MÖSSBAUER STUDIES ON THE RADIATION DAMAGE PRODUCED BY ELECTRON CAPTURE AND GAMMA RADIOLYSIS IN COBALT AND IRON FORMATES

(1)

HAL Id: jpa-00216721

https://hal.archives-ouvertes.fr/jpa-00216721

Submitted on 1 Jan 1976

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

MÖSSBAUER STUDIES ON THE RADIATION

DAMAGE PRODUCED BY ELECTRON CAPTURE

AND GAMMA RADIOLYSIS IN COBALT AND IRON

FORMATES

J. Ladriere, D. Apers

To cite this version:

(2)

JOURNAL DE PHYSIQUE Colloque C6, supple'ment au no 12, Tome 37, De'cembre 1976, page c6-913

MOSSBAUER

STUDIES ON

THE

RADIATION DAMAGE PRODUCED

BY ELECTRON CAPTURE AND

GAMMA RADIOLYSIS IN COBALT

AND IRON FORMATES

J. LADRIERE and D. J. APERS

Laboratoire de Chimie Nucltaire, Universitt de Louvain, 2, chemin du Cyclotron, B-1348 Louvain-la-Neuve, Belgium

R6sum6. - Les effets produits par capture electronique et radiolyse gamma dans les formiates de cobalt et de fer ont et6 Btudits par spectroscopie Mossbauer en tmission et absorption. Dans le formiate de cobalt dihydrate marque au 57C0, le fer est stabilise diffkremment suivant qu'il occupe le site cationique tetrahydrate ou le site ne comportant que des formiates : dans le premier site, le Fe se stabilise en Fe2+(40 %) et en Fe3+(lO

X),

tandis que dans l'autre, il se stabilise unique- ment sous forme de Fez+. Le formiate de fer (11) dihydrate est insensible a une irradiation gamma 298 K correspondant

a

une dose de 1,5 X 109 rads. L'absence de Fe'+ dans ce produit irradie est attribuee aux reactions de recombinaison entre les radicaux oxydants et rkducteurs. La radiolyse d'un formiate ferrique anhydre, Na3Fe(HCOz)6, induit, au contraire, une reduction rapide du Fe3+ en Fez+, en raisoil du pouvoir intercepteur de Pion Fe3+ vis-his des radicaux rkducteurs. Abstract. - The effects produced by electron capture and gamma radiolysis in cobalt and iron formates have been studied using emission or absorption Mossbauer spectroscopy. In 57Co labelled CO (11) formate dihydrate, the stabilization of the decayed iron is different in the two cationic sites. In the tetrahydrated site, iron is stabilized as Fe2+(40 %) and Fe3+(lO

X),

whereas in the site containing only formate ligands, iron is stabilized as Fez+ only. Iron (11) formate dihydrate is insensitive to an external gamma irradiation up to a dose of 1.5 X 109 rads at 298 K. The absence of Fe3+ in this irradiated compound has been attributed to recombination reactions between oxidizing and reducing radicals. Anhydrous ferric formate, Na3Fe(HC02)6 is, on the con- trary, readily reduced into a ferrous formate, owing to the scavenging ability of the Fe3+ versus reducing radicals.

1. Introduction.

-

Electron capture decay of 57Co to 57Fe and the subsequent Auger cascade produce a highly charged iron ion which is rapidly reduced to lower charge states by the surrounding electrons. In 57Co labelled cobalt complexes, it has been suggested that the stabilization forms of the iron atom are generally determined by the redox properties of the radicals produced by the autoradiolysis of the surrounding ligands of the decayed nucleus [l]. The determining mechanism of the after effects is therefore depending on the nature of the host compound. Radi- cals originated from water are well known to oxidize Fe2+ and then stabilize Fe3+ [2, 31. Radicals from

organic ligands are mostly reducing towards Fe3+, so that Fe2+ is preferably stabilized [4].

Similarly, the external gamma radiolysis of the iron homolog compounds should provide the same effects. However, in the autoradiolytic process, the stabilization of iron is observed a t 10-7 s after the E. C., when further reactions, such as radical diffusion and recombination are not yet effective. During the short interval between the E. C. event and the emission of the 14.4 keV gamma ray, only electron transfer reactions are to be expected. Therefore, different results can be obtained from these two processes, especially when both oxidizing and reducing radicals are present.

The purpose of this work is to compare the effects of the internal- and external radiolysis on cobalt and iron complexes containing both water and organic ligands. Mossbauer parameters obtained by emission and absorption spectroscopy are used in order to identify the oxidation statq, the coordination and the relative abundance of the radiolytic pro- ducts in the following compounds : 57Co labelled Co(HC02), 2 H 2 0 , Fe(HC02), 2 H 2 0 and Na,Fe(HC02)6.

