Surface Science 66 (1977) 1 - 13 0 North-Holland abashing Company

SURFACE SEGREGATION IN ALLOYS: DILUTE SOLID SOLUTIONS OF Cr,

Fe AND Ni IN Pt

J.J. BURTON and R.S. POLIZZOTTI Corporate Research Laboratories, Exxon Research and Engineering Company, Linden, New Jersey 07036, USA

Received 6 August 1976; manuscript received in final form 6 April 1977

The equilibrium surface compositions of dilute solid solutions of -2 at% Cr, Fe, and Ni in Pt were measured using Auger electron spectroscopy. No surface segregation was found in any of these systems. This result casts serious doubts on the validity of the widely used simple theories of surface segregation, based on heats of sublimation or size differences. The results are compared with the predictions of more sophisticated theoretical models.

1. Introduction

Gibbs [ 1] predicted a century ago that the equi~ibri~ composition of an alloy surface is not necessarily identical to its bulk composition; that is, one component of the alloy segregates to its surface. This phenomenon, known as surface segrega- tion, has considerable importance in metallurgy [2] and catalysis [3], and has at- tracted recent interest [4].

Rigorously, the excess concentration of one component on the surface over its bulk concentration, I’, is related to the composition dependence of the surface tension da/da by [S]

I’ = -(a/RT) (do/da) . (1)

This means that surface segregation of one component to the surface should occur if the surface tension decreases with increasing concentration of that component. The problem is applying Gibbs’ rigorous result is that generally very little is known about the composition dependence of the surface tension, do/da. Therefore, a number of alternative simple approaches to prediction of surface segregation have been attempted [3,6-91. For very dilute binary alloys the surface concentration of the solute, Xs has been related to its bulk concentration, Xn, by

.%/(I -xs)= ixBf@ -XB)I exp(~/R~, (2)

where Q is the heat of segregation. If Q is large and positive, than segregation should occur. Two simple approaches to estimating Q were suggested. One is based

1

2 J.J. Burton, R.S. Folizzotti f Surface segregrttion in alloys

on bond breaking and predicts that the lower heat of sublimation component of the alloy should segregate to its surface [ ‘7-91; here the driving force for segregation is maximization of the number of strong bonds in the system. The second is based on elastic strain and predicts that segregation should occur whenever the size difference between the constituents is large [6] ; here the driving force for segregation is the lowering of the elastic strain energy in the bulk which arises from lattice mismatch.

To date, most of the studies of surface segregation have attempted to establish merely whether segregation occurs [4]. There has been little success in experiment- ally measuring the heat of segregation on very clean well defined samples primarily because this exper~ent is considerably more difficult than merely establis~ng whether segregation occurs [lo,1 l].

So far the only reliable results for the heat of segregation are for segregation of Au to the (I 11) surface of Ni [ IO,1 I] and segregation of Fe to the surface of pofy crystalline Zr [ 121. There is, thus, as yet insufficient data to critically test the valid- ity of the quantitative predictions of the theoretical models. Comparison of the pre-

dicted heats of segregation which careful experimental results in a number of sim- ple systems is crucial to further testing the adequency of our theories of surface

segregation.

In this paper we report on measurements of the heats of segregation of Cr, Fe, and Ni to the surface of polycrystalline Pt. These alloys were chosen for the three

reasons: (1) We have already estabiished in a study of Au segregation to the surface of Pt [ 131 that the surface composition of polycrystalline Pt alloys is readily equilibrated at temperatures above 600°C. As some studies of alloy surfaces have failed to see segregation because of annealing at too low a temperature, knowledge of requisite

annealing temperature is essential. (2) In our study of Au segregation to the Pt surface f13], we found that the surface is easily cleaned of impurities. We have previously found that impurities can sign

Scantly affect segregation behavior [ 10,l l] . (3) The two current theories of segregation predict significantly different heats of segregation for these systems. The bond breaking theory 27-91 predicts heats of segregation of about 8-10 kcallmole for segregation of Cr, Fe, and Ni to the Pt surface. However, the size difference theory [6] predicts that the heat of segrega- tion of Cr to the Pt surface should be about I .5 kcal/mole while that of Fe and Ni to the Pt surface is predicted to be 4-5 kcal/mole. Therefore, a careful study of these systems may provide discrimination between the rival theories.

