ESR of X-irradiated potassium hydrogen sulfate

JOURNAL

OF MAGNETIC

RESONANCE

21,311-319

(1976)

ESR of X-Irradiated Potassium Hydrogen Sulfate C. RAMASASTRY AND C. S. SUNANDANA Department

of Physics,

Indian

Institute

of Technology,

MadrasdOOO36,

India

Received December 23.1974 Radiation damage in single crystals of KHSO., caused by X-irradiation at 300 K has given rise to several paramagnetic centers. The species S04- which has an orthorhombic g-tensor (2.0047, 2.0089, 2.0474) is trapped at four physically inequivalent sites. Anintense, isotropic peak of Gaussian shape at g = 2.0036 is assigned to SOB-, which shows power saturation effects and a co? B-dependence of intensity. Two peaks in another set are weak, have both the same orthorhombic g tensor (1.9965, 2.0275, 2.0368), and have been tentatively attributed to SO,-. An electron-excess center with an axially symmetric g tensor (g ,, = 1.9754, g, = 1.9502) is probably a metallic impurity ion that has become paramagnetic on capturing an electron released from the Sod*-. INTRODUCTION

ESR studies on irradiated single crystals of alkali and ammonium sulfates (l-7), alkali thiosulfates (8, 9), potassium persulfate (10) and ammonium bisulfate (II) have revealed the presence of a variety of sulfur-oxygen and oxygen radicals such as SO,-, SO:,-, SOZ-, and O,-. Moreover, electron-excess cation centers such as Sn3+ and Cd+ have been reported in y-irradiated sulfates with divalent cations (12-14). Similar studies made on y-irradiated KH,PO, have given valuable information (25) on the involvement of protons in stabilizing radiolysis products. We report in this paper, for the first time, ESR studies on X-irradiated KHSO, crystals at 300 K. KHSO, has an orthorhombic (Pbcp)unit cell with dimensions of 8.40 A, 9.79 A, and 18.!)3 A, which accommodates sixteen molecules. Two types of sulfur form sulfates with oxygen types I to IV and V to VIII, respectively. The positions of hydrogens are not yet established. They are considered to bridge oxygens of types I and II (to form HSO,- dimers) and oxygens of types V and VI (to form polymeric HSO,- chains) (16). EXPERIMENTAL

The crystals were grown at 25°C from saturated aqueous solutions of SarabhaiMerck GR grade KHS04, by slow evaporation. Transparent, hexagon-shaped tablets with an elongated edge were obtained. X-ray irradiation of the crystals (for times ranging between 4 and 8 hours) was done on a Philips PW 1130 X-ray unit with Cutarget operated at 40 kV, 20 mA. The crystals were colorless after X-irradiation but showed ESR spectra which were recorded on a Varian E-4 EPR spectrometer employing 100 kHz modulation. Angular variation studies were carried out for rotations about three mutually perpendicular crystallographic axes, a, b, and c which could be identified from morphological considerations. Copyright 0 1976 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain

311

312

RAMASASTRY

AND

SUNANDANA

RESULTS

Figure 1 shows typical ESR spectra of an X-irradiated KHSO, crystal at 300 K recorded at 20 mW power and 1 Oe modulation amplitude. Figure 2 shows the angular variation of g2 for crystal rotations about a, b, and c axes in the dc magnetic field. There are, in general, seven peaks for rotations about “a” and “c” axes. Six of them are anisotropic but the most intense one is isotropic with g = 2.0036, coinciding with

FIG. 1. ESR spectra of X-irradiated KHSO., crystal at 300 K when the dc magnetic field is: (i) inclined to the “c” axis at 20” in the bc plane, where A,, AZ, A1, A4 peaks are seen fully resolved; (ii) parallel to the a axis; (iii) parallel to the b axis, and (iv) parallel to the c axis. The marked g anisotropy of A and C peaks and the structure exhibited by the C peak is clearly brought out in the above spectra.

