Surface segregation in Pt-Au alloys studied using Auger electron spectroscopy

Applied Surface Science 26 (1986) 27-41 North-Holland, Amsterdam

27

SURFACE SEGRElGAd’ION IN Pt-Au ALLOYS STUDIED USING AUGER ELECTRON SPECTROSCOPY

Sven Erik HGRNSTRGM

and Leif I. JOHANSSON

Department of Physics and Measurement S-581 83 LinkZping, Sweden

Technology, Linkging

University,

and Anders FLODSTReM MAX Laboratory, Lund University, S-221 00 Lund Sweden Received 2 January 1986; accepted for publication

18 February 1986

Temperature dependent surface segregation studies using Auger electron spectroscopy have been performed on three different Pt-Au alloys, containing 2, 5 and 90 wt% Au. By utilizing Auger transitions of different kinetic energies and model segregation profiles, an estimate of the in-depth variation in composition was made. Strong surface segregation of Au was observed in the three alloys.

1. Introduction

The equilibrium composition of an alloy surface is generally different from the bulk composition. Many material properties, as corrosion resistance, catalytic activity and chemisorption properties, are determined by the composition in the outermost surface layers. A field which has gained an increased interest in recent years is alloy catalysts for hydrocarbon reactions. This is due to the possibility of improving the selectivity for multi-path reactions, increasing the activity and obtaining better resistance to deactivation [l]. The catalytic properties of the Pt-Au system for hydrocarbon reactions as hydrogenolysis, isomerization and dehydrocyclization have been extensively investigated [l-3]. In addition, the chemisorption of hydrogen [4] and carbon oxide [5] on Pt-Au films have been studied. A knowledge of the surface composition is highly important for the understanding of the reactions which take place at an alloy surface [3]. In the present paper we present the results from studies of the surface segregation and its temperature dependence in three different Pt-Au alloys 0169-4332/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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S. E. Hiimstriim et al. / Segregation in Pt - Au studied using AES

containing 2, 5 and 90% Au. The experimental investigations were made using Auger electron spectroscopy (AES). A study of the segregation profile in a Pt-Au alloy using atom-probe field ion microscopy has previously been reported by Tsong et al. [6], supporting earlier results [7] that indicated a strong surface enrichment of Au. We have estimated the in-depth variation of the composition in the alloys by utilizing the difference in probing depths between Auger transitions of different kinetic energy and model segregation profiles. For the strongly overlapping low kinetic energy Auger transitions i.e. the NO0 and NNN transitions, the quantitative evaluation was performed by computer adding pure Pt and Au spectra. This method was found to well represent the experimental Auger spectra. First layer concentrations of Au close to 100% were obtained for all samples and at all temperatures studied (< 800°C), but the extension of the segregation profiles into the bulk was found to vary both with alloy composition and temperature.

2. Experimental Polycrystalline samples of Pt-2wtSAu, Pt-Swt%Au, Pt-90wt%Au, pure Pt and Au were used. Prior to measurements carbon impurities were removed by annealing the samples in an oxygen atmosphere ( = 10e4 Pa) for several hours. The measurements were performed using a commercial scanning Auger elec-

LAYER.i

SAMPLE

SURFACE

Fig. 1. The orientation of the sample relative to the CMA. /3 is the angle between the CMA axis and the sample normal and a the acceptance angle of the CMA. I,( (p) is the path length travelled in the material by an Auger electron escaping from layer i as a function of the azimuthal angle cp of the CMA.

SE. Hiintstriim et al. / Segregation in Pt - Au studied using AES

29

tron spectrometer equipped with a single pass CMA and a coaxial electron gun. The samples were mounted on a resistively heated MO sample holder designed to avoid electron emission from the heating filament to reach the CMA. To allow sputter cleaning of the samples, the sample holder was tilted an angle of 30” relative to the CMA axis, see fig. 1. The annealing temperatures were measured using a Chromel-Alumel thermocouple spot-welded to the sample. The residual pressure in the spectrometer during annealing was below 1 x lo-’ Pa. If the equilibrium surface composition is to be measured it is important that the annealing temperature is not so high that the surface composition is significantly altered due to evaporation losses. Since the vapour pressure for Au is much higher than for Pt, a depletion of Au in the near-surface region can occur if the diffusion from the bulk is not sufficiently rapid compared to evaporation. An estimate of the evaporation rate, r;, for an element i in an alloy can be made by using the relation [8] ri = (~sM,RT)-“~C,S~P~*,