The heats of segregation of Cr, Fe, and Ni to the surface of dilute pol~c~stalline Pt alloys were measured and found to be about 0 k&/mole in all cases. NO SdiiCe

segregation was detected in any of these systems. Comparing these results with the predictions of the two currently used simple theoretical models, we conclude that both these theories are inadequate. However, we will show that our results are ade- quately explained by recent, more sophisticated, theoretical models.

J.J. Burton, R.S. Polizzotti /Surface segregation in alloys

2. Experimental methods

All of the samples used in this study were made by vacuum arc melting 99.999 at% Pt with Cr, Fe, or Ni of similar purity. The alloys all contained about 2 at% of the minor constituent (Cr, Fe, Ni) and 98% Pt. After melting, the alloys were spark cut into slices about 15 mil thick, polished, and then annealed in vacuum at 800°C

for several days. Measurements of the surface composition of the alloys were made with a Physical

Electronics cylindrical-mirror single-pass Auger analyzer using a beam current of 35 PA and a beam energy of 2000 eV. All measurements were made at chamber pressures below 2 X lo-r” Torr. The sample was heated using an electron beam heater which enabled Auger measurements at elevated temperatures without any of the magnetic field effects associated with resistive heating. We have shown previ- ously that Auger measurements at temperature are very important in measuring the heat of segregation [lo,1 11.

Considerable care was taken to obtain very clean sample surfaces as we have found that surface impurities can significantly affect segregation behavior [ 10,111. The samples were cleaned by an extended cycle of alternating annealing at very high temperatures (BOO-1200°C) and sputtering. During the cleaning, a variety of impurities, including C, Ca, P, and Sn, were observed to segregate to the sample surface. Eventually our sputter/anneal procedure purged the sample of these im- purities. All data reported in this paper will be from these cleaned samples with no contaminents visible with AES. (We estimate our limit of detection to be about 0.005-0.01 monolayer for most of the common impurities such as C, 0, P, and S.)

As we are interested in obtaining the heat of segregation, which is an equilibrium property, it is necessary to take considerable care that the equilibrium surface com- position is obtained. Previous experiments have indicated that temperatures above 600°C are sufficient to obtain equilibrium segregation of dilute impurities to the surface of Pt [ 131. All of the data reported below will be equilibrium data obtained at temperatures well above 600°C.

3. Equilibrium surface compositions

In fig. 1 we show Auger spectra of our samples at 900°C. It appears that the Auger peaks arising from the Cr, Fe, and Ni are quite small, whereas they are often large in heavily segregated samples [ 10-141.

We can get a rough estimate of the extent of the surface segregation by asking how large the impurity peaks in fig. 1 would be were the sample unsegregated. In pure samples, the Cr(529) eV Auger transition peak is about the same size as the Pt(64) peak [ 141. Therefore, assuming that the peaks in the alloy are like those in the pure metal, we would expect that in a completely unsegregated random alloy of about 2% Cr in Pt, the Cr(529) Auger peak should be only about 2% as large as the

4

EWERGY Wf

ys of Cr, Fe, and Ni in Pt at 900°C. (No and a~oy~ element peaks are labelled in

xcess incantation of the solutes on the

chases in The Auger

T F-C)

. 2. ~qu~b~i~~ Auger peak he~ht ratios as a fo~ction of tcm~rat~~ for alloys of Cr, and Ni. The ~lute/~ Auger p hi ratios shown are, ~i(a48)/~(64), 5 29)~~(64~~

and ~e(651~/~(64~. There is RO e~~de~~ of any tem~xat~ra decoders of the sofas corn- pa&ion.

