After-effects of electron capture of 57Co in frozen aqueous solutions of 57CoCl2

N u c l e a r I n s t r u m e n t s a n d M e t h o d s 199 (1982) 2 7 7 - 2 7 9 North-Holland Publishing Company

277

AFTER-EFFECTS OF ELECTRON CAPTURE OF S7Co IN FROZEN AQUEOUS SOLUTIONS OF 57CoCi 2 Jehad K. MULHEM *, Dezs6 HORVATH, B61a MOLNJkR and D6nes Lajos NAGY Central Research Institute for Physics, H-1525 Budapest 114, PO Box 49, Hungary

M 6 s s b a u e r spectra of 57C0 in dilute frozen aqueous solutions of 57COC12 and in 0.11 at.% frozen aqueous solutions of CoCI 2 with a n d w i t h o u t glass f o r m e r have been studied. The spectra reveal the presence of a small fraction of Fe 3+ ions besides the Fe 2+ as a c o n s e q u e n c e of the A u g e r cascade following the electron capture. The a d d i t i o n of glycerol to the 57COC12 solution resulted in d e c r e a s i n g the fraction of Fe 3+ . The q u a d r u p o l e splitting of the ferrous ions was found to be less by 13% than that of the Fe(H20)62+ c o m p l e x as m e a s u r e d in a b s o r p t i o n geometry. This is suggested as being the c o n s e q u e n c e of the a u t o r a d i o l y s i s of w a t e r molecules resulting in a m e t a s t a b l e state of the complex.

1. Introduction After-effects of ~7C0 electron capture decay have been observed in various kinds of hosts. The results have been interpreted in terms of different mechanisms. Auger cascades lead to highly ionized daughter iron ions which may be neutralized by the transfer of electrons from neighbouring atoms in the molecule. The electrostatic repulsion drives the positive charges to the periphery of the molecule. The charge distribution may produce a strong Coulombic repulsion between the atoms in the molecule leading to its explosion (explosion model) [1]. The Auger electrons and X-rays radiolyse the ligands to ions and free radicals. Oxidizing or reducing radicals ensure the stabilization of the aliovalent ions through the reaction with the central metal ion (radiolysis model) [2]. In a previous paper [3] we have shown that in ferrous perchlorate and tetrafluoroborate hexahydrate complexes having a known phase transformation the autoradiolysis model is appropriate. Here we present a study of the mechanism of after-effects of 57C0 in frozen aqueous solutions (FAS) of CoC12.

then diluted by 0.2 ml of 0.001 n HC1. The solution was put into a holder with a teflon middle ring and two Terylene foils. Then the solution was solidified by immersion in liquid nitrogen. 57CoC12(CoC12) was prepared using 14.5 mg CoCI2.6 H20 dissolved in,1 ml of distilled water and mixed with 0.08 ml 57COC12 solution with an activity of 18.5 MBq. Two further samples with 50 vol.% glycerol added were prepared as well.

3. Results The ~7CoC1~ FAS was measured after solidification at liquid nitrogen temperature. The same

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Fig. 1. M 6 s s b a u e r e m i s s i o n spectra for frozen aqueous solutions of 57CoC1z a n d 57CoC12(CoC12) at T - - 8 8 K.

0167-5087/82/0000-0000/$02.75 © 1982 North-Holland

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278

J.K. Mulhem et al. / After-effects of electron capture

Table 1 Quadrupole splittings, isomer shifts, line widths, and Fe z+ fractions obtained from emission M6ssbauer spectra of 57CoC1~ and 57CoCIz(CoCl~) frozen aqueous solutions at T=88 K. The upper values of AEQand IS are related to Fe 2~ , the lower ones to Fe 3~ AEQ (mm/s)

IS (mm/s)

F (mm/s)

Fe 2+ (%)

57C°C12

2.86±0.04 1.06±0.06

- 1.30±0.02 -0.38±0.04

0.91 ±0.04

66

57C°C12(COCI 2 )

2.72-+0.04 l. 10 ± 0.04

- 1.31 -+0.2 0.35 ± 0.04

0.78 ± 0.04

57

Table 2 Quadrupole splittings, isomer shifts, line widths, and Fe z+ fractions obtained from emission M6ssbauer spectra of 57COC12 and 57CoC12(CoC12) frozen aqueous solutions with 50 vol.% glycerol added at T = 88 K. The upper values of AEQand IS are related to Fe 2+, the lower ones to Fe 3+ A EQ(mm/s)

57C°C12

2.74--0.04 1.30 ± 0.08

57C°C12(C°C12 )

2.72 ± 0.04 1.24-+0.08

IS(mm/s)

F(mm/s)

Fe 2 + (%)

1.32±0.02 0.42 --+0.04

0.86 ± 0.04

83

- 1.29 ± 0.02 -0.37±0.04

0"79-+0"04

92

-

4. Discussion

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Fig. 2. M6ssbauer emission spectra for frozen aqueous solutions of 57COC12and 57CoC12(CoC12)with 50 vol.% glycerol added at T = 88 K.

m e a s u r e m e n t was m a d e w i t h the 57CoCl2(CoC12) F A S . T h e M 6 s s b a u e r e m i s s i o n s p e c t r a are s h o w n in fig. 1 a n d the p a r a m e t e r v a l u e s are listed in t a b l e 1. M O s s b a u e r e m i s s i o n s p e c t r a a n d p a r a m e ters m e a s u r e d o n the s a m p l e s w i t h a d d e d g l y c e r o l a r e s h o w n in fig. 2 a n d t a b l e 2, r e s p e c t i v e l y .

