STRUCTURE AND BONDING NATURE OF Pd-Si AMORPHOUS ALLOYS

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STRUCTURE AND BONDING NATURE OF Pd-Si AMORPHOUS ALLOYS

A. Matsuzaki, S. Nanao, H. Ino

To cite this version:

A. Matsuzaki, S. Nanao, H. Ino. STRUCTURE AND BONDING NATURE OF Pd-Si AMORPHOUS ALLOYS. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-104-C2-106.

�10.1051/jphyscol:1979237�. �jpa-00218635�

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JOURNAL DE PHYSIQUE Colloque C2, supplkment au n O 3, Tome 40, mars 1979, page C2-104

STRUCTURE

AND

BONDING NATURE OF Pd-Si

AMORPHOUS

ALLOYS A. Matsuzaki, S. Nanao and H. Ino

I n s t i t u t e of Industria2 Science, University of Tokyo, 22-1, Roppongi, 7-chome, Minatoku, Tokyo 106, Japan

R6sumd.- La structure atomique et la dynamique du r6seau d'alliages amorphes Pd-Si-Fe ontr bt6 Studies par spectrometric ~zssbauer. Les tempdratures de Debye ddduites de la fraction d'effet sans recul et de l'effet Doppler du second ordre sont 320 f 20 K et 300 f 20 K respectivement. On n'a observe aucun desaccord avec le modSle de Debye dans les limites de la precision experimentale.

L'effet quadrupolaire ddpend plus fortement de la temperature dans les alliages amorphes (-1,5 x mm/s/k) que dans Pd3Si (Fe) cristallis6.

Abstract.- The atomic structure and the dynamical lattice behavior of amorphous Pd-Si-Fe alloys were investigated by Mijssbauer effect. The Debye temperatures derived from the recoilless fraction and the 2nd-order Doppler shift are 320

+

20 K and 300 f 20 K, respectively. No obvious deviations from the Debye model were observed in the amorphous alloys within the experimental errors. The quadrupole splitting in the amorphous alloys has a larger temperature dependence (-1.5~10-'mm/s/~) than that in the crystalline Pd3Si(Fe).

1.Introduction.- Recent studies for the amorphous alloys have made asignificant contribution to the understanding of their structures. Few of them, however, were concerned with the local arrangement and dynamical lattice behavior of the amorphous Pd- Si alloy.

In this paper we will report the temperature and composition dependence of quadrupole splitting, recoilless fraction and 2nd-order Doppler sh%ft and discuss the validity of Debye approximation for the phonon spectrum in amorphous Pd-Si alloy.

2. Experimental.- Amorphous Pd-Si alloys including 0.3 and 1 at%Fe enriched with 5 7 ~ e were prepared by two piston splat quenching technique. The specimens were thin foils, about 20mm in diameter and 4 0 - 6 0 ~ in thickness. X-ray diffraction measurements of as- quenched specimens containing 15-20 at%Si exhibited only diffuse diffraction patterns corresponding to a single amorphous phase. The Mijssbauer spectra of 5 7 ~ e were measured for the specimens as a function of temperature between 4.2 K and 300 K.

3. Results and discussion.- The Mijssbauer spectra for the amorphous Pd-Si-Fe alloys consist of two distinct sharp absorption peaks with nearly equal intensities. Figure 1 illustrates the mean value of isomer shift and quadrupole splitting for the amorphous Pd-Si alloys and the crystalline Pd3Si containing 0.3 at%Fe. The increase of isomer shift with increasing Si content is consistent with usual alloying effect of metalloid atoms. The isomer shift of crystalline PdsSi(Fe) is,in contrast, significant- ly smaller than that of the amorphous phase.

Fig. 1 : The isomer shift and quadrupole splitting as a function of Si content for amorphous Pd-Si and crystalline Pd3Si alloys containing 0.3 at%Fe.

The quadrupole splitting for the amorphous phase decreases linearly with the concentration of Si, and the values extrapolated to 25 at%Si are nearly equal to thqse of crystalline PdsSi phase. The results show that the local atomic configuration around iron atom changes gradually with Si content and it is close to that of the crystalline Pd3Si phase. A detailed analysis of Si concentration dependence of the spectrum is given elsewhere (Ino H., Nanao, S. and Muto, T., submitted to 3 . Phys. Soc.

Japan).

