Electronic and atomic structure of AlxLa70−xNi30 amorphous alloys

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Journal of Non-Crystalhne Sohds 156-158 (1993)302-306 North-Holland

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Electronic and atomic structure o f A1 x La 70-x Ni 3o a m o r p h o u s alloys Itsuro Y a m a m o t o a, John Van Zytveld b and Hirohisa E n d o ~ a Department of Physics, Faculty of Science, Kyoto Umverstty, Kyoto 606-01, Japan b Department of Physics, Calwn College, Grand Rapids, MI 49506, USA

X-ray photoemlsslon spectroscopy (XPS) spectra, electrical resistivity, optical reflectwlty and extended X-ray absorption fine structure (EXAFS) of AlxLa70_xN130 (0 < x < 40) amorphous alloys have been measured The XPS spectra show that the peak of the NI 3d-band lies at a higher binding energy and has a narrower width than that of pure Nl, as a result of the hybridization of the NI 3d-band with La and Al bands The density of states at the Fermi level, N(EF), decreases as the A1 content Increases. The decrease of N(E v) is consistent with the results of the electrical resistivity. From the reflectlvtty spectra and EXAFS data, it was found that above x = 30 at.% the hybridization of Ni and Al becomes dominant rather than that of NI and La, implying that the interaction between Nl and AI is comparatively strong. These results have been discussed in relation to chemical short-range order

1. Introduction Inoue et al. [1] recently found that, for A1-LaNi systems, amorphous alloys are formed over a wide compositional range; they have a wide supercooled liquid region and a high glass transition temperature. It is also known that some A1-La-Ni amorphous alloys have a high strength and good ductility. Their practical use as new materials is expected to be developed in the near future. Studies from the microscopic point of view give helpful information on understanding the physical and chemical properties of these interesting amorphous alloys. In order to elucidate how the thermal and mechanical stability are related to the electronic and atomic structure, we prepared for this study four AlxLa70_xNi30(0 < x < 40) amorphous alloys by melt spinning; we measured X-ray photoemission spectroscopy (XPS) spectra, electrical resistivity, optical reflectivity and extended X-ray absorption fine structure (EXAFS).

Correspondence to Dr I Yamamoto, Department of Physics, Faculty of Science, Kyoto Umverslty, Kyoto 606-01, Japan Tel' + 81-75 753 3752. Telefax: + 81-75 753 3780.

2. Experimental From alloy ingots prepared by induction melting, the amorphous alloy ribbons with a typical size of 0.03 × 10 mm 2 were prepared by a singleroller melt-spinning technique in an argon atmosphere. The amorphous nature of the melt-spun ribbons was confirmed by X-ray diffraction. In fig. 1, the compositions of A1-La-Ni amorphous alloys used for our experiments are denoted by the open circles, together with the contour lines of crystallization temperature [1]. X-ray photoemission valence band spectra were recorded on a Shimadzu ESCA-850 spectrometer, using Mg K a radiation (1253 eV). The sample surfaces were cleaned by argon-ion sputtering in a high-vacuum preparation chamber. The optical reflectivity measurements were carried out on a modified Shimadzu UV365 spectrometer in the energy range 0.5-5.7 eV. The amorphous alloy ribbons were polished in air with alumina powder with a grain size as small as 0.3 tim. EXAFS measurements were performed with a spectrometer installed at BL-10B of Photon Factory in the National Laboratory for High Energy Physics. X-ray absorption spectra near the Ni

0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B V All rights reserved

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K-edge were measured at 80 K. Further details of the experimental procedure and data analysis were described in a previous paper [2].

narrower than that of pure N1. The XPS analysis for Ni alloys with an electropositive metal such as AI or La has revealed that the modification of the Ni 3d-band by alloying originates from the hybridization of the partner element bands [4]. The shift of the d-band centroid, the peak narrowing and the decrease of the density of states at E F, N(EF), observed for the amorphous alloy with x = 10 result from the hybridization of the Ni d-band with La and AI bands; the contribution of the La 5d-band which lies across E F is not so large because the photoelectron cross-section of the La 5d-electrons is comparatively small. As La atoms begin to be replaced by A1 atoms, the peak just below E v is reduced somewhat in height, and N ( E v) also decreases.

