Electronic structure of Ni-Cu alloys: The d -electron charge distribution H. H. Hsieh, Y. K. Chang, W. F. Pong,* J. Y. Pieh, ² and P. K. Tseng Department of Physics, Tamkang University, Tamsui, Taiwan T. K. Sham, I. Coulthard, and S. J. Naftel Department of Chemistry, The University of Western Ontario, London N6A 5B7, Canada J. F. Lee, S. C. Chung, and K. L. Tsang Synchrotron Radiation Research Center, Hsinchu Science-based Industrial Park, Taiwan Received 20 January 1998 This work investigates charge redistribution in a series of Ni-Cu alloys using x-ray photoemission spectros- copy XPS and Ni/Cu L 3,2 - and K -edge x-ray-absorption near-edge structure XANES. XPS results show that the constituent d bands are well separated and shifted to a slightly higher binding energy upon dilution into the other host, indicating that the atomic sites in the alloy are not as well screened relative to the pure metal. However, no significant d -band narrowing is observed, suggesting that there is modest d - d interaction in the alloys. In contrast to the XPS observation, XANES results show a reduction in white-line intensity at both edges relative to the pure metal suggesting that both Ni and Cu sites gain d charge. The unoccupied Ni d band is far from fully occupied even at infinite dilution. The discrepancy between the implications of the XPS and XANES results is dealt with using a charge redistribution model in which s - p - d rehybridization takes place at both sites within the framework of electroneutrality and electronegativity considerations. It appears that both Ni and Cu gain a small but measurable amount of d charge in alloy formation through rehybridization loss of non-d conduction charge. Possible connection between these results and the disappearance of ferromagnetism in Ni 1-x Cu x alloys at x 0.6 is discussed. S0163-18299804024-7 I. INTRODUCTION The electronic structure of binary nd -transition-metal n =3, 4, 5 solid-solution alloys has traditionally received ex- tensive attention. 1 More recently, particular emphasis has been placed on the evaluation of the nature of the alloy d band and d -charge redistribution at the constituent sites. 2–8 The energy distribution of the 3 d electrons in Ni-Cu alloys is a classical example of alloying and has been described by the so-called rigid-band RB model. 9 In this model, the conven- tional wisdom has been that Ni and Cu form a fcc alloy in which the Ni d band becomes progressively filled with in- creasing Cu concentration. Ni and Cu are both fcc metals with atomic electronic configurations of 4 s 1 3 d 9 and 4 s 1 3 d 10 , respectively, in the solid state and are traditionally described as d 9 and d 10 metals. In Cu the d band is nomi- nally full. The RB model assumes that the one-electron den- sity of state wave function is the same for Ni, Cu, and Ni-Cu alloys; that is, that the constituents donate their valence elec- trons to a common band structure. This model predicts that the Ni d band is completely filled at a Cu composition of 60%. This finding correlates with the experimental observa- tion of the disappearance of ferromagnetism at this composition. 10 However, investigations of a series of Ni-Cu alloys at the Ni and Cu L 3,2 edge using x-ray-absorption near-edge structure XANES by Cordts, Pease, and Aza ´ roff 11 and Meitzner, Fischer, and Sinfelt 12 show that the RB model fails in predicting the d -charge counts. Their re- sults indicate that there is no significant decrease in the num- ber of d holes as Cu concentration in these alloys increases. On the other hand, photoemission spectroscopy 13,14 and the- oretical calculations 15,16 showed that while the binding en- ergy of the Cu d band remains unchanged, the densities of states of the Ni 3 d band are modified significantly in both width and line shape due to rehybridization. Unfortunately, owing to the limitations in the energy resolution in these earlier measurements, results of Cordts, Pease, and Aza ´ roff and Meitzner, Fischer, and Sinfelt were not conclusive. Fur- thermore, a resonant photoemission study 17 showed that electron transfer occurs from Ni 3 d states to Cu 4 s states in contrast to the direction proposed by the RB model. It should be noted that in metals the charge count at the atomic site Wigner-Seitz volume tends to remain neutral. Thus only a small fraction of net charge transfer will occur upon alloy- ing. The cohesion is accomplished by d - d and itinerant sp type conduction electron interaction modified by the elec- tronegativity difference of the constituent metals. Granted that electroneutrality is the rule, charge redistribution can still occur via hybridization without any significant net charge flow on and off a site. For example, in Au-transition and metalloid binary alloys, 2 Au loses d charge and over- compensates its loss by gaining non-d ( sp ) charge so that Au, the most electronegative element, gains a small net charge typically, 0.1e , in accord with electroneutrality considerations. In this paper we report results from two sets of experi- ments for a series of Ni-Cu alloys with the following objec- tives in mind: a To reexamine the validity of the RB model with high-resolution results and b to investigate the system- atic of d - d interaction and charge redistribution in binary transition-metal alloys in Cu and Ni with nominally full and nearly full d bands, respectively. We have carried out high- PHYSICAL REVIEW B 15 JUNE 1998-II VOLUME 57, NUMBER 24 57 0163-1829/98/5724/152047/$15.00 15 204 © 1998 The American Physical Society