Figure 2 shows the temperature dependence of quadrupole splitting for the amorphous Pd-2OSi-O.3Fe and Pd-20Si-1Fe alloys. These values were derived by

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979237

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the best fitting with the assumption that the spectra consist of two Lorentzian curves. For Pd-20Si- 0.3Fe alloy, the half widths of the peaks were 0.35-0.37 mm/s and the X2 values were 1.1-1.4.

I ^I I

0 50 100 150 200 250

TEMPERATURE ( K)

Fig. 2 : The temperature dependence of quadrupole splitting for the amorphous Pd-Si-Fe alloys.

For Pd-2OSi-1Fe alloy, the half widths were 0.41- 0.46 mm/s and the X2 values were 1.2-1.5. Systematic deviations of the experimental data from the assu- med two Lorentzian fitting were observed, which show that the atomic configuration of Fe inPd-2OSi-1Fe alloy is presumably more complicated than that of Pd-2OSi-O.3Fe alloy. The difference in quadrupole splitting between two specimens is smaller with the visual estimation than the difference due to nume- rical analysis shown in figure 2. A remarkable increase of quadrupole splitting with increasing Fe content has been observed in higher concentration range of Fe / I / .

The quadrupole splitting decreases linearly with increasing temperature at the rate of 1.5 x

~ / s / K . The temperature dependence of quadrupole splitting is larger than in the case of crystalline PdaSi (see figure 1). The origin of the large temperature dependence in the amorphous phase is not clear. The effect of thermal expansion and vibration will cause the decrease of electric field gradient. The contribution of the change in the electronic density of virtual bound state at Fermi level /2/ may be possible.

Figure 3 shows the normalized areas of Mzss- bauer absorption line as a function of temperature for the amorphous Pd-Si-Fe alloys, which are linear with t?he recoilless fraction in a thin foil approximation. There are also illustrated two curves based on the Debye model with the Debye temperature 300K and 350K. The absorption areas and the recoilless fractions are normalized by the data at 77K.

The variation with temperature for both specimens can be fitted by the Debye model within the experi-

mental errors. The estimated Debye temperature is 320+20K.

Fig. 3 : The normalized absorption areas as a function of temperature for the amorphous Pd-Si-Fe alloys.

9

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The value of the datum at 4.2K seems to be lower than the fitting curve, but under the present situa- tion we cannot discuss further on this point.

In figure 4, the variations of the 2nd-order Doppler shift with temperature are analyzed for Pd-20Si-0.3Fe and Pd-20Si-1Fe alloys. Using the fol-

lowing approximation in high temperature region :

--1.

0 79.7Pd-20 Si-0.3Fe

1. 79Pd-2051-1Fe

1 L.

. 0 -

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.

-

DEBYE TEMP.

-

350 K - ---300 K

I I I I I

the Debye temperatures are estimated to be 300f20K for Pd-20Si-0.3Fe alloy and 330f20K for Pd-2OSi-1Fe alloy.

0 50 100 150 200 250

TEMPERATURE ( K )

Fig. 4 : The vaifation of the 2nd-order Doppler shift with temperature for the amorphous Pd-Si-Fe alloys.

The variation of chemical shift with temperature was neglected here. We can also estimate the Debye temperature from the datum at 4.2K to be 300f20K for Pd-20Si-0.3Fe alloy, using the next relation in low temperature limit

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JOURNAL DE PHYSIQUE

The MGssbauer recoilless fraction and the 2nd- order Doppler shift are related to the mean square amplitude and velocity of the MGssbauer nuclei, respectively. Therefore, the Debye temperatures given by the recoilless fraction and the 2nd-order Doppler shift are different from each other in averaging phonon mode / 3 , 4 / . In our results, the values of Debye temperature derived from both physical quanti- ties are nearly the same, indicating that the Debyc approximation is valid in the amorphous Pd-Si-Fe alloy.

The Debye temperatures obtained from the measurements of specific heat at low temperature (Mizu- tani, U., Hartwig, K.T., Hopper, R.W. and Massalski, submitted to Phys. Rev. Lett.) and the elastic modu- lus / 5 / were reported as 236K and 250K, respectively. The values are lower than those of present expe- riment.

Acknowledgements.- The authors express their sincere thanks to Pr. S. Nishikawa for his discussion and

References

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encouragement and to Mr.Sassa for his advice in data