THE ISOMER SHIFT VALUES OF IMPLANTED /133Xe/133Cs IN VARIOUS HOSTS

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THE ISOMER SHIFT VALUES OF IMPLANTED /133Xe/133Cs IN VARIOUS HOSTS

I. Dézsi, R. Coussement, G. Langouche, H. Pattyn, S. Reintsema, M. van Rossum, J. de Bruyn

To cite this version:

I. Dézsi, R. Coussement, G. Langouche, H. Pattyn, S. Reintsema, et al.. THE ISOMER SHIFT

VALUES OF IMPLANTED /133Xe/133Cs IN VARIOUS HOSTS. Journal de Physique Colloques,

1979, 40 (C2), pp.C2-573-C2-575. �10.1051/jphyscol:19792200�. �jpa-00218579�

THE ISOMER SHIFT VALUES OF IMPLANTED /1 3 3Xe/ 1 3 3C s IN VARIOUS HOSTS

I.Dezsi , R. Coussement, G. Langouche, H. Pattyn, S. Reintsema, M. Van Rossum and J. De bruyn Institut voor Kern-en Stealingsfysika, University of Leuven, B-3030 Leuven, Belgium

Abstract.- The isomer shift values of 1 3 3Cs implanted in graphite, diamond, Te, Si, Ge, Cu, Zn, Mo, and W are found in a larger interval than those found in Cs and Cs-compounds. The extension to lar- ger values are explained by the increase of electron densities due to the compression of the over- sized implanted atom.

1. Introduction.- Recent Mossbauer studies of im- planted /1 3 3X e / l 3 3Cs atoms showed some very inte- resting results /l,2,3,4/. The implanted Xe atoms were found, depending on the implantation tempera- ture, partly in substitutional positions and in impurity-vacancy associations. The Mossbauer spec- tra of the daughter element Cs could give informa- tion on these impurity sites. Some surprisingly large isomer shift values observed for Cs in the implanted samples required further studies in a broader range of materials. The results of these studies are presented in this paper.

2. Experimental.- The 1 3 3Cs Mossbauer spectra were recorded using the 81 keV transition of 1 3 3Cs re- sulting in the decay of 1 3 3Xe sources implanted in various materials. The implantations were performed at 70 keV using the Leuven Isotope Separator. The doses were in the range of 5x1013 to SxlO11* atoms/cm2. In some metallic samples the implantations were made at 120 K. The Mossbauer spectra were measured at 4.2 K. A CsCl single line absorber was used with 450 mg/cm thickness.

The samples were single crystals of the va- rious elements except for diamond and graphite. The diamond samples were mosaics of small ^3x3 mm sized crystals while the graphite was in pyrolitic form.

3. Discussion of the experimental results.- Some of the measured Mossbauer spectra are shown in figure 1.

The spectra had a shape of broadened single lines except for the case of diamond, Mo and W where more details could be seen.

The fits of spectra with single lines for gra- phite, Si, Ge, Te and Zn samples gave satisfactory

rel. x2 values between 1.0 and 1.6. The origin of the broadening cannot be known for sure.

The spin values of 33Cs first excited state and ground state is 5/2 and 7/2, respectively. The qua- drupole moment (Q) of the ground state is very small:

-0.025 b /5/, and recently Q = -0.22 b was measured for the first excited state /6/. Thus in the case of the broadened spectra, taking into account also the large value (-121.3) of the Sternheimer factor for Cs, /ll the presence of quadrupole interaction cannot be excluded. A fit of the broadened single line line spectra of graphite, Si, Ge, Te and Zn Fig. 1 : The Mossbauer spectra of 1 3 3Xe 1 3 3Cs im- planted in graphite, diamond, Te, Cu(n.a. means : not annealed). Negative velocities correspond to a lowering the emitted gamma energy.

On leave from Central Research Institute for Phy- sics, Budapest, Hungary.