(1)

where Mi is the atomic weight of element i, R the universal gas constant and T the evaporation temperature. ci, Si and Pi* are respectively the molar fraction, the activity coefficient and the vapour pressure at the actual temperature of element i. The activity coefficient can be estimated from the equation ai = exp(AGy/RT),

(2)

where AG,Fs is the relative partial molar excess free energy of component i. Using vapour pressures for pure Pt and Au from ref. [8] and excess free energies for the Pt-Au system from ref. [9], the evaporation rates can be calculated. In the most unfavourable case a loss of approximately two atomic layers per hour is obtained for a temperature of 800°C. Compared to the time required to reach equilibrium (= 15 min) when annealing a sputter cleaned surface at 800°C this should be acceptable losses. We have not used annealing temperatures higher than 8OO”C,since increasing the temperature further leads to a rapid increase in the evaporation rate of Au. The Auger spectra were recorded in the common differentiated form, digitized and stored in a computer. Detailed Auger spectra, at different annealing temperatures, were recorded in kinetic energy regions covering the Pt and Au N,,O,,O,, transitions (Pt: 64 eV, Au: 69 ev), the Pt and Au N,,Nh7Nh7 transitions (Pt: 150-158-168 eV, Au: 141-150-160 eV) and the Pt M,N,7N,7 (1967 eV) and the Au M,N,,N,, (2111 eV) transitions. The surface cleanliness was checked at the different annealing temperatures by recording wide scan spectra. No impurities could be detected except for traces of In and Sb in the Pt-90%Au sample (< 3% of a monolayer). After sputter cleaning the samples were annealed for 40 min at the highest temperature in the series (see table 1) before the spectra were recorded. The temperature was then lowered

30

S.E. Hbrnstriim et al. / Segregation in Pt - Au studied using AES

in steps of about 40°C and held at the new temperature for 20 min before measuring. The normal way to make a quantitative analysis from differentiated Auger spectra is to relate the peak-to-peak heights of the Auger kuctures in the spectra to the concentration of the elements. This procedure was used when evaluating the high kinetic energy spectra, where the Pt M,N,,N,, and the Au I

I

I

I

Reference

spectra ,Au: _.__.__

pt:-

I

-1

i

Alloy spectrum ------ Fit 0.24Pt+0.76

40

50

60 KINETIC

I 70

I 80

At

I 90

ENERGY (eV)

Fig. 2. Upper curves: pure Pt and Au N6,04s04s Auger spectra used in the fitting procedure. Lower curves: the N,,0,s04, alloy spectrum recorded at a temperature of 76O“C and the corresponding fitted spectrum.

S. E. Hiirnstriim et al. / Segregation in Pi - Au studied using AES

dNldE

Reference Pt: _,Au:

dNldE

31

spectra ..a . . . .

Alloy spec hum ------Fit 0.33 Pt+0.67Au

KINETIC

ENERGY (eV)

Fig. 3. Upper curves: pure Pt and Au N,,N6,N6, Auger spectra used in the fitting procedure. alloy spectrum recorded at a temperature of 760°C and the Lower curves: the N,,N,,N6, corresponding fitted spectrum.

M4N6,N6, peaks are well separated in energy. For the two lower kinetic energy regions this was not possible due to large overlaps between the Pt and Au transitions, see figs. 2 and 3. The spectra in these energy regions were evaluated by computer adding Auger spectra from pure Pt and Au. These computer generated alloy spectra were visually compared to the experimental spectra for different weighting factors of Pt and Au.

S.E. Hhstriim

32

ei al. / Segregation in Pt - Au studied using AES

3. Results

In figs. 2 and 3 the Ns7045045 and the Nd5N6,N6, spectra, respectively, are shown for the Pt-5!%Au sample together with the computer generated spectra. The alloy spectra which were recorded at a temperature of 760°C are plotted by solid lines and the corresponding fits as dotted lines. Also shown in the figures are the pure Pt and Au spectra used in the fitting procedure. Fig. 2 shows that the fitting procedure works surprisingly well also for the N67045045 transitions where one could expect problems due to alloying effects in the 5d valence band and since the transitions are superimposed on a steeply sloping background. The accuracy of the compositions obtained by fitting the data with computer generated spectra is better than f3%. In figs. 4 and 5 the concentration of Au obtained for the three different transitions is shown as a

looI 90

Pt-5%

t

N,,

Au

O,, O,, I-

70 eV)

Nk5 Nb7 Nb7 (- 160 eV

L

A”

I

500 Fig. 4. The Au concentration N 45N 67N 67 and M,sN6,N6,

I 600

I 700

TEMPERATURE

1 ‘C 1

in the Pt-5WAu sample obtained Auger transitions.