J.J. Burton, R.S. Polizzotti / Surface segregation in alloys 5

Pt(64) peak. This estimation is, of course, somewhat uncertain. However, our naive

expectation is similar to the data in fig. 1 suggesting that if segregation of Cr does occur to the Pt surface, the extent of the segregation is small. Likewise, if Fe does

not segregate to the Pt surface, we would expect an Auger peak height ratio, Fe- (651)/Pt(64) of about 0.15, which is similar to our result. Finally, if Ni does not segregate, we would expect Ni(848)/Pt(64) to be about 0.15, also much like our ex- perimental result. Therefore, our tentative conclusion must be that Cr, Fe, and Ni

do not segregate to the surface of Pt. In strongly segregating samples, we have found that at equilibrium the relative

Auger peak heights of the solute and matrix atoms depend strongly on temperature [lo-131. Thus, looking for temperature effects is another good test for segrega- tion. In fig. 2 we show an Arrhenius plot of the equilibrium Auger peak height ratios, Cr(529)/Pt(64), Fe(651)/Pt(64), and Ni(848)/Pt(64) as a function of the in- verse temperature. The slope of the straight lines through the data is related to the heat of segregation [ 10,111. Fig. 2 shows that there is no temperature dependence of the equilibrium surface composition in our samples, again suggesting that there is no segregation.

4. Discussion of the data

In the previous section, we have reported that we have failed to find any evidence of segregation of Cr, Fe, or Ni to the surface of polycrystalline Pt. There are two possible interpretations of our results: either segregation really does not occur or it does but we did not perform the experiment correctly. The latter possibility should not be ruled out as a number of other workers’ experimental conclusions have been

invalid because of poor experimental technique. In the remainder of this section we will argue that the results in figs. 1 and 2 re-

present the equilibrium properties of the system. There are two reasons that investigations have failed to see segregation when the

sample should segregate at equilibrium: (a) inadequate annealing of the sample to produce segregation and (b) use of inadequately surface sensitive high energy Auger transitions.

We have a number of reasons for thinking that our samples were adequately an- nealed: (1) We have found that polycrystalline samples similar to ours with a few percent Au in Pt readily segregate and reach equilibrium within an hour at 6OO’C [13]. Therefore, we would expect to be able to obtain segregation in our samples at tem- peratures above 600°C unless Au diffuses abnormally rapidly in Pt, which is not ex- pected. (2) We have found in studies of segregation of 1% Au in single crystal Ni that segre- gation can be observed below l/2 the melting temperatures (750°C in Pt) and that at 2/3 the melting temperature (1125°C) equilibrium was obtained within seconds

6 J.J. Burton, R.S. Polizzotti / Surface segregation in alloys

[ll]. As, in the single crystal, the solute diffuses to the surface via dislocations [15], whereas, in the polycrystalline sample, we expect grain boundary diffusion which is faster than dislocation diffusion to predominate, our samples should have been equilibratable at lower temperatures than expected from our studies of Ni. (3) Knowing that oxygen can draw Fe to the surface of Pt [ 161, we exposed our Fe containing sample to 1 X lo-’ Torr of 02 at 700°C. Fig. 3 shows the time depend- ence of the surface composition during this exposure. Note that the sample reaches

its final equilibrium value within an hour, suggesting that the relaxation time of the sample surface at 700°C is of the order of an hour. (4) We found that when the sample was maintained at 1000°C for as long as 20 h, no charge in the surface composition occurred.

We believe that the Auger transitions used were adequately surface sensitive to detect large changes in the surface concentration for the following reasons: (1) We were able to see Fe being brought to the surface of 02, fig. 3. (2) Fain has found that very thin overlayers of Au on Ag can be seen even with the Au (2024 eV) Auger transition [ 171. (3) Helms has found that the effects of surface segregation of Cu to the surface of Ni can be seen even with the Cu (920 eV) Auger transition [ 181. (4) Finally, we expect no difficulty in using high energy Auger transitions to study surface segregation provided that dilute alloys are being studied. The reason for this can be seen from a simple example. Imagine that we were trying to observe segrega- tion using a solute Auger transition which samples a region of about 10 atomic

planes (such as the Au (2024 eV) Auger transition). Imagine also that segregation produces a full monolayer in the surface plane but that the other sample planes

retain their bulk compositions. Then, if the alloy contained only 1% of the segregat- ing solute in the bulk, segregation would produce an Auger signal from the solute

TIME (hrs)

Fig. 3. Time dependence of the Auger peak height ratios, Fe(651)/Pt(64) of a 2% Fe in Pt alloy during exposure to lo-’ Torr of 02 at 700°C. Oxygen draws Fe to the surface of the sample.

