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THE EFFECT OF FISSION TRACK FORMATION ON
THE DEFECTS AND ELECTRONIC STRUCTURE
OF IRON (II, III) METAPHOSPHATE GLASSES
Y. Takashima, T. Nishida
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, De'cernbre 1976, page C6-919
THE EFFECT OF FISSION TRACK FORMATION
ON THE DEFECTS AND ELECTRONIC STRUCTURE
OF IRON (11, 111) METAPMOSPHATE GLASSES
Y. TAKASHIMA and T. NISHIDA
Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka 812, Japan
R6um6.
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On a fait des recherches a l'aide de Ia spectroscopie Mossbauer sur Ies effets de l'irradiation sur du verre de metaphosphate de fer (11,111) contenant de l'uranium dans un reacteur. Les variations dans Yetat d'oxydation du fer dues 2 l'irradiation ont kt6 attribuees aux trous et aux electrons rksultant des rayons y.D'autre part, les changements des paramktres Mossbauer relatifs au temps d'irradiation et a I'uranium contenu ont kte attribuks aux phenomknes qui accompagnent les lacunes et qui sont en 6troites relations avec la formation de la trace de fission dans le verre.
Abstract.
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Irradiation effects of uranium containing iron (11, 111) metaphosphate glasses with a reactor were investigated by Mossbauer spectroscopy. The irradiation induced changes of the oxidation state of iron were attributed to holes and electrons created by y rays. On the other hand, the changes in Mossbauer parameters with irradiation time and uranium content were ascribed to vacancy association phenomena which are closely related with fission track formation in the glasses.1. Introduction.
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Recently, an increasing interest is being shown in the application of the Mossbauer technique to the studies of radiation induced structural change in solids. Considerable progress has been made in this field as has been shown in several review papers [l-31. Until now, however, such investigations were most frequently concerned with influences of radiation on structural defects in metal and alloys. The reasons are attributed to the fact that : a) there is already plenty of information available from other techniques ; b) theoretical treatments of irradiation effects are less difficult in the metallic solids.In the present study an attempt to elucidate the consequences of heavy damage in ionic compounds by means of Mossbauer spectroscopy has been done. Mossbauer spectroscopy is now known as a well established technique for the study of solid-state properties of glasses [4-51 as well as of crystalline compounds. On the other hand, it is also well known that fission and other heavy particle tracks are register- ed in non-metallic compounds such as glass, plastic and some minerals [6]. In view of the above considera- tions, our study was undertaken to delineate the possible influences of fission track formation on the defect and electronic structure of iron (11, 111) meta- phosphate glasses.
2. Experimental. - 2.1 MATERIALS.
-
The iron (11, 111) metaphosphate: samples were prepared by melting the mixtures of weighed quantities of iron oxide, potassium metaphosphate and uranium oxide in air or under a nitrogen stream at 1 200 OC for2 hours in
an
electric furnace. The melt was quenched quickly dy compressing it between two copper plates [7]. The atomic ratio of P/Fe was kept to 5 for all the samples and uranium content in the glasses was varied from 0 to 10 000 ppm. The unquenched samples were also prepared for comparison. A slightly lowertemperature was applied for the case of iron (11) metaphosphate glass preparation as compared with iron (111) metaphosphate glass preparation. Enriched iron (57Fe) was used for only one case. The samples thus obtained were usually transparent glasses of brownish color. A greenish color was seen in the samples with high uranium content. The products were analysed for iron and phosphate by the wet chemical techniques.
2.2 MOSSBAUER SPECTROSCOPY.
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The Mossbauerspectra of these samples were measured at room temperature before and after neutron irradiations. The apparatus useg is virtually identical with that described elsewhere 181. Cobalt-57 ( 5 mCi) diffused into a palladium foil was used as a source. The spectrometer was calibrated using a pure iron foil and the isomer shift values were taken with respect to metallic iron. The transmitted counts were stored up to 150 000 counts per channel in order to minimize the statistical error. All spectra were fitted to Lorentzian line shapes using the computer program developed in our laboratory.
2.3 NEUTRON IRRADIATION.
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Some hundreds mgof each sample were sealed in polyethylene capsules in the presence of air, and sera ies of the samples were
C6-920 Y. TAKASHIMA AND T. NISHIDA
irradiated in the Research Reactor of the Atomic Energy Institute of Rikkyo University for 30 minutes
or 6 hours. The thermal neutron flux and the y-dose rate in the reactor were 5.5 X 10'' n/cm2.s and
1.5 X 106 R/hour at 100 kW operation, respectively.
3. Results and Discussion.
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Four characteristic Mossbauer spectra for both Fe,03-KPO, and FeO- KPO, systems are shown in figures 1 and 2. A marked change was observed in the spectra between pre-and post-irradiation samples, especially in case of samples having higher uranium content.
