L-edge Xanes Studies of Charge Redistribution upon Compound Formation in Gold and Nickel Metallic Compounds

L-edge XANES of a series of gold (AuA12, AuGa and Au2Ag) and nickel (NiSb, Ni Fe NiA1, and NiAl ) intermetallic compounds have been measured. !?t6Ys f86%gYthat the LI1 I : edges of AuA12 and AuGa2 exhibit very intense whiteline indicating a sign$flcant depletion of d character at the Au site of these compounds. This observation is interpreted in terms of a mechanism in which s-d interplay determines the overall charge redistribution. In the case of Ni compounds, the effect of s-d interplay is found to be less dramatic. Introduction Charge redistribution upon alloying and formation of intermetallic compounds is of particular interest in the studies of the electronic structure of intermetallic systems containing noble metals [ I ] . Two questions often arise: one concerns the behavior of the d band, the other concerns the nature of the actual electron redistribution (charge transfer or rehybridization) and its correlation with the difference in the electronegativity of the constituents. Here we report a L edge XANES study of a series of gold (AuA12, AuGa and Au2Ag) and nickel (NiSb, Ni Fe NiAl and NiAl ) intermetalllcs. ~ i i s study 3 is an extention of our rece8t62tu9f38; pd L edges in Pd intermetallics [z]. Since LIII II XANES arise primarily from p to d edge XANES (part~cularly the whiteline features) of compounds is a direct measure of the density of the unoccupied d states above the Fermi level. Experimental The intermetallics investigated in this study were prepared with standard procedures. Au Ag was rolled to thin foils and AuAl and AuGa were in the form 2 of 'fine powders? All the nickel compounds were in tfe form of ingots except Nio.6UFeo.26 (Zcc structure, y phase) which was a foil. L edge absorption measuremen s o the Au compounds were made in a transmission mode at the C2 station of the Cornell High Energy Synchrotron Source (CHESS) using a channel-cut Si(220) monochromator. The measurements of the Ni series were carried out in a total-electronyield mode at the U15 beamline of the National Synchrotron Light Source (NSLS). U15 is equipped with a Toroidal Grating Monochromator (TGM) that covers an extended range of soft x-ray energies (200 to >1000eV). The energy resolution was a moderate QeV at the Au L-edge and was %6eV at the Ni L edge. The Ni samples were Ar ion etched in a vacuum chamber prior to measurement. The specimen current was measured as the total electron yield [3] . Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868215 C8-1098 JOURNAL DE PHYSIQUE Results and Discussion The Au L edge XANES of AuAl,, AuGa, and Au,Ag are shown in Fig. 1 where the L L spectra of Au Ag are very similar to th6se of pure Au except for a small increase in the absorp$ion coefficient just above the threshold (within %20eV). The spectra of AuA12 and AuGa2 clearly exhibit whiteline features. PHOTON ENERGY ( e V PHOTON ENERGY ( e V ) Fig. 1. Au LIII, LII edge XANES of AuA12, AuGa and Au2Ag. 2 The Ni LIII,II edge XANES of the Ni compounds are shown in fig. 2 together with a theoretical spectrum of Miiller et a1 [4]. The LIII, LII whitelines are separable despite the poor resolution. The theoretical spectrum has been broadened with the core hole width [4]. THEORY (Mullor el 011 I , , , , , 20 0 20 40 60 60 Fig. 2, L-edge XANES spectra (raw data) of NiSb, Ni, Ni 0.64 Fe0.36s NiAl and NiA13 in the order of increasing electronegativity difference (from top to bottom) and the theoretical spectrum of Ni by Miiller et al. Let us first discuss the Au results. From fig. 1 the whitelines in the AuAl and AuGa2 spectra are clearly observed while only a small increase in the 2 absorption coefficient is seen in the Au Ag XANES relative to pure Au. This 2 observation immediately indicates the presence of high density of unoccupied d states above the Fermi level in AuA12 and AuGa . The implication is that the d states at the Au site which are full in elemengal Au are depleted upon alloying with electropositive elements. This is in contrast to Paulings