J.J. Burton, R.S. Polizzotti / Surface segregation in alloys I

atoms about 11 times larger than expected in the unsegregated sample. However, if this same sample started with 20% solute in the bulk, segregation would enhance the solute Auger signal by only about 1.5.

In summary then, we think that our failure to see segregation reflects the fact that our alloys do not segregate at equilibrium.

5. Heat of segregation - experiment

It is clear that we have seen only very small effects due to segregation. Assuming, as we have argued, that our surfaces were equilibrium surfaces, we now want to try to set some bounds on the heats of segregation.

The ratio of the Auger signal arising from solute atoms to that from the solvent atoms is given by an expression of the form [ 11 ,171

I(solute) = Cr {X, + Xn(eA/hr ‘OS e + e-2dlhl cos e + +..)} -.--

I(solvent) C2{(l -X,)+ (1 _ x,)(e-dXxT~~2 c0se + ._.)} ' (3)

where Cr and C2 are constants which give the relative strengths of the two Auger transitions of the solute and solvent, Xa is the surface concentration of the solute, Xn is its bulk concentration, d is the planar spacing in the lattice, X1 and A2 are the mean escape depths of the two Auger electrons, and 8 is the angle of acceptance of

the Auger analyzer relative to the surface normal. In writing eq. (3), we have also assumed that the segregation, if it occurs, is confined to the surface plane [7,9,17] and that the Auger transitions are identical to those in the pure materials. We have also neglected backscattering effects, which can be important in quantitative inter- pretation of Auger experiments [ 191. The possible effect of backscattering will be discussed below.

We have used eqs. (2) and (3) to calculate the likely behavior of the Fe(651)/ Pt(64) Auger peak height ratio as a function of the heat of segregation. The predict- ed temperature dependence of Fe(651)/Pt(64) is shown in fig. 4 for various values

of Q. In these calculations we have assumed Xn = 0.22, d/[Xr+(esr) cos 01 = 0.2,

dl ]hpt(e4) cos 0 1 = 0.5, CF~(C,S 1) = 9 and CPt(64) = 39. The values of h were obtained from the Riviere’s compilation of escape depths [20] and the coefficients, Cpt and CF~ were chosen to give roughly the right Auger peak heights in the pure metals

[ 141. XS was calculated from Xn and Q using eq. (2).

It would appear from fig. 4 that our calculated Auger peak height ratios, Fe- (651)/Pt(64), are compatible with experiment for heats of segregation less than 2 kcal/mole. However, in drawing fig. 4, we have had to choose our coefficients in eq. (3) somewhat arbitrarily; there is also uncertainty on the bulk compositions of our samples. Therefore, in fig. 5 we have plotted both our data and our predicted tem- perature dependence normalized to 900°C. Again, we see that the data are best re- presented by a heat of segregation of Q = 0 kca.l/mole and are clearly incompatible with Q > 2 kcal/mole.

We have carried out similar analyses of our Cr and Ni data. The results of these analyses are in table 1.

J.J. Burton, R.S. PoUzzotti /Surface segregation in alloys

__--- /--- G 3 cc

___C-- s 2 _.,_. -.-.-. _._._.“‘;.-’

-.

g I 1 -m--S_-_m_-_m__--_-_m_--_-_ +Ol .O -m-m 1 l -m_

I f

.0051 .bO .75 -90 1.05

1000/T ( K)

Fig. 4. Temperature dependence of the Fe~6Sl)Pt(64) Auger peak height ratio as measured ex- perimentally and predicted for various values of the heat of segregation, Q = 0, 1, 2 and 3 kcai/ mole. The data are best fit by Q = 0 kcal/mole.