The change is also remarkable when a higher neutron dose is applied to the samples. This fact is confirmed by comparing the spectra obtained with the samples irradiated for 30 minutes and 6 hours, respectively. Tables I and I1 summarise the Mossbauer data for pre- irradiation and 6 hour irradiation samples in which uranium content varies from 0 to 10 000 ppm.
As seen in figures 1 and 2, before irradiation the
. . .- .- m . . , . . m m .- IJl E '.. _. . Fe* . , -2.0 -1.0 0 1.0 2.0 3.0 4.0 Retat~ve velocity (mm/sJ Relative velocity (mm/s)
FIG. 1. - Mossbauer spectra of Fe203-KP03 glass before and FIG. 2. - Mossbauer spectra of FeO-KPO3 glass before and after neutron irradiations (irradiation time = 6 hours) (a) before after neutron irradiations (irradiation time = 6 hours) (a) before irradiation (6) after irradiation. The solid line refers to compu- irradiation (b) after irradiation. The solid line refers to compu-
ter analysis. ter analysis.
Mossbauer parameters of Fe,03-KPO, system before and after irradiation, 6 and A represent the isomer shqt
and quadrupole splitting, respectively. The error is respectively
+
0.01 for the isomer shifts and 0.02 for the quadru-pole splittings in mm/s
(mm/s> Before irradiation After irradiation
U content ( P P ~ >
-
0 10 100 1 000 10 000Mdssbauer parameters of FeO-KPO, system before and after irradiation. 6 and A represent the isomer shift and
quadrupole splitting, respectively. The error is respectively
+
0.01 for the isomer shifts and 0.02 for the quadrupolesplittings in mm/s
(mm/s> Before irradiation After irradiation
U content Fe3 + Fe2+ Fe3
*
~ e ' + Fe+FISSION TRACK FORMATION ON THE DEFECTS AND ELECTRONIC STRUCTURE OF IRON (11, ITI) C6-921
spectra consist of simple Fe3+ line for Fe,03-KPO, system and of Fe3+ and ~ e ' + superimposed lines for FeO-KPO, system. The ~ e , ' line in the latter case is due to a partial oxidation of ferrous iron in the course of glass preparation. A weak line was seen on the rightest side in the spectrum for both systems. Although it is supposed to be the one hand of the quadrupole splitting due to Fe2+, the computer analysis for these lines was not successful. After irradiation new peaks appeared in the spectra ; their positions and intensities change according to the irradiation conditions, etc. One doublet is characteristic of ionic Fe2* with an octahedral symmetry. The other doublet was tentatively assigned to be Fe+ charge state since it has very large isomer shift value. With respect to anomalous charge state, Fe+ species are reported on the 57Fe doped MgO and KMgF,, etc. [9]. These irradiation induced changes of oxidation state would be better to ascribe to y rays exposure rather than the fission track formation in the glass, because the change is seen regardless of the presence or absence of uranium. The experimental evidence for radiation induced redox reaction of Sb, MO, and U by trapping holes and electrons has already been made [10]. As described above, two doublets in Mossbauer spectra after irradiation were clearly observed, the one assigned as Fe2+ and the other as Fe'. By measuring the areas under each line, it is clarified that the irradiation induced Fe2+ or Fef line increases in intensity at the expense of the original Fe3 C
line. Namely, the ratio, Fe2+(+ Fe+)/Fe3+, increases with increasing irradiation time for Fe203-KPO,, while the ratio decreases with increasing irradiation time for FeO-KPO,. These findings support the conception that the following reactions have taken place upon irradiation.
and
+
e- + Fef for Fe203-KPO, system. andFe2+
+
e- + Fe+ for FeO-KPO, system. With respect to the influence of fission track formation, it could be considered in connection with the values of isomer shift and quadrupole splitting. It is to be noted that the isomer shift, as a rough tendency, increases with increasing irradiation time and increasing uranium content in glasses. If we accept the ion explosion spike model for track formation, it becomes easier to under- stand the above results. Namely, the atoms are ionized by the massive charged particle, and the mutual repulsion of the ions has forced them into the lattice leaving a large number of vacancies. The vacancies will be easily trapped by iron atoms, and will contribute to the isomer shift increase. There is a published paper concerning electron density evaluation of vacancy associated iron atoms in which the isomer shift wasalso observed to increase due to the vacancy trapp- ing [ll]. It may be naturally thought from our data that such phenomena are extended into almost whole solid matrix because the change in Mossbauer spectrum is observed even in such a low iron concentration. There is another evidence to support this idea, i. e., the Mossbauer parameters obtained from the chemically etched samples essentially the same as those of the unetched samples.
