it is seen that one of the principal g values for the A center (g = 2.0091) and one of those for the B center (g = 2.0135) lie in the bc plane along the same direction. From crystal structure data it can be calculated that the vector Or-O,, has direction cosines (I) = (0.0, 0.5790, and 0.8153) referred to the abc axis system. This suggests that one of the principal values of the two centers A and B is along this direction, while the other two should be in a plane perpendicular to Or-On. It may be calculated that the vector passing through S and OnI (m) and the perpendicular from O,, to Or-Ott (n) in the (0, On Or,,) plane form an orthogonal system with the Or-On direction. The direction cosines referred to the abc axis system are m = (0.8179, 0.4692, 0.3333) and n = (0.5753, 0.6668, 0.4732) and by a comparison with Table 1, these agree with the direction cosines for the other two principal values within the combined error of the present investigation and crystal structure data. Since OrOII is common for the two ellipsoids the other two principal values form an ellipse in a plane perpendicular to the Or-O,, direction and the two ellipses are inclined to each other. It can also be seen that this plane contains the crystallographic a axis. The principal directions obtained by Morton (II) and Hukuda (2) are very much different and differ with the values in the present investigation. Aiki and Hukudu observe that the A center in their investigation is oriented such that the defect occurs on the mirror plane containing S, namely the be plane, while the other center is in a different orientation as in the case of Morton’s investigation. In the present investigation, it is observed that both the centers corresponding to SO, are oriented such that the defect occurs in the same plane, namely the be plane. CONCLUSION
Electron spin resonance investigations on ultraviolet and X-irradiated single crystals of K,S04 indicated the presence of several paramagnetic centers. Optical and thermal bleaching experiments have been done to identify the different paramagnetic species in the crystal. (i) The six line spectrum is found to be due to hyperfine splitting of the unpaired electron with a potassium nucleus of spin 312 and a proton of spin l/2. This hypothesis is tested by thermal bleaching experiments, in which the two lines due to proton splitting disappeared at 140” leaving only four lines of equal intensity. (ii) The isotropic line at g = 2.0125 attributed to 0; on the basis of a comparison of its g value by earlier investigators, is replaced at 77°K by an anisotropic spectrum having principal values 2.0139, 2.0127, and 2.0102. The assignment is supported by bleaching experiments, particularly the thermal bleaching at 120” that is characteristic of 0;. (iii) The isotropic line at g = 2.0025 is similar to the one observed in the y-irradiated crystal by Morton and Hukuda and is attributed to SO;. No optical
IRRADIATED
POTASSIUM
SULFATE
647
absorption band characteristic of SO; or 0; is observed in the present investigation. (iv) The isotropic line at g = 1.996 could not be assigned to any specific radical. However, it is thought to be an electron type center unaffected either by optical or thermal bleaching. (v) The two paramagnetic centers giving highly anisotropic spectra are attributed to SO; and its g values are compared with the corresponding values in the y-irradiated crystal. Both the centers are found to disappear with either thermal bleaching or optical bleaching. ACKNOWLEDGMENT The authors are grateful to Prof. C. Ramasastry for his interest in the work and to Prof. Putcha Venkateswarlu, I.I.T. Kanpur, for facilities extended in the use of their ESR spectrometer.
1.
2. 3. 4. 5.
6. 7. 8. 9.
IO. 11.
REFERENCES V. V. GROMOV AND J. R. MORTON, Can. J. Chem. 44,521(1966). KUNIO AIKI AND KENZI HUKUDA, .I. Phys. Sot. Japan 22, 663 (1967). R. W. G. WYCKOFF, “The Structure of Crystals,” 2nd Ed., p. 291. Chemical Catalog Co. Inc., New York (1931). G. W. CHANTRY, A. HORSFIELD, J. R. MORTON, J. R. ROWLANDS AND D. H. WHLFFEN, Mol. Phys. 5, 233 (1962). N. HARIHARAN AND J. SOBHANADRI, Mol. Phys. 1968 (in press). P. W. ATKINS, J. A. BRNATI, N. KEEN, M. C. R. SYMONS AND P. A. TREVALION, J. Chem. Sot. 4785 (1962). N. HARIHARAN AND J. SOBHANADRI, J. Phys. Chem. Solids 30, 778 (1969). T. ANDERSON, H. E. LUNDAGER MADSEN AND K. OLESEN, Trans. Faraday Sot. 62,2409 (1966). C. RAMASASTRY AND S. B. S. SASTRY, Solid State Communications 5,199 (1967). N. HARIHARAN AND J. SOBHANADE?I, Znd. J. Pure and Appl. Phys. 6, 73 (1968). J. R. MORTON, DAVID M. BISHOP .AND M. RANDIC, J. Chem. Phys. 45,1885 (1966).