DPPH. From the equality of the intensities and the similarity of the angular dependence of their positions, the peaks Al, AZ, AS, and A4 can be considered as a related set. B is a weak peak, often hidden under the A peaks. The C peak is also weak, has g < 2.0023 for all crystal orientations, and is considerably anisotropic. For some orientations it splits into three components with approximate intensity ratio of 1 : 2 : 1 and a separation of 4 Oe. The D peak which is isotropic is also the most intense. It shows change in intensity by about a factor of 2 for rotations about the a or the c axis but no change either in position or in half-width. However, for rotations about the b axis, there is no

IRRADIATED

POTASSIUM

HYDROGEN

313

SULFATE

Rotations

obout’c

axis

trJgJ/( ‘fJ--J--y-j 30

60

90

ii0

150

60

180’

8-m

FIG. 2. Angular variation

90

120

IS0

180

8-d

of the ESR of X-irradiated

KHSO,

crystal

for rotations about the a, b,

and c axes.

change in intensity. For all orientations, the D peak shows saturation effects, even at 300 K, for microwave powers greater than 2 mW. The experimental g2 values are fit-ted, by the method of least squares, to the relation g2(0) = c(+ p cos 28 + y sin 20.

PI where 8 is the angle between the dc magnetic field and an axis of the crystal, measured in the clockwise direction, and CI, j?, y are the least-square parameters. The angles corresponding to g,&,, and g&in are found from the relation 8 ~XtFXll”lIl = + tan-’ (v//I). The maximum and minimum g-value for each peak is then determined using Eq. [I].

PI

314

RAMASASTRY

AND

SUNANDANA

The curves obtained for rotations about the a axis are combined with their counterparts in the ac and ub planes by making use of the fact that ESR spectra for magnetic field along each of the three crystallographic axes occur twice. The combinations thus obtained are diagonalized by Schonland’s method (17) to get the principal g values and the more consistent set is chosen. The criterion used is that the highest and the lowest of the principal g values should lie outside or coincide with the extreme values observed in the recorded spectra. The principal g values and their direction cosines for all the species obtained are given in Table 1. TABLE PRINCIPAL

Species Al

g VALUES AND DIRECTION

1

COSINES FOR PARAMAGNETIC RADICALS CRYSTAL AT Rook TEMPERATURE

Direction cosines

Principal g value

and angles

with

IN X-IRRADIATED

respect

to axis

c

b

a

KHS04

2.0027 2.0100 2.0480

0.5240 -0.7401 0.4235

(58’) (138) (65)

0.7521 0.6333 0.1759

(41”) (51) (80)

-0.3997 0.2263 0.8887

(114”) (77) (27)

2.0060 2.0080 2.0470

0.6713 0.6185 0.4023

(48) (52) (66)

0.6832 0.7336 0.0256

(47) (43) (89)

0.2873 -0.2816 0.9160

(73) (106) (24)

2.0034 2.0092 2.0474

-0.3045 0.2224 0.9270

(108) (77) (22)

0.8312 0.5308 0.1433

(34) (58) (82)

0.4652 -0.8178 0.3466

(62) (145) (70)

2.0047 2.0085 2.0473

-0.3087 0.1959 0.9312

(108) (79) (21)

0.7657 0.6320 0.1164

(40) (51) (83)

0.5643 -0.7498 0.3455

(56) (139) (70)

1.9965 2.0275 2.0368

-0.2207 -0.5739 0.6670

(103) (125) (48)

0.9371 0.5206 0.0209

(20) (59) (89)

-0.2701 -0.6324 0.7434

(106) (129) (42)

1.9965 2.0276 2.0368

-0.2078 -0.5740 0.6665

(102) (I 25) (48)

0.8788 0.5189 0.0159

(28.5) (59) (89)

-0.2519 -0.6334 0.7428

(105) (129) (42)

1.9502 1.9749 1.9759

-0.2237 -0.6444 0.9502

(103) (130) (18)

0.9653 0.0913 0.2005

(15) (85) (78)