3.2. Electrical reststwity The electrical resistivity, p, was measured by a standard dc four-probe technique. The temperature variation of p for the AlxLaT0_,Ni30 amorphous alloys is plotted in fig. 3. p increases gradually with increasing A1 content. At x = 40, the

13. Results

3.1. XPS valence band spectra The XPS valence band spectra for the AI xLa70_xNi30 amorphous alloys are shown in fig. 2. The Fermi energy, EF, was determined with Au as the position where the 6s-band is down to half its height. The intensities of the spectra are normalized by the Ni 2pl/2 core level peaks, which do not change in position or in shape with concentration. The split peaks of the La 5P3/2j/2 core levels lie at 16.8 and 18.7 eV, and the peak intensities decrease with decreasing La concentration. The spectra are dominated by the peak centered around 1.5 eV below EF, which mainly arises from the Ni 3d-band. The 3d-band of pure Ni determined by XPS measurements has a maximum at 0.6 eV below E F [3]. The peak observed for the amorphous alloy containing 10 at.% A1 lies at a higher binding energy and has a width

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value of p at room t e m p e r a t u r e is 370 i~fl cm. The t e m p e r a t u r e coefficient of p is small and negative. These characteristic behaviors are consistent with Mooij's empirical rule [5] in disordered alloy systems. Since the XPS data show that N ( E v) is small, and the values of p are rather large, the following model [6] may be applicable for analyzing the electronic properties. The electrical conductivity, o-, is given as

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3.3. Opttcal reflecttvity Figure 5(a) shows the reflectivity spectra for the Al~LaT0_~Ni30 amorphous alloys. A steep rise of the reflectivity due to the Drude term is seen in the low-energy range ( E < 1.6 eV), and 10 (.o) O8 >,,

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where a is the atomic distance and J is the interatomic transfer integral. In these amorphous alloys, the mean free path of electrons is comparable with the atomic distance. In fig. 4, N(EF) taken from XPS data is plotted versus the square root of the electrical conductivity, ~r ~/2. As the model predicts, N ( E F) is almost proportional to cr 1/2. The extrapolation of the straight line tends to zero as indicated by the dashed line. The chemical short-range order formed by the strong hybridization of Ni and La, or Ni and AI, causes the deep minimum of N(EF). This is consistent with the observation by Mizutani et al. [7] of a reduced N ( E F) for these amorphous alloys, where they measured the electronic specific heat.

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305

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the reflectivity decreases gradually with further increasing photon energy. Since the energy range of the measurements was restricted to 0.5-5.7 eV, the reflectivity spectra were analyzed by using an oscillator fitting method [8] instead of a Kramers-Kr6nig analysis. As shown in fig. 5(b), the resulting optical conductivity, o-(w), has two broad humps around 2.3 and 4.2 eV for the amorphous alloy with x = 10 at.% A1. The observed humps are discussed in terms of an interband transition. It should be noted that in the amorphous state the interband transition reflects the density of the initial and final states, because the k-conservation rule fails. It is likely that the humps in tr(to) in the amorphous alloys with x = 10 and 20 arise mainly from the hybridization of Ni and La bands; i.e., the transition from E F to the anti-bonding states of the Ni 3d-band (try,) causes the hump around 2.3 eV and the transition from the Ni 3d-band to or* causes the hump around 4.2 eV. The small shifts of the humps to higher energy with increasing A1 content may come from the increase of the hybridization. The EXAFS data show that, under the same conditions, the distance between La and Ni decreases, as is discussed below. For the amorphous alloys with x -- 30 and 40 which contain nearly the same content of Ni and La, both humps become more prominent. In this concentration range, the transition may depend on the hybridization with AI, rather than La, which implies that the interaction between Ni and A1 atoms is comparatively strong. For the amorphous alloy with x = 40, the transitions from E F to try, and from the Ni 3d-band to cry, cause the humps around 2.5 and 5.2 eV, respectively.

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content, and tends to a constant value of 2.73 in the range of higher A1 content, while the Ni-AI bond length is about 2.46 A and is independent of the A1 concentration. For A1-Ni stoichiometric compounds such as AI3Ni, A13Niz, A1Ni and AINi3, it is known that the Ni-A1 bond length has almost the same values, ranging from o 2.46 to 2.52 A, despite the different crystalline structure [10,11]. The Ni-AI bond length for the A1-La-Ni amorphous alloys is close to those values. The shortening of the Ni-La bond length may give an indication that the local structure changes to a more close-packed configuration with increasing AI content.