Regarding the change in quadrupole splitting with irradiation time and uranium content, it is also explain- ed by the vacancy associated phenomenon, i. e., the vacancy located nearest or next nearest neighbor of Mossbauer atom may give rise to an electric field gradient. Thus, one observes, in a rough sense, increasing quadrupole splittings with increasing irra- diation time and uranium content of the glasses for the irradiation produced new lines (Fe2+ and Fe'). In contrast with that, the quadrupole splittings of Fe3+ are decreased with increasing irradiation time and uranium content. The lowest value of the quadrupole splittings is similar to that obtained for unquenched crystalline sample. This suggest that crystallization partly occured during irradiation and our x-rays diffraction experiment supported this assumption, though the diffraction patterns did not completely agree with those of unquenched sample. One possible reason for this crystallization may arise from the ambient temperature in the reactor ( W 50 O C ) . But this
is denied by the annealing experiment as described below.
The fact that irradiation induced Fe2+ and Fe' species are observed as stable species even at room temperature, can also be interpreted as a vacancy association phenomenon. The idea of vacancy asso- ciated metal ion complex is proposed by Watkins [12]. In the light of his idea, we believe that Fe2+ and especially Fef ions are stabilized in the glass by combining with the negatively charged vacancies.
An attempt to confirm this assumption has been done by using annealing experiments. The results which show a relationship between the extent of annealing and the Mossbauer parameters of the annealed sample are given in table 111.
In a previous paper we have studied the behavior of annealing reaction of fission tracks for the same glass system and have found an activation energy of
C6-922 Y. TAKASHIMA AND T. NISHIDA
Results of annealing. Mossbauer parameters for Fe3 + in Fe,O,-KPO, system containing 10 000ppm of uranium.
The jigure in parenthesis indicates the degree of fission track annealing. The error is respectively f 0.01 for the isomer shift, 0.02 for the quadrupole splittings and f 0.01 for the line widths in mmls
(mm/s) Temp. ("C)
-
130 160 160 160 210 210 210 210 Time (min) 93 15 36 100 46 100 230 385 Isomer shift 6-
0.44 0.38 0.41 0.45 0.43 0.45 0.45 0.44 Quadrupole splitting A 0.45 0.47 0.49 0.48 0.49 0.48 0.47 0.48 Line widthr
-
0.43 0.51 0.47 0.44 0.42 0.42 0.40 0.42 demonstrate that a local rearrangement of atoms Acknowledgement.-
The authors are indebted to around the Mossbauer atom has taken place following the staff of Rikkyo University for their assistance the oxidation reaction owing to vacancy migration. in carrying out the neutron irradiation.[l] SCHROEER, D., LAMBE, R. L., SPENCER, C. D., MiiSsbauer [7] KMO, M., TAKASHIMA, Y., OHASHI, S., Bull. Chem. Soc
Effect Methodology, Vol. 7, ed. I. J. Gruverman (Ple- Japan, 40 (1967) 2812.
num Press, New York) 1971, - D.
-
3. [g] TAKASHIMA, Y., MAEDA, Y., J. Znorg. Nucl. Chem., 31 (1969) WERTHEIM, G. K., The Electronic Structure of Point Defects,ed. G. K . Wertheim, A. Hausmann, W. Sander (North- Holland, Amsterdam) 1971, p. 12.
VOGL, G., J. Physique Colloq. 35 (1974) C6-165.
FLUCK, E., KERLER, W., NEUWIRTH, W., Angew. Chem. 2 (1963) 227.
GOSSELIN, J. P., SHIMONY, U., GRODZINS, L., COOPER, A. R.,
Phys. Chem. Glasses, 8 (1967) 56.
1337.
[9] TRAUTWEIN, A., REGNARD, J. R., HARRIS, F. E., MAEDA, Y.,
Phys. Rev. B, 7 (1973) 947.
[l01 KOBAYASHI, Y., MATSUURA, N., Bull. Chem. Soc. Japan,
46 (1973) 1346.
[l11 MATSUI, K., HASIGUTI, R., SHOJI, T., OHKAWA, O., Proc. Internat. Conf. on Lattice Defects in Semiconductors, Freiburg (1 974).
[l21 WATKINS, G., Phys. Rev., 113 (1959) 79.
[6] FLEISHER, R. L., PRICE, P. B., WALKER, R. M., Science, [l31 NISHIDA, T., TAKASHIMA, Y., J. Sol. Stat. Chem., 18 (1976)