As we remarked above, backscattering corrections can significantly affect the in. ter~retation of Auger experiments [19]. Qu~titat~ve correction for backscatte~ng is uncertain. However, it is certain that if a light element (such as Cr, Fe, or Ni) were segregated to the surface of a heavy metal (Pt), backscattering effects would cause a larger Auger signal to be seen from the light element than we would expect from our simple model neglecting backscattering. Inclusion of backscattering into

T (“Cl

:: & 2.0 1200 950 700 I I I

3

2

d

s

Y” O 9

g

a

!i “u .5

I I

t.“h .bO .75 .90 1.05

1000/T ( K)

Fig. 5. Temperature dependence of the Fe(651)/Pt(64) Auger peak height ratio divided by its value at 900°C as measured experimentally and as predicted for various values of the heat of segregation, Q = 0, 2, 3 and 4 kcaI/mole. The data are not compatible with Q greater than 2 k&/mole.

J.J. Burton, R.S. Polizzotti /Surface segregation in alloys 9

Table 1 Heats of segregation, Q, of various elements to the surface of polycrystalline Pt; the numbers in parenthesis under “experiment” are upper bounds on the experimental heat of segregation; the fist number in this column is the best fit to the data

Element

Cr Fe Ni

Experiment

O(2) O(2) O(3)

Q (kcal/mole), predicted

Bonds Size

10 1.5 9 4 8 5

our theoretical calculations with eq. (3) would then raise the theoretical curves in fig. 4 (for Q > 0) and steepen those in fig. 5 (for Q > 0). This consideration of back-

scattering corrections to the theoretical curves in figs. 4 and 5 strengthens our con- clusion that Cr, Fe, and Ni do not segregate to the surface of Pt.

6. Heat of segregation - theory

In the previous sections we have shown that we obtained no evidence for segre- gation of Cr, Fe, or Ni to the surface of polycrystalline Pt and that the heats of segregation for all three metals is 0 kcal/mole. In this section, we compare our results with the predictions of the two widely used simple theories of surface segre- gation. We will show that neither theory adequately explains the experimental re- sults. Then we will show that our results are in satisfactory agreement with several recent, more complicated, segregation theories.

In the bond breaking picture for surface segregation, the heat of segregation is given approximately by [3]

Q = (u/z) (Ha - Hb) ,

where 2 is the coordination number in the bulk, A? is the coordination number de- ficit in the surface, and Ha and Hb are the sublimation energies of the solvent and solute. We have used eq. (4) to predict heats of segregation. These are in table 1.

In the size difference picture, Q is given by [6]

Q = 24rrKG r3 e2/(3K + 4G) , (9

where K is the bulk modulus of the solute, G is the shear modulus of the solvent, r is the average of the radius of a solvent and a solute stom, (rsolvent t r,&/2 and E

is (rmlute - rwlvent)/rsolute. The predictions of eq. 5 are also in table 1. Our results, table 1, are in very poor agreement with either simple theoretical

model. This result is not entirely surprising. In this paper we have compared quan-

10 J.J. Burton, R.S. Polizzotti /Surface segregation in alloys

titative predictions of these simple theories with our experimental results. However, it is actually known that these simple theories do not even satisfactorily answer the question “Does A segregate to the surface of B?” [21] while our question, which is quantitative, should be expected to be even more difficult. Basically, the problem is that surface segregation is a manifestation of the full complexes of forces in a solid. AS these forces can be quite complicated, quite bizarre segregation effects can occur [22,23]. A number of improvements have been made on the simple theories discussed SO far. These advances attempt to deal with the greater complexity of the forces in solids than was assumed in the simple theoretical predictions of eqs. (4) and (5).

In developing the bond breaking picture for the heat of segregation, the bond strengths are estimated from the heats of sublimation, which leads to the presence of the heats of sublimation in eq. (4). As the driving force for segregation, in the bond breaking picture, is the lowering of the surface energy, it is more correct to

estimate bond energies from surface energies than from heats of sublimation [4,24]. When this is done, the assumed bond energies are reduced by about 50% and thus

the predicted heats of segregation are also reduced by about 50%. Thus, in this picture, the theoretical heats of sublimation in table 1 under “bonds” should be re- duced by about a factor of 2. While this does not bring any of these numbers into perfect agreement with experiment, it certainly does reduce the discrepancy be- tween theory and experiment.