3.4. E X A F S

From the Ni K-edge EXAFS measurements for the AlxLa70_xNi30 amorphous alloys, we obtained the radial distribution function of La and A1 around the central Ni atom. Figure 6(a) shows Ni-La and Ni-AI bond lengths as a function of A1 concentration. At x = 0 at.% AI, the Ni-La bond length for the LavNi 3 stoichiometric compound is also plotted in fig. 6 [9]. The Ni-La bond length becomes shorter with increasing A1

4. Discussion

Le et al. [12] have recently discussed the stability of transition-metal-based amorphous alloys, based on the minimization of the electronic energy. They found that chemical short-range order (CSRO) is essential, and occurs when E v coincides with a minimum of the density of states. Long-range order is not expected unless the full

306

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lattice energy is also minimal, because this would produce a crystalline phase. We assume that the crystallization temperature, Tx, can be taken as a measure of the stability for these amorphous alloys. As shown in fig. 1, Tx increases from 460 K to 660 K with increasing Al content. Our results for XPS spectra indicate that N ( E v) also decreases with increasing Al content. This suggests that C S R O is important in A l - L a - N i amorphous alloys. In the limit of x = 0, the AlxLa70_xNi30 amorphous alloys correspond to the stoichiometric compound La7Ni 3. LaTNi 3 has the Th7Fe3-type crystalline structure (hexagonal, C4,, ( P 6 3 m c ) ) , and is composed of trigonal pyramid units [9]. Each trigonal pyramid has four La atoms around a Ni atom. From our E X A F S analysis, at x = 10 the sum of Al and La coordination numbers around a Ni atom is about four (fig. 6(b)). This may imply that, at x = 10 and probably x = 20, the local structure is composed of the trigonal pyramid-like configuration. In the other extreme, the AlxLa70_xNi30 amorphous alloys contain no La atom at x = 70. There exists the Al3Ni stoichiometric compound near this concentration. Al3Ni is orthorhombic (D~ 6 ( P n m a ) ) [10]. The characteristic feature of this structure is the presence of distorted hexagonal rings consisting of alternate Al and Ni atoms. In these rings, the AI coordination number around a Ni atom is three. Also, at x = 40 our data show that the sum of the Al and La coordination numbers around a Ni atom is about three. These results encourage us to believe that the local structure changes from the trigonal pyramid-like configuration to the distorted hexagonal ring above x = 30.

5. Conclusions We have measured XPS spectra, electrical resistivity, reflectivity and E X A F S for the AlxLaT0_xNi30 amorphous alloys. We found that the hybridization of Ni and A1 becomes dominant

rather than that of Ni and La above x --- 30 at.%. E X A F S data suggest that the local structure transforms from the La7Ni3-1ike configuration to the Al3Ni-like configuration, which is accompanied by the change of the electronic states. The authors would like to thank Professor A. Inoue for his valuable comments, and are grateful to Drs. K. Maruyama and M. Yao, and Messers S. Kawakita, Y. O m a s a and T. Tsuzuki for measurements and analysis of EXAFS, Dr. M. Ozawa of Kawasaki Steel Corporation for sample preparation, and Drs. H. Okashita, M. Tanabe and Yoshimi of Shimadzu Corporation for measuring XPS spectra. J.V.Z. acknowledges the kind hospitality of Professor Endo's group at Kyoto University and the support of the US National Science Foundation (Grant No. I N T 90-22624) while a portion of this work was completed.

References [1] A. Inoue, T. Zhang and T Masumoto, Mater. Trans Jpn Inst Met 30 (1989)965. [2] M Inui, M. Yao and H Endo, J Phys. Soc. Jpn. 57 (1988) 553. [3] See, for example, N. Martensson and B. Johansson, Phys Rev. Lett. 45 (1980) 482 and refs. thereto [4] J.C Fuggle, F.U Hdlebrecht, R Zeller, Z Zolnierek, P.A Bennett and Ch Frelburg, Phys. Rev B27 (1982) 2145. [5] J H Moou, Phys Status. Soh& (a)17 (1973) 521 [6] N F. Mon and E A. Davis, Electromc Processes m NonCrystalhne Materials, 2nd Ed. (Clarendon, Oxford, 1971) p 181 [7] U. Mlzutam, S. Ohashl, T Matsuda, K Fukamlchl and K. Tanaka, J Phys. Condens. Matter 2 (1990) 541 [8] H P Seyer, K. Tamura, H Hoshlno, H. Endo and F. Hensel, Ber Bunsenges. Phys Chem. 90 (1986) 587. [9] P Fischer, W Hfilg, L Schlapbach and K. Yvon, J Less-Common Met 60 (1978) 1 [10] A.J. Bradley and A Taylor, Phdos. Mag 23 (1937) 1049 [11] W.B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys (Pergamon, London, 1958) p 377 [12] D H Le, C. Cohnet and A Pasturel, Phllos Mag. B63 (1991) 1299