I 800

using respectively

1

1

1

the N67045045,

33

SE. Hiknstriim et al. / Segregation in Pt - Au studied using AES I

I

Pt-2%

80 -

Au

70 N,,O,,O,, (-70eVl 60 -

N,,N,,N,,(-160eVI

30 M,,N6,N6, (- ZOOOeV) 20-

lo,t

”

I

I

500

600

I 700

TEMPERATURE

('I:1

Fig, 5. The Au concentration N,,N,,N,, and M4sN6,NS7

I 800

in the Pt-2%Au sample obtained using respectively Auger transitions.

the N6,04s04sr

function of the temperature for the Pt-5SAu and the Pt-2%Au sample respectively. The highest concentration of Au is obtained for the N,,O,,O$, Auger electrons (E,, = 70 eV), having a probing depth of approximately 5 A. Successively lower concentrations are obtained for the N45N67N67 (EL,, = 130-170 eV) and the M,,N6,N6, (Ekin f: 1950-J150 eV) Auger electrons, having probing depths of approximately 7 and 25 A respectively [lo]. It is thus obvious that Au has segregated to the alloy surfaces. It is also clear from figs. 4 and 5 that, for the Pt-SC&Au and Pt-2%Au samples, the enrichment of Au decreases with increasing temperature. An enrichment of Au to the alloy surface was also found for the Pt-9OSAu sample. Au concentrations of 92, 95 and 97% were obtained using the M,,N,,N,,, N4sNs7N6, and N67045045 transitions respectively. No variations in surface composition with temperature could, however, be detected for this sample.

34

S.E. Hiimstriim

et al. / Segregation in Pt - Au studied using AES

4. Calculated Pt/ Au intensity ratios

In order to estimate the in-depth composition profile, Pt to Au intensity ratios were calculated using an approach where the Auger intensity is expressed as a sum of contributions from the different layers in the sample. The intensity ratio between Pt and Au in a Pt-Au alloy, normalized to pure element standards, can be expressed as

where Xi is the Pt concentration in layer i and P,(E) the probability that a created Auger electron in layer i, with the kinetic energy E, will escape from the alloy surface without losing energy. K is a constant containing the backscattering correction and the atomic densities of the pure elements. Since Pt and Au are neighbours in the periodic table, their backscattering factors are almost the same. This correction was therefore neglected. The escape probability [ll] is given by

(4 where l,(p) is the path length traveled in the material by an electron escaping from layer i and X(E) is the inelastic mean free path of an electron with kinetic energy E. We assumed that the electron mean free path is matrix independent and that the same values can be used for Pt and Au, since their Auger transitions are close in energy. Since the sample was tilted relative to the CMA axis, the path length depends on the azimuthal angle, (p, of the CMA, see fig. 1. The following expression for ri( (p) can be derived

4(cp>= cos

(i - l)d

/3 cos ff + sin /3 sin (Ysin cp’

(5)

where a is the acceptance angle of the CMA (o = 42.3’), B the angle between the sample normal and the CMA axis (/3 = 30“) and d the interplanar distance The inelastic electron mean free paths were calculated using the formula X(E) = (A/E2)

+ BE”2

(6)

of Seah and Dench [lo], in which A and B are material dependent parameters. We have used A = 117 and B = 0.054, their tabulated values for Au. The interplanar distance d enters in eq. (5). It is not obvious which value to use, since the samples studied were polycrystalline and it is the interplanar

S. E. Hiirnsrriimet al. / Segregation in Pt - Au studied using AES

35

distance for the alloys that is required. The grain size in the Pt-S%Au and the Pt-2%Au samples was so large (= 300 pm) that the electron beam could be focussed on a single grain. We were not, however, able to determine the actual crystal orientation of the grains measured. We did therefore use the mean value of the Pt(ll1) and Au(ll1) interplanar distances, since this generally is the most preferred orientation [12]. The shape of the segregation profile was assumed to be exponential or Gaussian. The best agreement with the experimental data was obtained using a Gaussian profile which can be expressed as X.=Xb+(XS-Xb) exp{ -[(i-1)/D12}, (7) where X, and X, are the bulk and surface (first layer) atomic fractions of Pt and D is a measure of the extent of the concentration profile expressed in atomic layers into the sample. By combining eqs. (3), (4) and (7) Pt/Au I

Pt-5% 2.5 -

l

1

I

I

I

Au

0

1.610.01

185” c

1.810.00

A 725°C 0 69O'C l 660°C o 625'C

’

I

D/X,(Pt)

0 76O'C 2.0-

1

2.6/0.01

I

I

I

25 30 5 10 15 20 INELASTIC ELECTRON MEAN FREE PATH (A)

Fig. 6. Comparison between measured (symbols) and calculated (lines) Pt/Au intensity ratio versus inelastic electron mean free path for different temperatures. D times the interplanar distance (d = 2.31 14) gives a measure, in A, of the extent of each segregation profile.

