Mössbauer investigations of amorphous metal-metal alloys

Mössbauer investigations of amorphous metal-metal alloys

Journal of Non-Crystalline Solids 61 & 62 (1984) 427-432 North-Holland, Amsterdam 427 MUSSBAUER INVESTIGATIONS OF AMORPHOUSMETAL-METAL ALLOYS H.-G. ...

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Journal of Non-Crystalline Solids 61 & 62 (1984) 427-432 North-Holland, Amsterdam

427

MUSSBAUER INVESTIGATIONS OF AMORPHOUSMETAL-METAL ALLOYS H.-G. Wagner, M. Ghafari, H.-P. Klein, U. Gonser Angewandte Physik, Universit~t des Saarlandes, D-6600 SaarbrUcken, Federal Republic of Germany M ~ s s b a u e r m e a s u r e m e n t s of a m o r p h o u s ZrFe and NiZr(Fe) alloys are presented. It is found that the spectra of both m a t e r i a l s are v e r y similar. F i t t i n g a d i s t r i b u t i o n of q u a d r u p o l e splittings to the e x p e r i m e n t a l data one observes that the d i s t r i b u t i o n s are also almost identical for both alloys. This suggests that the Fe e n v i r o n m e n t is the same in both cases. A c o m p a r i s o n with spectra of c r y s t a l l i n e phases allows a tentative i d e n t i f i c a t i o n of the local units around Fe.

i . INTRODUCTION In the common amorphous t r a n s i t i o n metal-metalloid alloys the coexistence of magnetic and e l e c t r i c hyperfine i n t e r a c t i o n s g r e a t l y complicates the analysis of M~ssbauer spectra and therefore r e l i a b l e conclusions about the local Fe environment are almost impossible. By contrast, zirconium rich ZrFe and ZrNiFe alloys o f f e r some i n t e r e s t i n g p o s s i b i l i t i e s f o r M~ssbauer spectroscopy. Spectra of ZrFe and ZrNiFe show d i s t r i b u t i o n s of isomer s h i f t s and quadrupole s p l i t t i n g s only. They are therefore easier to i n t e r p r e t and, since the quadrupole i n t e r action is sensitive to changes in local order, these spectra o f f e r a b e t t e r chance to e x t r a c t information about the environment of the Fe atoms. E a r l i e r M~ssbauer investigations of these materials have led to d i f f e r i n g views of the question of local order. Vincze and coworkers claimed that the short range order around Fe was that of metastable c r y s t a l l i n e ZraFe I obtained as a f i r s t

metastable c r y s t a l l i z a t i o n product on heating the amorphous sample.

By comparison of the d i s t r i b u t i o n of quadrupole s p l i t t i n g s in amorphous and metastable c r y s t a l l i n e ZrFe we showed that the large differences observed in the d i s t r i b u t i o n s made this quite u n l i k e l y 2. In a recent paper the concentration dependence of the quadrupole s p l i t t i n g and isomer s h i f t in amorphous Feloo_xZr× alloys was said to be in agreement with a random a l l o y without chemical short range order 3. This disagrees with results from d i f f r a c t i o n experiments by Ruppersberg and Wagner who suggest chemical short range order in these alloys 4. We w i l l show in t h i s paper that the results from M~ssbauer spectroscopy of amorphous Zrloo_xFex and Zrgs_×Ni×Fes are in agreement with the existence of short range order around the Fe atoms. 0022-3093/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

H.-G. Wagner et al. / MSssbauer investigations o f amorphous metal-metal alloys

428

2. EXPERIMENTAL 5 mm wide melt spun ribbons of amorphous Zrloo-xFex (25.5 ~ x ~ 29) and Zrgs_xNixFes (25 ~ x ~ 40) of 25 ~m thickness were arranged as M~ssbauer absorber of i cm2. They were measured at room temperature in standard transmission geometry using a constant acceleration spectrometer. The source was svCo in Rh. All isomer shifts are given r e l a t i v e to a-Fe. Calorimetric measurements were done in a Perkin Elmer d i f f e r e n t i a l scanning calorimeter DSC 2.

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X'=25j ~ 09

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[mm/sl

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0

Velocity

1

Imrn/sl

FIGURE 1

(~9

&EaImm/sl

1.8

FIGURE 2

M~ssbauer spectra and quadrupole distributions of amorphous Fel00-xZr×.

M6ssbauer spectra and quadrupole distributions of amorphous Zrgs-×NixFes.