Again, in developing eq. (4) an ideal solution model of the alloy was assumed. More correctly, a regular solution picture is required. When the alloy is considered as a regular solution, the predicted heat of segregation is modified by a regular solu- tion parameter term which increases the heat of segregation in systems with a mis- cibility gap but lowers it in ordering systems [4]. Pt-Cr, Pt-Fe, and Pt-Ni are all ordering systems and so this regular solution parameter term should further reduce the predicted heats of segregation. When the surface energy consideration discussed in the previous paragraph is combined with the regular solution parameter term, as has been done by Overbury [4], then the predicted heats of segregation of Cr, Fe, and Ni to the surface of Pt are all below 2 kcal/mole, in good agreement with

our experimental results. The theories for surface segregation discussed so far have all attempted to model

the microscopic forces active in the solid and then directly predict thermodynamic quantities from these forces. As we have seen, the broken model adequately ac- counts for our data provided surface energies are used to calculate bond strengths and the tendency of the alloy to order is considered. We will now discuss two theor- ies with relate surface segregation to the bulk alloy phase diagrams. These theories do not attempt to describe the microscopic forces. Rather, they identify features of the phase diagram which are expected to correlate with surface segregation.

Seah and Hondros [25] have argued that enrichment of grain boundaries and free surfaces with solute should be related to the terminal solid solubility of the solute. In their view, the more soluble a solute is, the less it should segregate. Con-

J.J. Burton, R.S. Polizzotti /Surface segregation in alloys

(a) NON-SEGREGATING

I

.

(b) SEGREGATING

.

11

0 5 10 0 5 10

“! SOLUTE “! SOLUTE

Fig. 6. Typical phase diagrams for dilute alloys. The region between the solidus (dashed curve) and liquidus (solid curve) is a two phase region.

versely, very insoluble solutes should segregate strongly. They have shown that segregation correlates well with insolubility for a number of Cu and Fe based alloys. Fe and Pt and Ni and Pt are completely miscible while Cr is soluble in Pt up to about 70% [26]. Therefore, from Seah and Hondros terminal solid solubility ideas, there should be little or no tendency for Cr, Fe, or Ni to segregate to the Pt surface, in agreement with our experimental results.

Burton and Machlin [21] have looked at a different aspect of bulk alloy phase diagrams, the solid-liquid equilibrium. They have identified two major types of phase diagrams for dilute solutions, shown in fig. 6. In their view, alloys with phase diagrams in the dilute limit like fig. 6b are expected to segregate solute to the sur- face if the separation between the solidus and liquidus is large while those with small separation between the solidus or liquidus or like fig. 6a will not show segregation. This simple rule has been found to account for the segregation proper- ties of a wide range of alloys [21]. The high temperature Ni-Pt phase diagram is shown in fig. 7. Following Machlin and Burton’s rule, we do not expect segrega- tion of Ni to the Pt surface as the separation between the solidus and the liquidus is small. This is in agreement with our experimental result, namely that Ni does not

i,i--“1. 0 20

Ni Pt ATOMIC % Pt

Fig. 7. The high temperature Ni-Pt phase diagram [ 261. Note that at the Pt rich end the separa- tion between the solidus and the liquidus is very small. Therefore segregation of Ni to the Pt surface is not expected.

.I2 J.J. Burton, R.S. Polizzotti /Surface segregation in alloys

gregate to the Pt surface. The Pt-Fe and Pt-Cr phase diagrams [26] are similar to that of I’-Ni in the Pt rich region and similarly Burton and Machlin’s rule would suggest no segregation, in agreement with our experimental data.