-0.1349 -0.7592 0.2387

(98) (139) (76)

2.0036 (isotropic)

DISCUSSION

The intense isotropic peak, D, coinciding with the DPPH resonance, is assigned to SOS-. Whiffen et al. (28) observed an isotropic peak withg = 2.004 in several y-irradiated salts containing SO_?‘-which they attributed to SO,-. They also observed the four hyperfine components due to the interaction of the unpaired electron with the 33S nucleus (I= 4). Molecular orbital considerations predict g NN2.004 and a spin density of 0.13 at the sulfur nucleus both of which were found to agree with the experiment at values. Gromov and Morton also found an identical radical in X-irradiated KzS04 crystals (4).

IRRADIATED POTASSIUM HYDROGEN SULFATE

315

Although the D peak has an isotropic g, it exhibits cos’ e-dependence of intensity for rotations about the a and b axes but not for rotation about the c axis. Saturation effects are noticed even at 300 K for microwave powers in excess of 2 mW, the line shape is Gaussian and the linewidth is about 3.2 Oe. Both the shape and width do not change either on saturation (Fig. 3) or on lowering of temperature, a feature similar to that of the F center ESR peak in X-irradiated KC1 (19). These interesting features suggest that the ESR peak is an envelope of unresolved superhyperfine structure, as first pointed out by Gromov and Morton (4). ENDOR measurements on this easily saturable ESR pea.k (D) may help in identifying the nuclei giving rise to the superhyperfine structure. In Fig. 1 (i) the four A,, A,, A,, A4 peaks are of nearly the same intensity. They also exhibit similar angular variation of their positions (Fig. 2a, b, c). Their g tensors have the same principal g values (2.0047, 2.0089, 2.0474) but different cosines. They are therefore due to the same paramagnetic radical trapped at four physically inequivalent sites in the KHS04 lattice. The principal g values are fairly close to those of SO,- in x-irradiated K,SO, (2.0037, 2.0082,2.0486) as reported by Gromov and Morton (4). Molecular orbital considerations of SO4- (IO) based on earlier work (20) on isoelectronic radicals, namely CrO, 2- , MnO,- and ClO,,, indicates that none of the three principal g values is expected to be less than 2.0023 while one of them should be considerably higher. The A species satisfy these requirements. In view of the above considerations, we propose that the A,, A,, A,, and A, peaks are due to the free radical SO,-. KHSO, crystals grown from three different source materials have given, on Xirradiation, three different g tensors attributable to SO,- (Fig. 4, Table 2). Crystals grown from S. Merck “GR” and S. Merck “extra puriss” (EP) materials give four and

FCC. 3. Saturation behavior of the isotropic D peak (SO,-) at 300 K. The (Gaussian) line-shape and half-width are unaltered on saturation indicating that the peak is inhomogeneously broadened.

316

RAMASASTRY

AND

TABLE

PRINCIPAL g-VALuEs OF SO,-

Al

gz

2

IN THREE X-IRRADIATED KHSO,

EP El

SUNANDANA

CRYSTALSFROM DIFFERENTSOURCES

GR g3

A2

2.0079 2.0061

2.0137 2.0136

2.0236 2.0244

Average

2.0070

2.0137

2.0240

671

A, A2 A3 A4

2.0027

PC

g2 2.0100

g1 2.0480

2.0087

82 2.0087

g3 2.0371

2.0034 2.0080 2.0470 2.0082 2.0082 2.0354 2.0047 2.0085 2.0474 2.0091 2.0091 2.0361 2.0060 2.0092 2.0473 2.0079 2.0079 2.0360 2.0047