S. E. Hiirnstriim et al. / Segregation in Pt - Au studied uring AES

36

intensity ratios could be calculated for a given combination of X, and D, which were used as fitting parameters. Fits to the experimentally determined Pt/Au ratios were obtained using numerical calculations, where X, was varied in steps of 0.01 and D in steps of 0.1. The results of these calculations are shown for the Pt-5%Au sample in fig. 6. Each solid line corresponds to the Pt/Au ratio, calculated for a given combination of X, and D as a function of the inelastic electron mean free path. The experimental data, at a series of temperatures, are marked with different symbols at the appropriate electron mean free paths. As can be seen the agreement between calculated and experimental values is quite good. The surface concentration of Pt is close to zero at all temperatures, but the extent of the profile, D, decreases as the

100 -

Pt-5% Au T = 785’ C

80 60 -

40 -

20 -

:

o-1

1

2

"

3

4

"

5

6

"

7

8

"

9

10

5 e 100

Pt-5 % Au T= 625’ C

80 -

-

80-

60 -

-

60-

40 -

-

GO-

0

1

““‘I”‘2 3

&

5

6

7

8

9

10 ATOMIC

0

Pt-4% Au T= 600°C (Tsong et a0

"I 1231r567

I

I

I

I

8

I

9

10

LAYER

Fig.,?. The in-depth &I concentration obtained from the fits, for three different temperatures. The atom-probe field ion microscopy results of Tsong et al. [6] are also shown.

S. E. Hiirnstriim et al. / Segregation in Pt - Au studied using AES

31

Table 1 Fitting parameters giving the best agreement between experimental and calculated iniensity ratios when assuming a Gaussian composition profile and an interplanar distance of 2.31 A

D

xs WV

&I 800 780 745 710 675 640 600 555 510 25

Pt-9O%Au

Pt-5SAu

Pt-2%Au

D

XsW

T W)

D

X,(W

1.6 1.8 2.3 2.6 3.1 3.1 3.0 3.3 3.1

0.01 0.00 0.03 0.01 0.02 0.00 0.00 0.00 0.00

25-770

0.9

0.00

(760 0.7 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.5 1.4

0.04 0.00 0.02 0.02 0.02 0.01 0.01 0.00 0.04 0.00

785 760 725 690 660 625 590 525 480

,

temperature is increased. These results are also illustrated in fig. 7 where the concentration of Au in successive atomic layers is plotted for three different temperatures. For comparison the atom-probe field ion microscopy data of Tsong et al. [6] are also presented. In table 1 the calculated X, and D values are given for the three alloys studied. Also for the Pt-2SAu sample a Pt surface concentration close to zero and a decrease in the extent of the segregation profile with temperature were obtained. For the Pt-!N%Au sample no temperature dependence was found. If a smaller interplanar distance than what corresponds to the (111) orientation is used, the segregation profile will be extended to more layers but the absolute depth remains approximately constant. Changing the interplanar distance from 2.31 A ((111) orientation) to 2.00 A ((200) orientation) gives, for the Pt-S%Au sample at 625’C, an increase in the D-value from 3.1 to 3.8. If instead an exponential segregation profile is used when fitting to the experimentally determined intensity ratios shown in fig. 6, the calculated Pt/Au ratios show larger deviations than for the Gaussian profile. The exponential profile yields Pt/Au ratios that are somewhat larger for the Auger transitions with the lower inelastic mean free path. 5. Discussion The relationship between the surface and bulk composition binary alloy can be expressed as [13]

in a dilute

(8)