2. Experimental.

-

Iron (11) and Cobalt (11) formate dihydrates were prepared by mixing a 2M sodium formate solution and a 1 M solution of FeC1, or CoCl,, under nitrogen atmosphere. Labelling of the cobalt formate was performed by adding 0.5 mCi 57CoC12 carrier-free to the cobaltous chloride solution. Sodium hexaformato ferrate, Na5Fe(HC02)6, was precipitated from a solution of concentrated sodium formate (20 M) and ferric nitrate (1 M), as a pale green crystalline powder [5]. The chemical composition of these compounds was checked by thermogravimetric analysis and infrared spectroscopy. The samples were irradiated a t room temperature and under vacuum with a 6 0 ~ o gamma rays source a t a dose rate of 0.65 Mrads per hour. The Mossbauer spectra were

(3)

C6-914 J. L A D R I ~ R E AND D. J. APERS recorded with a constant velocity spectrometer using

a "Co(Pd) source for the absorption spectra and a stainless steel single line absorber in the emission experiments. The Mossbauer parameters were estimated by a least squares computer fit. The relative abundance of the different iron species has been estimated from the peak areas by assuming that the iron Lamb- Mossbauer factors are identical. In the case of the iron compounds, this supposition has been found to be in good agreement with the results obtained by potentio- metric titration of Fe2+ and Fe3' with Ce4+ and Cr2+ solutions as reagents.

3. Results and discussion.

-

The Mossbauer parameters of the investigated compounds are listed in table I. The corresponding spectra are shown in figure 1 and figure 3.

The Mossbauer spectrum of the iron (11) formate dihydrate (Fig. Za) exhibits four equally intense lines which have been resolved into two quadrupole doublets having approximately the same center shift [6]. This is

-

VELOCITY ( m m 1 S)-

FIG. l.

-

(a) Mossbauer absorption spectrum of F e ( H c 0 ~ ) ~ . 2 H20 at 298 K (source = 57Co(Pd)). (6) MOSS- bauer emission spectrum of 57C0 labelled Co(HC02)2.2 H20

at 298 K (Absorber = stainless steel).

in agreement with the crystalline structure of metal (11) formate dihydrates [7, 81 which contain two basically different types of cations per unit cell : one cation is at the center of an octahedron of six formate oxygens (site 2) whereas the other is coordinated to two formate oxygens in a trans-configuration and four water molecules (site 1). The different octahedra are linked together by formate ions each of which takes part in the coordination of two ~ e ions. Figure 2 shows the ~ + coordination symmetry at the two sites.

FIG. 2.

-

Coordination symmetry at the two Ferrous-ion sites in Fe(HC02)z. 2 H20.

The larger quadrupole splitting (AA') has been assigned to the more asymmetrical site in which the Fe,' ion is tetrahydrated (site 1).

In the emission spectrum of the cobalt (11) formate dihydrate (Fig. lb), the intensity of the larger quadrupole doublet (AA') is decreased relatively to the intensity of the inner doublet (BB'), while a new doublet (CC'), characteristic of Fe3

',

appears. Since the inten-

VELOCITY (rnmls)

-

FIG. 3.

-

Mossbauer absorption spectra of unirradiated (a)

and irradiated Na3Fe(HC02)6 up to a dose of (b) 150 Mrads,

(4)

RADIATION DAMAGE PRODUCED BY .ELECTRON CAPTURE AND GAMMA RADTOLYSIS C6-915

Mossbauer parameters of irradiated and unirradiated forrrtates

Dose

T(")

A(d) 6 (bld)

I'

(c,d)

%

Fe2 + (3

Compound Mrads K mm.s-l mrn.sw1 mm.s-' (+ 5

%)

-

Fe(HC02)2. 2 H 2 0 0 298 2.96 1.51 0.27 50 0.65 1.47 0.29 50 80 3.44 1.64 0.35 50 1.32 1.59 0.35 50 1 500 298 2.97 1.51 0.28 50 0.66 1.48 0.29 50 Co(HC02), .2 H 2 0 E. C . 298 2.90 1.48 0.70 40 0.70 1.46 0.70 50 0.37 0.69 0.70

-

E. C. 80 3.20 1.63 0.83 39 1.40 1.58 0.83 50 0.29 0.73 0.83

-

0 298 0.38 0.66 0.60

-

105 298 1.83 1.50 0.31 30 (29) 0.43 0.67 0.54 150 298 1.82 1.51 0.32 40 (38) 0.43 0.68 0.56

-

270 298 1.80 1.48 0.28 58 (60) 0.42 0.68 0.65 330 298 1.82 1.50 0.27 63 (64) 0.38 0.66 0.61

-

460 298 1.82 1.49 0.30 78 (79) 0.41 0.65 0.54 a-Fe(HC02)2 0 298 1 .S6 1.50 0.35 100

(a) Absorber or source temperature in absorption or emission measurements.