3. RESULTS AND DISCUSSION Figures 1 and 2 show spectra of amorphous Zrloo_xFex and amorphous Zrgs-xNi×Fes, The continuous lines are the results of f i t s of distributions of quadrupole s p l i t t i n g s to the experimental data points. The corresponding distributions are shown in the right-hand side of Figs. 1 and 2. The f i t t i n g procedure was a constrained Hesse-RUbartsch method as introduced by Le Ca~r and Dubois5. The asymmetry of the quadrupole doublet shows that there is a correla~ ion between the isomer s h i f t IS and the quadrupole s p l i t t i n g QS. As a f i r s t approximation this was taken to be linear: IS : a(QS - QSo) + ISo

H-G. Wagner et al. / MSssbauer investigations o f amorphous metal-metal alloys

429

where a is a c o r r e l a t i o n parameter and ISo and QSo are the smallest values allowed f o r IS and QS. The QS d i s t r i b u t i o n function obtained f o r amorphous ZrFe and ZrNiFe a l l o y s are remarkably s i m i l a r : the average QS positions and d i s t r i b u t i o n widths and the average IS a l l agree very w e l l , suggesting t h a t the Fe environment might be the same in both a l l o y s . I t is however necessary to p o i n t out t h a t to show a complete i d e n t i t y i t would be necessary to know the d i s t r i b u t i o n s of the biggest Eigenvalue of the quadrupole i n t e r a c t i o n Vzz, and of the asymmetry parameter n. These cannot be simultaneously obtained from the d i s t r i b u t i o n of QS f o r the M~ssbauer isotope SVFe.

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85o

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FIGURE

D S C - P l o t of Z r g s - x N i x F e s r e c o r d e d with a heating rate of 40 K/min.

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i

il 550 65o

~

4

M ~ s s b a u e r spectra and q u a d r u p o l e distributions of m e t a s t a b l e c r y s t a l l i n e Feloo-xZrx.

Both ZrFe and ZrNiFe were c r y s t a l l i z e d on heating at a rate of 40 K per minute. This is most conveniently monitored in a d i f f e r e n t i a l

scanning c a l o r i m e t e r .

Fig. 3 shows the exothermal peaks, i n d i c a t i n g c r y s t a l l i z a t i o n ZrFe the f i r s t

crystallization

f o r ZrNiFe. For

product is metastable ZraFe. I t s M~ssbauer spec-

t r a and the corresponding QS d i s t r i b u t i o n s are shown in Fig. 4. A comparison of the d i s t r i b u t i o n of the metastable c r y s t a l l i n e state with t h a t of the amorphous sample (Fig. i ) shows t h a t these are completely d i f f e r e n t .

I t is t h e r e f o r e un-

l i k e l y t h a t the Fe environment should be s i m i l a r 2. From the M6ssbauer parameters and X-ray data the main component of the meta-

H.-G. Wagner et al, / M6ssbauer investigations o/amorphous metal-metal alloys

430

stable crystalline phases was identified as Zr2Fe with NiTi2 structure. Further heating leads to the appearance of the crystalline phase Zr3Fe with a concentration dependent admixture of Zr2Fe

(Fig. 5). The metastable crystalline phase

which is obtained on heating Zr6sNi3oFes could not yet be identified.

0 9 6LL" ~±_ ~ ~_. __~. ~ 5 1~L~zL:.~ 00 -

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0 Velocity [mm/sl

1

FIGURE 6 of the stable of Feloo-xZrx.

spectra of the stable crystalline phase of Zr9s-xNixFes.

M~ssbauer

Spectra recorded of samples heated above the highest temperature crystallization peak (780 K) show M~ssbauer parameters typical of the Zr:Ni(Fe) phase with CuAl2 structure (Fig. 6). This was corroborated by X-ray diffraction. The M~ssbauer parameters of this phase and the average parameters observed for amorphous ZrFe and ZrNiFe are very similar, which suggests that the average Fe environment in amorphous ZrFe and ZrNiFe might be like that of Fe in crystalline Zr2Ni(Fe), i.e. each Fe atom is surrounded by Zr atoms only with the average coordination of 8 Zr atoms arranged in an archimedian antiprism

capped by

2 Fe atoms. ACKNOWLEDGEMENTS This project was partially supported by NATOgrant, no. RG 128 (80). We are

H.-G. Wagner et al. / M~ssbauer investigations o f amorphous metal-metal alloys

grateful to Dr. G. Le Ca~r f o r providing us with his d i s t r i b u t i o n f i t

431

program.

REFERENCES 1) I. Vincze, F. van der Woude, M.G. Scott, Sol. State Com. 37 (1981) 567. 2) M. Ghafari, U. Gonser, H.-G. Wagner, M. Naka, Nucl. I n s t r . Meth. 199 (1982) 197. 3) P.M.L.O. Scholte, G.A. Fokkema, P. Dorenbos, F. van der Woude, I. Vincze, K.H.J. Buschow, Proceedings of the Sixth International Conference on Hyperf i n e Interactions (Groningen, 1983) in press. 4) C.N.J. Wagner, H. Ruppersberg, in: Application of Nuclear Techniques to the Studies of Amorphous Metals, ed. U. Gonser ( I n t . Atomic Energy Agency, Vienna, 1981) p. 101. 5) G. Le Caer, J.M. Dubois, J. Phys. E12 (1979) 1083.