T h e e m i s s i o n s p e c t r a s h o w a s i g n i f i c a n t dec r e a s e in the F e 2+ q u a d r u p o l e s p l i t t i n g (13%) relative to the a b s o r p t i o n case: A E Q ( e m ) = ( 2 . 8 6 + -0.04) m m / s , A E Q ( a b s ) = (3.30--+ 0.05) m m / s [4]. T h e M 6 s s b a u e r s p e c t r a also s h o w a s m a l l f r a c t i o n o f F e 3+ w h i c h m e a n s the p a r t i a l c h a n g e o f the o x i d a t i o n state w i t h r e s p e c t to the c o b a l t p a r e n t atom. U s i n g 50% g l y c e r o l of the s o l u t i o n v o l u m e as a glass f o r m e r n o c h a n g e c o u l d b e o b s e r v e d in the q u a d r u p o l e s p l i t t i n g a n d i s o m e r shift of F e 2+ w i t h i n the limits of e x p e r i m e n t a l error. T h e q u a d r u p o l e s p l i t t i n g of the a b s o r p t i o n s p e c t r a of the F A S o f ferrous c o m p o u n d s s h o w s slight devia t i o n s (0.2 m m / s ) ; t h e r e are r e m a r k a b l e diff e r e n c e s in the AE(2 v a l u e s m e a s u r e d o n p u r e c r y s t a l l i n e h y d r a t e s [5]. F o r e m i s s i o n s p e c t r a of F A S such small d i f f e r e n c e s m a y also a p p e a r , b u t as a result of the line b r o a d e n i n g t h e y c a n n o t be observed. T h e o n l y effect of a d d i n g 50% g l y c e r o l was that the r e l a t i v e f r a c t i o n of i r o n (III) d e c r e a s e d significantly. In the h y d r a t e s of C o 2+ c o m p o u n d s a r e g u l a r i n c r e a s e of the p r o p o r t i o n of ferric ion w i t h h y d r a t i o n n u m b e r was o b s e r v e d [6]. T h i s is

J.K. Mulhem et al. / After-effects of electron capture

expected from the autoradiolysis model. The radicals originating from H 2 0 ligands will oxidize Fe 2+ and thus stabilize Fe 3+ ions. The reaction Fe 2+ + O H ~ Fe 3+ + O H is a well known process in radiation chemistry [7]. The increase in the yield of Fe 3+ with the number of H 2 0 ligands in the nearest coordination shell is due to the higher density of the oxidizing radicals. In conventional radiolysis experiments the presence of alcohols in aqueous solutions of ferrous salts is known to increase the yield of Fe 3+ ions by about a factor of 3 [7]. This is explained by the assumption that OH radicals produce organic peroxy radicals rather than oxidizing Fe 3+ . The peroxy radicals will, in turn, oxidize three Fe 2+ ions: RH+OH~R+H20, R ÷O 2--+RO2, R O 2 + Fe 2+ ~ Fe 3+ + R O 2 ,

R O 2 + H + -+ RO2H , RO2H + Fe 2+ + RO + O H - + F e 3+ ,

279

adding glycerol to the solution. This result shows that glycerol molecules enter at least the second ligand sphere of the metal ion thereby considerably affecting the first ligand shell. The decrease of AEQ can be explained as a result of a metastable intermediate state of the molecule produced by the autoradiolysis of the surrounding water ligands i.e. by the formation of a transient form of Fe(H20)62+ [3]. On increasing the concentration of CoC12 in water a slight increase of the Fe 3÷ fraction is observed. Although the quenching procedure shifts the effective salt concentration towards higher values [4] the final concentration is expected to follow the initial concentration of the solution. With this in mind, the increase of the Fe 3+ fraction might be attributed to the increased concentration of C1 ions. If these anions are radiolysed, C I - --, CI + e - , the nascent chlorine may contribute to the oxidization of the Fe 2-. In contrast to this, an increase of the CoC12 concentration in the water/glycerol solution seems to decrease the Fe 3÷ fraction. This point needs further clarification.

H + + F e 2+ + R O - - . R O H + Fe 3+ " In this case R H denotes glycerol. The observed decrease in the ferric yield seemingly contradicts the above expectations. Taking into account, however, that (l) the above model is applicable to frozen solutions only to a limited extent since it supposes a strong diffusion of the R radicals thereby enabling them to oxidize three Fe 2+ ions; (2) the first reaction decreases the direct Fe 3+ yield by depriving Fe 2+ ions of OH radicals while the other reactions are certainly much slower and are complete only beyond the M6ssbauer lifetime; (3) the produced peroxy radicals do not necessarily oxidize the same Fe 2+ ion to whose ligand sphere the OH radical belonged; one expects in fact a decrease in the ferric yield on

References [1] T.S. Srivastava and A. Nath, Radiochem. Radioanal. Lett. 16 (1974) 103. [2] Y. Llabador and J.M. Friedt, Chem. Phys. Lett. 8 (1971) 592. [3] J. Mulhem, J. Balogh, I. Demeter, D. Horv~th, B. Moln~, D.L. Nagy and I.S. Sziics, Preprint KFKI-1981-87, Budapest. [4] I. D6zsi, L. Keszthelyi, B. Moln~ and L. P6cs, KFKI 9/1967, Budapest. [5] I. D6zsi, L. Keszthelyi, G. Nagy and D.L. Nagy, Proc. Conf. Application of the M6ssbauer effect, Tihany, 1969 (Akad6miai Kiad6, Budapest, 1970) p. 607. [6] J.M. Friedt and J.P. Adloff, C. R. Acad. Sci., Paris, 264C (1967) 1356. [7] J.K. Thomas, Advances in radiation chemistry, Vol. I (1969) 103.

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