2.0089

2.0474

2.0085

2.0085

2.0362

two peaks, respectively, having orthorhombic g tensors. On cooling the EP crystal to 77 K the two S04- peaks split into four, with an increase in the magnitude of principal g values. However, crystals obtained from Dr. Sharon of Poona University (PC) gave four peaks with axial g tensors (g ,, = 2.0362, g, = 2.0085) just as in the case of BaSO, (g,, = 2.0307, g, = 2.0076) (14). In fact, different principal g-values have been reported for S04- in X-irradiated K,S04, from different laboratories (3-5). Also, unlike the SO,- radical which has an isotropic g of about 2.0036 in all the lattices, SO,- has its g tensor axially symmetric in BaSO, (24) and orthorhombic in LizSOb. H,O (I, 2, 6a), Na$O, (6b), K,S04 (d-6), K&O, (ZO), and CdSO, ice (13) the principal g values varying from 2.008 to 2.0091, 2.0076 to 2.0271 and 2.0187 to 2.0572. Further, Atkins and Symons have pointed out (22) that the ESR spectrum of SO,- would be particularly sensitive to both temperature and environment. The above data on S04- in various systems reported from different laboratoriesjustifies out assignment of the A peaks to SO,- in X-irradiated KHSO,. It may be pointed out here that the unidentified centers “a” and “b” in the paper by Aiki and Hukuda on K,S04 (5), which have principal g values 2.003, 2.009, 2.057/ 2.048 may be attributed to SO4- while their center “d” could be due to Sod-. Their assignment of center “c” (2.002,2.030,2.040) to SO,- is questionable; it could, however, be due to SO,- as the g-values compare favorably with those of center B of the present work. The B peak splits into two for rotations of the crystal about the b axis. The principal g values are 1.9965, 2.0275, 2.0368 for both components, suggesting that the radical is trapped at two physically inequivalent sites. SOz- which is detected in X-irradiated Na,SO,, (NH&SO,, and BaSO, has axial g tensors with g,, ranging from 2.0218 to 2.0085 and g, from 2.002 to 2.0081 It is a 19-electron system isoelectronic with SeO,-, CIOz, O,- and NO,-. Chen and Das (22), on the basis of MO calculations, predicted 2.0023,2.0077,2.0267 for SeO,-, in agreement with the experimental data of Cook et al. (23) for SeO,‘- in y-irradiated NaHSeO, (1.9967, 2.0062, 2.0268). There is a wide variation of the experimental principal g values of SeOz- (1.9974 to 1.9986,2.0059 to 2.0118 and 2.03 to 2.0310) in y-irradiated Na,Se04, Na,SeO,, and K,SeO, (24). In view of this, it appears reasonable to assign the B peaks to SO,-. The electron-excess center (C) has an axially symmetric g tensor with g,, = 1.9754 and g, = 1.9502. The peak shows a triplet structure with intensity ratio 1 :2 : 1 and a

IRRADIATED

POTASSIUM

HYDROGEN

317

SULFATE

separation of about 4 Oe. The structure probably arises from superhyperfine interaction of the electron with two equivalent hydrogen nuclei. This center has to be associated with either an electron trapped at a lattice defect (such as an anion vacancy) or an impurity metal ion that can easily trap electrons to become paramagnetic. The latter assignment is more reasonable as anion vacancies may not exist in KHSO,. However, impurity ions such as Pb2+ in alkali halides are known to be efficient electron traps. We have also studied the effects of X-irradiation on the ESR of copper-doped KHSO, powder. In addition to the trapping of electrons by CL?+ to become Cu+, S03seems to have been formed in abundance @isotropic= 2.0048). However, there is another center with an orthorhombic g-tensor (1.9985,2.0100,2.0381) which islikely to be SO,-. Storage effects Cu+ --f Cu*+ reconversion. These results demonstrate the role played by impurities in stabilizing certain radicals in radiation damage. O,- is a rather stable radical formed readily in X-irradiated alkali and alkaline earth

(4

FIG. 4. ESR spectra of x-irradiated of KHS04 crystals grown from: (a) S. Merck “extrapuriss” (EP); (b) :S. Merck “GR” grade materials; (c) crystal obtained from Poona University (PC). The dc magnetic field makes 60” with the c axis in the bc plane. It is seen that considerable differences exist in the number and g anisotropy of free radicals formed in the three crystals.