7. Conclusions

We have measured the equilibrium surface composition of three dilute solid solu- tions of Cr, Fe, and Ni in Pt. We have found no evidence of surface segregation of either Cr, Fe, or Ni to the surface of polycrystalline Pt. The widely used bond breaking model [7-9] in its simplest form predicts strong segregation in all three

systems (table 1). The size difference model [6] predicts significant segregation of Fe and Ni to the Pt surface but not of Cr (table 1). Our results are incompatible with either of these sets of predictions and we suggest that neither theory is ade- quate for generally predicting segregation behavior in a wide range of systems.

While one might speculate that some combination of these two simple theories might be more successful than either theory by itself, our results also argue against this possibility as we failed to see segregation of Fe or Ni to the surface of Pt,

though both theories predict it. Our results are, however, in good agreement with the predictions of three more

sophisticated models. When surface energies rather than heats of sublimation are use to predict bond strengths in the broken bond model and ordering effects are considered [4] little segregation of Cr, Fe, or Ni is expected to the surface of Pt, in good agreement with our results. Two theories based on bulk alloy phase diagrams also would predict littIe or no segregation in our systems. Seah and Hondros [25] have related segregation to solid solubility. As Cr, Fe, and Ni are all very soluble in Pt, little or no segregation is expected. Similarly, Burton and Machlin [21] have used the shape of melting curve to predict whether segregation should occur. Their arguments also suggest little or no segregation of Cr, Fe, or Ni to the Pt surface.

References

[I] J.W. Gibbs, Trans. Connecticut Acad. Sci. 3,108 (1875/76); 343 (1977/78). [2] M.P. Seah, Surface Sci. 53 (1975) 168. [3] J.J. Burton and E. Hyman, J. Catalysis 37 (1975) 114. [4) S.H. Overbury, P.A. Bertrand and G.A. Somorjai, Chem. Rev. 75 (1975) 547. [S] A.W. Adamson, Physical Chemistry of Surfaces (Wiley, New York, 1967). [6] R. McLean, Grain Boundaries in MetaIs (Clarendon Press, Oxford, 1957). [7] F.L. Williams and D. Nason, Surface Sci. 45 (1974) 377. [S] R.A. van Santen and W.M.H. Sachtler, J. Catalysis 33 (1974) 202. [9] J.J. Burton, E. Hyman and D. Fedak, J. Catalysis 37 (1975) 106.

[lo] J.J. Burton, C.R. Helms and R.S. Polizzotti, J. Vacuum Sci. Technol. 13 (2976) 204. [ll] J.J. Burton, C.R. Helms and R.S. Polizzotti, J. Chem. Phys. 65 (1976) 1089. [ 121 R.S. Polizzotti and J.J. Burton, J. Vacuum Sci. Technot. 14 (1977) 347.

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[13] J.A. Schwarz, J.J. Burton and R.S. Polizzotti, J. Vacuum Sci. Technol. 14 (1977) 457. [ 141 P.W. Palmberg, G.E. Riach, R.E. Weber and N.C. MacDonald, Handbook of Auger Spectro-

metry (Physical Electronics, Edina, Minn., 1972). [ 151 J.J. Burton and R.S. Polizzotti, to be published. [16] R.L. Garten, Mossbauer Effect Methodology 10 (1976). (171 S.C. Fain, Faraday Disc., Chem. Sx. 60 (1975) 310. [18] C.R. Helms, J. Catalysis 36 (1975) 114. [19] S.H. Overbury and G.A. Somorjai, Surface Sci. 55 (1976) 209. [20] J.C. Riviere, Contemp. Phys. 14 (1973) 513. 1211 J.J. Burton and E.S. Machlin, Phys. Rev. Letters 37 (1976) 1433. [22] K. Binder and P.C. Hohenboerg, Phys. Rev. 89 (1974) 2194. [23] K. Binder, D. Stauffer and V. Wildpaner, Acta Met. 23 (1975) 1191. [24] R.A. van Santen and M.A.M. Boersma, J. Catalysis 34 (1974) 13. [25] M.P. Seah and E.D. Hondros, Proc. Roy. Sot. (London) A335 (1973) 191. [26] M. Hansen, Constitution of Binary Alloys (McGraw-Hill, New York, 1958).