JOURNAL DE PHYSIQUE Colloque Cl, supplément au n° 3, Tome 40, mars 1979, page C2-573

Résumé.- Les valeurs du déplacement isomérique de Cs implanté dans le graphite diamant, Te, Si, Ge, Cu, Zn, Mo, W couvrent un domaine plus étendu que celui correspondant au Cs et aux composés du

Cs. L'extension vers les valeurs plus grandes peut s'expliquer par l'accroissement de la densité électronique causée par la compression de l'atome implanté.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792200

JOURNAL DE PHYSIQUE

with quadrupole triplet gave Gvalues very close to those obtained with the single line fit. The diffe- rences were less than 0.02 mm/s.

Zn and Cu samples were implanted at 120 K and measurements were also made after annealing these samples at 300 and 500 K for 1 hour, respectively.

The spectra of Zn did not show any appreciable chan- ge after nnealing while for Cu a significant change could be observed. The spectrum of Cu shifted to smaller velocities. The 6 values of Cu and W are in good agreement with those reported in reference 181.

The Mzssbauer spectrum of diamond was rather complex. A good fit could be obtained only by assu- ming three spectral components : two single lines and one quadrupole split triplet. The existence of quadrupole splitting in I2'xe implanted in diamond was suggested earlier 191. The 6 values measured are shown in figure 2. These values are given as absorber values (6= -6 -0.27) relative to BaAlr

exp

source for a direct comparison of our 6 values with earlier 6 values for Cs and Cs-compounds given in reference / ^101.

Fig. 2 : The isomer shift. (6) values of 1 3 3 ~ s in va- rious hosts. ns, = the effective number of 6s elec- trons. The 6 values are given relative to BaAlr.

s : substitutional; a : after annealing. The error is A6 = +0.01.

The outer electron configuration of a Cs atom is 5 ~ ~ 5 ~ ~ 6 s and the isomer shift can be given as :

6 = K

R

AR A I ' $ ( ~ ) ~ ~ I ~ ,

where K is a constant depending on the nucleus, AR is the fractional change of the nuclear charge ra- dius between the ground ant the excited state and A1$(0) 6s

1

is the difference in 6s electronic den- sity at the nucleus in source and absorber. A cali- bration curve was given in reference /IO/by connec- ting the 6 values of CsMnFs and Cs metal vs. the number of 6s electron with a straight line (the dashed line in figure 2). It can be seen that some 6 values lay well above the Cs and BaA14 6 values indicating a higher 6s electron density than those found in Cs and in BaA14. These large values can be explained only by the reduction of the atomic volu- me of Cs. Such a reduction can take place at sites where the available space is too small for the Cs

atom which has a higher atomic or ionic volume than the host atoms. Therefore, the Cs atom is very much compressed in these lattices. Two processes can de- crease the pressure. One is lattice relaxation and the second is delocalization of some electrons from the Cs atom. If 5p electrons are delocalized their delocalization increases the 5s and 6s electron den- sities because of the decrease in screening.The de- localized electrons are probably bound in a tight binding fashion to the impurity atom. The 6 values do not show a simple correlation with the host ato- mic volumes (e.g. the Mo atomic volume is larger

than that of Cu). Thus the actual 6 values appear as a consequence of more complex processes. On annea- ling the Xe atoms may trap more vacancies in the va- cancy recovery stage III., and consequently the pres- sure decreases at the impurity atom. The observed change in the spectrum of Cu can be explained by this process, since recovery stage I11 in Cu is around 250 K /II/. No similar change in the Zn sam- ple could be observed. Probably the vacancies are already mobile in Zn at 120 K and all the impurity atoms form vacancy associations. The presence of the high pressure on the implanted atoms can be seen in the diamond, Si and Ge samples too, where the 6 va- lues are near to, or above the 6 value of Cs metal.

The electronic configuration of Cs in these lattices is not known but probably is not the same as in Cs metal because of the large difference in the electro- negativities of Cs and these elements.

By using the calibration curve of reference /lo/

the ionization state of Cs in graphite and Te is found to be 6s0 and 6 s o e 3 respectively. The graphite value is in agreement with the value for the CsC2t+

intercalation compound published in reference 161.

The authors wish to thank Mr. R. Vanautgaerden for performing the implantations.

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