318

RAMASASTRY

AND

SIJNANDANA

chlorates as well as alkali sulphates but it is conspicuous by its absence in X-irradiated KHSO,. Figure 5 shows at a glance the principal g values obtained from the ESR spectra of three KHSO, crystals and KHSO, powder on X-irradiation. The powder spectrum does not show any g less than 2.000 and also there is considerable overlapping in the region 1.999 to 2.010 which make comparison with single crystal data difficult. The powder spectrum appears to depict the prominent features of the ESR of the three single crystals although there is a greater correspondence with that of crystal 3. The present ESR study of X-irradiated KHSO, thus reveals that background impurities and conditions of growth influence the ESR spectra in the number and symmetry of the paramagnetic radicals formed by radiation damage.

I I

‘, 1 I

g

A

I

I

R

I I

I

0

I I

A

II II

61 4

I

0 A

I I I I I 2 060

I

I

2.040 c-

I

2.030 2 020 PRINCIPAL

I

83 63 0

I

2.010 2 000 g VALUE(S)

I 1.990

FIG. 5. Comparison of the principal g values obtained in X-irradiated GR, EP, and PC, (crystals 1,2 and 3) and KHSO., powder. The data of crystal 3 agree rather well with the powder data.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. II. 12.

REFERENCES P. E. WIGEN AND J. A. COWEN,J. Phys. Chem. Solids 27,26 (1960). C. L. ASELTINEAND Y. W. KIM, J. Phys. Chem. Solids, 28,867 (1967); 29,531(1968). N. HARIHARAN AND J. SOBHANADRI,1. Mugn. Resonance 1,637 (1969). V. V. GROMOVAND J. R. MORTON, Can. J. Chem. 44,527 (1966). K. AIKI AND K. HUKUDA, J. Phys. Sot. Japan 22,663 (1967). N. HARIHARAN AND J. SOBHANADRI, (a) J. Phys. Chem. Solids 30,778 (1969); (b) Mol. Phys. 17, 507 (1969); (c) Mol. Phys. 18,713 (1970). I. SUZUKI AND R. ABE, J. Phys. Sot. Ju~un 30,586 (1971). N. Gem AND 0. MATUMURA, J. Phys. Sot. Japan 18, 1702 (1963) ; R. M. GOLDING AND J. M. DELISLE, 1. Chem. Phys. 43,329s (1965). R. L. EAGER AND D. S. MAHADEVAPPA, Can. J. Chem. 41,2106 (1963). P. W. ATKINS et al., 6th Int. Symp. on Free Radicals, Cambridge (1963), Proc. Chem. Sot. 1963, p. 222. 1. BARBUR, Phys. Stat. Sol. (b) 45, K 129 (1971). M. C. R. SYMONS et al., J. Phys. Chem. 76,141 (1972).

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P. N. M~~RTHY AND J. J. WEISS, Nature 201,1318 (1964). 14. M. B. D. BLOOM et al., J. Chem. Sot (A) 1235 (1970). IS. W. E. HUGHES AND W. G. MOULTON, J. Chem. Phys. 39,1359 (1963). 16. L. H. L~~PSTRA AND C. H. MACGILLAVRY, Acta Cryst. 11,349 (1958). 17. D. S. SCHONLAND, Proc. Phys. Sot. 73,788 (1959). 18. D. H. WHIFFE:N et al., Mol. Phys. 5,233 (1962). 19. A. M. PORTIS, Phy.s. Rev. 91,107l (1953). 20. M. WOLFSBERG AND L. HELMHOLZ, J. Chem. Phys. 20,837 (1952). 13.

21.

P. W. ATKINS AND M. C. R. SYMONS, “Structure 1967. 22. I. CHEN AND T. P. DA&J. Chem. Phys. 45,3526 23. R. J. Coon et al., Mol. Phys. 8,195 (1964). 24. P. W. ATKINS et al., J. Chem. Sot. 5215 (1964).

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(1966).

Radicals”,

Elsevier,

Amsterdam,