( B ) Relative to sodium nitroprusside.

(c)

r

is the full width at half mzximum.

(a) All values are given with an accuracy of =t 0.05 mm/s. ( G ) The values in brackets are obtained from chemical analysis.

sity lowering of the larger doublet (AA') is exactly equal to the intensity of the new doublet (CC'), it is assumed that the decayed iron is stabilized as Fe2+ (50

%)

in site 2, whereas, in site 1, it is stabilized as Fe2" (40

%)

and Fe3+ (10

X).

This difference can be attributed to a predominant amount of oxidizing radicals in the hydrated site and reducing radicals in the other, within 10-I S after the E. C . event. The ferric ion

may exist in the site 1, as long as the OH oxidizing radicals originated from the four water molecules are in excess relatively to the reducing radicals from the two formate ligands. In site 2, any Fe3+ will be imme- diately reduced into Fe2+ because this site contains only reducing radicals.

In contrast with these observations, it is seen from table I, that at room temperature, the iron (11) formate dihydrate is insensitive to an external gamma irradiation up to a dose of 1 500 Mrads. The absence of Fe3+ in this irradiated compound may result from the recombination of the oxidizing and reducing radicals produced in the two sites. Since the Fe2' ion is unable to scavenge the reducing radicals, the latter are free to react with the oxidizing radicals from the other site or to

recombine in their own site. As a result, the concen- trations of both radicals are strongly lowered, which ensures the stability of this compound. It should be noticed, however, that for irradiation at very low temperature, radical recombination and diffusion are strongly reduced or inhibited, so that room temperature unstable iron species might be observed [9, 101. As a proof for this mechanism, it is shown (Fig. 3)

that the irradiation of an anhydrous ferric formate, Na,Fe(HCO,),, gives indeed rise to a reduction of Fe3 + into FeZ+, with a yield increasing as a function of

(5)

C6-916 J. LADRIERE AND D. J. APERS indicating that it corresponds to a stable species. In

this process, carbon dioxide and hydrogen have been analyzed as escaped gases. The corresponding weight loss of the completely radiolysed compound, determined by thermogravimetric analysis, has been found to be 45 gr per mole, which is exactly the molecular weight of the escaped gas. In agreement with these observations, the following mechanism is proposed :

- excitation and decomposition of the formate ligand :

H C O O - m * H C O O - - + H + C O O - ;

-

reduction of Fe3+ by the coordinated ligand fragment :

Fe3+

+

COO- -, Fe2+

+

C O 2 ;

-

liberation of hydrogen :

H + H + H 2 .

The initial G,,,, value, which denotes the number of iron atoms reduced for each 100 electron-volts of energy absorbed, has been estimated from the initial slope of the curve in figure 4 ; G,,,, = 7.6. This value is rather high for a solid but is well explained by the fact that in this compound the ferric ion is bound to six formate ligands which ensure a high reduction proba- bility.

As a conclusion, the comparison of the chemical effects of the E. C . and gamma radiolysis in solids may lead to different results because radical diffusion and reactions between radicals are ineffective within the time scale of observation in the first case, whereas these reactions are readily seen in the second case.

FIG. 4. -Relative abundance of Fez+ produced by radiolysis in Na3Fe(HCO2)6, as a function of the dose.

Nevertheless, it has been confirmed that the stabilization forms of iron, as well in the autoradiolytic as in the radiolytic processes, are strongly dependent on the redox properties of the radicals which are produced in the immediate vicinity of the iron atom.

Acknowledgements.

-

We are profoundly indebted to Dr. J. Meyers who provided a valuable assistance

in computer analysis.

References [l] FRIEDT, 3. M. and DANON, J., Radiochim. Acta 17 (1972)

173-190.

[2] WERTHEIN, G. K. and BUCHANAN, D. N. E., Phys. Let-

ters 3 (1969) 87.

[3] HEIDRICH, W. , 2. Phys. 230 (1970) 418.

[4] SAITO, N., TOMINAGA, F. and MORIMOTO, T., J. Znorg. Nucl. Chem. 32 (1970) 2811.

[S] WEINLAND, R. F. and REIHLEN, H., Ber. 46 (1913) 3145.

[6] HOY, G. R. and DE S. BARROS, F., Phys. Rev. 139(1965)

A 929-934.

171 KROGMAN, K. and MATTES, R., 2. Kristall. 118 (1963)

291.

[S] DE WITH, G., HARKEMA, S. and VAN HUMMEL, G. J.,

Acta Crystallogr. 32 (1976) 1980.

[9] FRIEDT, 3. M., BAGGIO-SAITOVITCH, E. and D a m , J.,

Chem. Phys. Lett. 7 (1970) 603.

[l01 BAGGIO-SAITOVITCH, E. and DANON, J., Chem. Phys. Lett.

39 (1976) 296.