38

SE. HCmstriim et al. / Segregation in Pt- Au studied using AES

where X, and X, are respectively the surface and bulk composition of the solute. AHseg is the heat of surface segregation which includes energy contributions from the heat of solution, the difference in pure metal surface energy and the elastic size mismatch energy. Surface energies of solid metals [14] and heats of solutions for different combinations of metals [15] have been calculated. By using these results and also including the elastic size mismatch energy contribution Miedema [16] has calculated the surface heat of segregation for a large number of transition metal binary alloys. His value for Au diluted in Pt is -0.71 eV/atom. This corresponds to a strong surface segregation of Au, the main driving force being the difference in surface energy between Pt and Au. For a temperature of 800°C (the highest annealing temperature used in our work), eq. (8) yields surface concentrations of 98 and 99% Au for bulk concentrations of 2 and 5% Au respectively, in agreement with our results. By utilizing the heats of sublimation of Pt and Au to determine the difference in surface energy Wynblatt and Ku [13] have calculated a somewhat lower surface heat of segregation, -0.55 eV/atom. This value gives, at 800°C a surface concentration of 89 and 95% Au for the 2 and 5% Au alloys respectively. Another way in which the surface heat of segregation can be determined is to use the surface core level shift (SCS), i.e. the difference in core electron binding energy between surface and bulk atoms. It has been shown that for alloys between a Z and a Z + 1 metal (2 being the atomic number), the SCS of the Z component is a direct measure of the surface heat of segregation [17]. Experimentally determined SCS’s of - 0.40 eV for the Pt(ll1) surface [18] and of - 0.30 eV for a polycrystalline sample [19] have been reported. These values have the right sign in that segregation of Au is predicted, but are of smaller magnitude than the values of Miedema [16] and Wynblatt and Ku [13]. Eq. (8) can be generalized to yield the composition in deeper layers into the bulk [20]. This gives the expression xi =- xb 1 -Xi 1 -X,

-AH,

exp (

kT

1

’

(9)

where AHi is the heat of segregation to layer i. Due to a reduced coordination number at the surface the first atomic layer will show the largest heat of segregation. For an alloy which forms a regular solution, the heat of segregation will decrease rapidly with distance into the bulk. It has been proposed that if the alloy is formed exothermically oscillations of AHi in successive layers will occur, while for an endothermically formed alloy AH, decreases monotonically with depth [20]. For an alloy system in which the miscibility is reduced, it will be energetically favourable for like atoms to be neighbours. An enrichment of one of the components to the surface layer can in such systems be followed by a relatively large enrichment in the second and in deeper layers into the bulk. This leads to a larger extension of the segregation profile

S.E. Hiirnstriim et al. / Segregation in Pt - Au studied using AES

39

compared to what is obtained for a system with complete solubility of the constituents. From eq. (9) the heat of segregation to the various atomic layers can be determined, if the true equilibrium in-depth profile is known. A plot of In[ Xi/(1 - Xi)] versus l/T should give a straight line if eq. (9) is applicable. This is, however, rarely observed due to a temperature dependence in the heat of segregation and to entropy effects [13]. By using the in-depth composition profiles obtained in our fitting procedure (table l), values of the heats of segregation can be determined. These values give the order of magnitude and the differences for the first few layers. For the first layer only an approximate lower limit of the heat of segregation can be estimated, ( -A Hi > 0.6 eV/atom) since the surface composition is close to 100% Au at all temperatures studied. Higher annealing temperatures could not be used because of increasing evaporation losses. The values obtained for the second layer using eq. (9) are between - 0.2 and -0.3 eV/atom for the Pt-2SAu sample and between - 0.34 and - 0.40 eV/atom for the Pt-5SAu sample, while a plot of ln[ X,/(1 - X,)] versus l/T yields apparent values of - 0.9 eV/atom for both alloys. A possible explanation for the strong temperature dependence and the difference in enrichment of Au in the two alloys can be found from the equilibrium phase diagram for the Pt-Au alloy system. The phase diagram contains a miscibility gap ranging from a composition of approximately 5 to 80% Au at a temperature of 700°C [21]. Close to and within the immiscibility region one could not expect eq. (9) which assumes a regular solution, to be valid. The surface concentration and its temperature dependence may be influenced by the solubility of Au at the actual temperatures. The larger extension of the concentration profile in the Pt-5SAu alloy compared to the Pt-2%Au alloy is probably due to the fact that a bulk concentration of 5X Au is closer to the solubility limit and the strong temperature dependence due to an increasing solubility with increasing temperature. For an overall composition within the miscibility gap one could expect the Au-rich phase, which has a lower surface energy than the Pt-rich phase, to precipitate at the surface. The composition of the outermost surface layers is then governed by the properties of the precipitated Au-rich phase [22]. The Pt-9O%Au alloy has a composition which falls on the Au-rich side of the miscibility gap. In this region the heat of solution and the elastic strain mismatch energy terms will favour segregation of Pt, but since these contributions are small compared to the difference in surface energy, enrichment of Au is expected. The experimental results are in agreement with this. No variations in composition with temperature is, however, observed. This is probably due to the fact that at high bulk concentrations of Au the changes in composition with temperature are too small to resolve.

