VOLUME 57, NUMBER 6 P H Y S I C A L R E V I E W L E T T E R S 11 AUGUST 1986

Gibbsian Surface Segregation in Cu-Ni Alloys

In a recent Letter, Sakurai et al.1 reported results from an atom-probe study of Gibbsian surface segrega­tion in Cu-Ni alloys. They observed strong Cu enrich­ment on the surface at elevated temperatures for several Ni-rich specimens, in agreement with a large number of previous investigations.2 However, their results indicated that Ni segregated to the surface when the bulk composition fell below 16 at.% Ni. The authors pointed out that this first observation of a crossover in the segregating species disagrees with all existing theories of Gibbsian adsorption in Cu-Ni al­loys, including advanced microscopic approaches,3,4

and concluded that no satisfactory explanation exists for the unexpected Ni enrichment. In this Comment, we show that no crossover in the segregating species occurs in Cu-Ni alloys cleaned by intermittent sputter­ing at elevated temperatures.

We employed ion-scattering spectroscopy (ISS) to measure Gibbsian segregation in two alloys, Cu-60 at.% Ni and Cu-10 at.% Ni. Detailed descriptions of experimental procedures can be found elsewhere.5,6

Because low-energy ions are quickly neutralized in the bulk, the ISS signals come almost exclusively from the outermost surface layer, where Gibbsian segregation dominates. Ratios of the Cu to Ni ISS intensities mea­sured at several different temperatures in the two al­loys are shown in Fig. 1. The bulk ISS ratios were determined to be 0.7 and 10 for the 60- and 10-at.%-Ni specimens, respectively. Clearly, Gibbsian adsorption of Cu occurs in both alloys. No crossover is found as the bulk Ni concentration drops from 60 to 10 at.%. Furthermore, the enthalpies and entropies of adsorp­tion extracted from the two sets of measurements are, as expected from modern microscopic theories, identi­cal within experimental accuracy, —0.42 eV and — 2.60/c, respectively.

We offer a possible explanation for the crossover observed in the atom-probe work. Our experience shows that intermittent sputtering during long anneal­ing periods at elevated temperatures ( > 600 °C) is needed to purge the specimen of unwanted impurities such as S and O, which can induce surface composi­tional changes that overwhelm the Gibbsian segrega­tion effect. Both S and O have been reported to enhance the near-surface Ni concentration during an­nealing of Cu-Ni alloys at elevated temperatures.7 The low-temperature, field-evaporation cleaning procedure employed in the atom-probe work cannot remove these species from the bulk of the specimen. Contam­inants in the bulk can migrate to the surface during the subsequent, elevated-temperature annealing treat­ment, and dominate the observed segregation behavior. This interpretation for the crossover effect explains the apparent conflict of the atom-probe

IOOO

T (°C)

800 700 500 200 h

g lOOh < en

1.0 T"1 ( I03 K"1)

FIG. 1. Temperature dependence of the Cu/Ni ISS inten­sity ratio measured on 60- (lower line) and 10-at.% (upper line) Ni specimens.

results with our measurements (Fig. 1), and with the theoretical predictions. Additionally, it leaves intact the important further observation by Sakurai et al. of a strong similarity between their experimental findings, and measurements by Sinfelt, Carter, and Yates8 of catalytic activity of Cu-Ni alloys. Because the ca­talyzed reaction occurs in an environment that con­tains S and O, the atom-probe measurements of sur­face composition by Sakurai et al. may be more per­tinent for comparison with catalytic activity than the compositional changes resulting from Gibbsian segre­gation under ultraclean conditions.

This work was supported by the U. S. Department of Energy, Basic Energy Sciences, Materials Sciences, under Contract No. W-31-109-Eng-38.

L. E. Rehn, H. A. Hoff, and N. Q. Lam Materials Science and Technology Division Argonne National Laboratory Argonne, Illinois 60439

Received 22 January 1986 PACS numbers: 68.35.Fx, 61.16.Fk

lT. Sakurai, T. Hashizume, A. Jimbo, A. Sakai, and S. Hyado, Phys. Rev. Lett. 55, 514 (1985).

2P. Wynblatt and R. C. Ku, Surf. Sci. 65, 511 (1977). 3S. Mukherjee, J. L. Moran-Lopez, V. Kumar, and K. H.

Bennemann, Phys. Rev. B 25, 730 (1982). 4V. Kumar, Phys. Rev. B 23, 3756 (1981). 5L. E. Rehn, S. Danyluk, and H. Wiedersich, Phys. Rev.

Lett. 43, 1764 (1979). 6N. Q. Lam, H. A. Hoff, H. Wiedersich, and L. E. Rehn,

Surf. Sci. 149, 517 (1985). 7Y. Takasu and H. Shimizu, J. Catal. 29, 479 (1973);

T. Inui and R. Shimizu, Technol. Rep. Osaka Univ. 29, 373 (1979).

8J. H. Sinfelt, J. L. Carter, and D. J. C. Yates, J. Catal. 24, 280(1972).

780 © 1986 The American Physical Society