electronegativity considerations since Au is the most electronegative metallic element and charge is expected to flow onto the Au site upon compound formation. The electronegativity (E.N.) values of elements of interest are listed in table 1. Table 1. Paulings Electronegativity Values of Elements of Interest Element A1 Ga Fe Ni Ag Sb Pd Au E.N. values 1.61 1.81 1.83 1.91 1.93 2.05 2.20 2.54 The same situation was encountered in the Pd L edge study [2] and the photoemission and Mdssbauer studies of Au alloys [ 5 ] . These observations can be interpreted in terms of a charge compensation (rehybridization) model described earlier [5] in which d charge depletion at the Au site is overcompensated by s-p (conduction) charge gain so that the direction of the net charge flow (of the order of 0.1 electron) is in agreement with Paulings electronegativity. The order of the change of d counts at the Au site, An (a negative number in all Au metallic compounds), was previously estimated [5] to be I Andl (AuA12) > ]Andl (AuGa2) > /Andl (Au2Ag). This trend is confirmed by the relative intensity of the whitelines shown in fig. 1. The area under the whiteline is known to be a good measure of unoccupied d states [6,7,8]. We now turn to the Ni results. It is apparent from fig. 2 that alloying has a reverse but less significant effect on the L III.II edge of Ni compared to that -,-of Pd and Au (partly because of the poor resolution). Some crude parameters are listed in table 2. A qualitative trend is nevertheless obtained from these results. Table 2. Parameters for the LIII whiteline of fig. 2. Sample Ni NiSb NiO. 64Fe0. 36 NiAl NiA13 $ WIDTH^ (20.2e~) 4.5 4.4 4.9 6.1 5.9 b Height (20.05) 1.0 0.89 0.84 0.88 0.87 a. halfwidth of the rising LIII edge whiteline profile b. relative to Ni (the height is normalized to absorption at 40eV) C8-1100 JOURNAL DE PHYSIQUE What emerges from table 2 is that the L edge changes slightly with an apparent decrease in the white line intensity in the compounds, the height of the whiteline drops in all compounds but substantial broadening is observed in the case of NiAl and NiA13 indicating a spreading out of the d character in these compounds. Further analysis using the smooth Cu LIII,II edge as a step function shows that the area under the Ni L,, white line in all the compounds is indeed 111 decreased indicating a gain of d character at the Ni site upon compound formation. This behavior is in contrast to that of Pd and Au compounds. Summary It is found that the L 111,11 edges of AuA12 and AuGa2 exhibit intense whitelines and that a small increase in intensity is observed at the L II1,II edges of the Au Ag, indicating a depletion of d character at the Au site upon 2 compound formation. This observation is interpreted in terms of a mechanism in which s-d interplay (rehybridization) determines the overall charge redistribution (a gain of s charge is accompanied by a loss of of d charge). In the case of Ni compounds, the effect of s-d interplay is found to be less dramatic. AcknowledgementI am in debted to W. Kunmann for preparing the samples, J. Kirz for thebeamtime at U15, NSLS and 5.X. Yang for technical assistance. X-ray measurementswere made at CHESS which is supported by US NSF. Research carried out atBrookhaven National Laboratory under constract DE-AC02-76CH00016 with the U.S.Department of Energy and supported by its Division of Chemical Sciences, Office ofBasic Energy Sciences. References1. "Theory of Alloy Phase Formation" edited by L.H. Bennett (AIME, Warrendale,Penn 1980).2. T.K. Sham. Phys. Rev. B.2,1888 (1985); 31, 1903 (1985).3. T.K. Sham and R.G. Carr. J. Chem. Phys. 83, 5914 (1985).4. J.E. MUller, 0. Jepsen and J.W. Wilkins. Solid State Comm. 2, 365 (1982).5. B.E. Watson, 3. Hudis and M.L. Perlman. Phys. Rev. 5. 6, 4139 (1971). T.K.Sham, R.E. Watson and M.L. Perlman. Phys. Rev. B2, 539 (1979).6. F.W. Lytle, P.S. Wei, R.B. Greegor, G.H. Via and J.H. Sinfelt J. Chem. Phys.70, 4849 (1979).-7. M. Brown, R.E. Peierls, and E.A. Stern Phys. Rev. 5 15,738 (1977).8. L.F. Mattheisiss and R.E. Dietz, Phys. Rev. B 22, 1663 (1980).