40

SE. Hiirnstriim et al. / Segregation in Pt - Au studied using AES

6. Summary

Surface segregation studies using AES have been performed on three Pt-Au alloys, containing 2, 5 and 90% Au. The quantitative evaluation of the Auger spectra from this alloy system is aggravated by the large overlap between some of the Pt and Au Auger transitions. We found it possible to simulate the recorded alloy spectra and hence deduce the Pt/Au intensity ratios quite reliably by computer adding pure Pt and Au spectra. By utilizing Auger transitions of different energies, thus having different probing depths, a strong surface segregation of Au was found for all three alloys. This is in agreement with theoretical predictions and with other experimental results. Estimates of the in-depth variation in composition were performed by fitting model segregation profiles to the experimentally determined Pt/Au intensity ratio for different Auger transitions. The best agreement with the experimental data was obtained for a Gaussian shaped concentration profile. In all samples and at all temperatures studied (< SOO’C) the first layer composition was found to be close to 100% Au. For the Pt-2%Au and the Pt-5%Au samples the extent of the segregated region decreased with increasing temperature, while no changes in composition with temperature were detected for the Pt-90%Au sample. Acknowledgement

Sven-Erik Karlsson is acknowledged for his support and interest in the project and Kennet Larsson for his technical assistance. References [l] [2] [3] (41 [5] [6] [7] [8]

D.I. Hagen and G.A. Somorjai, J. Catalysis 41 (1976) 466. J.W.A. Sachtler, J.P. Bib&an and G.A. Somorjai, Surface Sci. 110 (1981) 43. J.W.A. Sachtler and G.A. Somorjai, J. Catalysis 81 (1983) 77. J.J. Stephan, V. Ponec and W.M.H. Sachtler, Surface Sci. 47 (1975) 403. J.J. Stephan and V. Ponec, J. Catalysis 42 (1976) 1. T.T. Tsong, Yee S. Ng and S.B. McLane, Jr., J. Chem. Phys. 73 (1980) 1464. R. Bouwman and W.M.H. Sachtler, J. Catalysis 19 (1970) 127. L.I. Maissel and R. Glang, Handbook of Thin Film Technology (McGraw-Hill, New York, 1970). [9] R. Hultgren, P.D. Desai, D.T. Hawkins, M. Gleisser and K.K. Kelly, Selected Values of the Thermodynamic Properties of Binary Alloys (American Society for Metals, Metals Park, Ohio, 1973). [lo] M.P. Seah and W.A. Dench, Surface Interface Anal. 1 (1979) 2. [ll] A model of Gallon has been utilized earlier by F.J. Kuijers and V. Ponec, Surface Sci. 68 (1977) 294. This model is, however, almost identical to the exponential model we use, as discussed in the thesis work of F.J. Kuijers, Auger Electron Spectroscopy of Alloys, Leiden (1978).

S.E. Hiirnstrijm et al. / Segregation in Pt - Au studied using AES

41

[12] L.E. Murr, Interfacial Phenomena in Metals and Alloys (Addison-Wesley, Reading, MA, 1975). [13] P. Wynblatt and R.C. Ku, Interfacial Segregation (American Society for Metals, Metals Park, OH, 1977). [14] A.R. Miedema, Z. Metal& 69 (1978) 287. (IS] AR. Miedema, R. Boom and F.R. DeBoer, J. Less-Common Metals 41 (1975) 283. [16] A.R. Miedema, Z. MetaUk. 69 (1978) 455. (171 A. Rosengren and B. Johansson, Phys. Rev. B23 (1981) 3852. [18] G. Apai, R.C. Baetzold, E. Shustorovich and R. Jaeger, Surface Sci. 116 (1982) L191. [19] S.E. Hornstrom, L. Johansson, A. Flodstriim, R. Nyholm and J. Schmidt-May;Surface Sci. 160 (1985) 561. [20] V. Kumar, Phys. Rev. B23 (1981) 3756. [21] M. Hansen, Constitution of Binary Alloys (McGraw-Hill, New York, 1958). [22] W.M.H. Sachtler, Vide 164 (1973) 64.