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Surface energy anisotropy of iridium
Surface Science 50 (1975) 399-406 0 North-Holland Publishing Company
SURFACE ENERGY ANISOTROPY
OF IRIDIUM
Rajinder KUMAR and Helen E. GRENGA School of Chemical Engineering and Metallurgy, Georgia Institute of Technology, Atlanta, Georgia 30332, U.S.A. Received 18 February 1975
Field-ion microscopy was used to study the faceting behavior and surface energy anisotropy of iridium in vacuum and in hydrogen. In vacuum below approximately 1300 K the order of faceting and the activation energy for growth of (111) facets agreed with previously published results of FIM studies. The unexpected faceting behavior of (210) planes was examined in terms of the geometry of field evaporated specimens. The observed anisotropy at temperatures above 1300 K was in qualitative agreement with Morse and Mie potential calculations and in nearly quantitative agreement with the pairwise bonding model using 02 = 0.4, 03 = 0.2. The observed maximum anisotropy of 8.8% for iridium at 1360 K fell within the range of extrapolated values for other metals at one-half the absolute melting temperature. Hydrogen appeared to lower the surface energy of each plane by only about 0.1%. An anisotropic effect of hydrogen on the faceting behavior, however, was observed and suggested that surface diffusion rates in (110) and (3 11) regions were preferentially increased in the presence of hydrogen.
1. Introduction Field-ion microscopy has been used previously to study the surface energy anisotropy of tungsten [l] and iridium [2]. In the case of iridium, Brenner observed that thermal faceting in vacuum at temperatures up to 1273 K produced { 11 l}, { 100) and (210) facets. He pointed out that these results were not in agreement with those predicted by surface energy considerations, since {1 lo}, which should have a lower surface energy than (2101, did not facet. Brenner also reported that impurities, notably oxygen, affected the faceting rate and the surface energy anisotropy. The purpose of the present paper is to report additional observations on vacuum faceting of iridium, particularly at temperatures greater than 1300 K where the observed order of faceting and surface energy anisotropies agree with theoretical models. Results on the effect of hydrogen on the surface energy anisotropy are also reported.
2. Experimental FIM specimens were prepared
[3] from annealed high purity (99.99%) iridium wire
400
R. Kurnar, H.E. Grenga/Surfbcc- energy anisotropy of‘lr
of 0.005 inch diameter. Each specimen was further polished in an ammonium carbonate solution and mounted in a conventional bakeable metal microscope. The residual gas pressure after bake-out was approximately 1 X 1O--9 torr. A tungsten wire loop was used to heat the specimen, and two additional tantalum leads were used to measure the temperature from resistivity changes in the loop. The specimen was cleaned by flash heating to about 2200 K in a vacuum of low9 torr and by field evaporation at 78 K in helium. The helium flow and the field were cur off, and the specimen was heated in a vacuum or in hydrogen at selected temperatures for varying periods of time. After each heat treatment, the specimen was reimaged in helium at 78 K, and a series of photographs were taken.
3. Results and discussion 3.1. Faceting in vacuum at 900~-1300 K Figs. 1 and 2, respectively, are typical helium-ion micrographs of specimens before and after annealing in a vacuum of 1OS9 torr at 1130 K for 65 min. It is seen that annealing at this temperature produces {I 1I}, { 100) and (210) facets, in agreement with earlier results by Brenner [2]. The activation energy for (111) facet growth in
Fig. 1. Typical
iridium
specimen
before
annealing,
18.0 kV.
R. Kumar, H.E. GwngajSurface
energy arCsotropj*
oflr
Fig. 2. Iridium after vacuum annealins at 1130 K for 65 min, 16.6 kV. the temperature 45.7 kcal/mole,
range of 900-1200 K was also determined [2] and was found to be again in agreement with Brenner’s earlier results of 45 kcal/mole.
3.2. Fucetirzg in Valium above 1300 K Fig. 3 shows a typical helium-ion micrograph of an iridium specimen which had been heated for 32 minutes at 1360 K in a vacuum of approximately lo-’ torr. This end form was essentially the same as that obtained after 16 min at 1360 K and revealed {ill}, {loo}, {llO}, {3ll},and {210} facets. Specimen blunting did not occur, since the best image potential before and after annealing was the same. The angular width of each facet was measured and corrected [l] for local differences in magnification (correction factor expressed as M”/iVr) and for the top ring contraction effect (correction factor expressed as MO/Mk) to obtain the corrected facet size. These values were then used to determine [ 1,2] the surface energy anisotropy, A,,,, which is the ratio of the surface energy of {hkf} planes to that of { 111) planes. From these results, which are summarized in table 1, it is seen that the order of increasing surface energy is {ll l}, {loo}, (1 lo}, {31 l}, (210). The maximum surface energy anisotropy, &ax 7was calculated [2] from the (111) facet size. Table 1 also gives the theoretical surface energy anisotropies predicted by the following methods: (i) using a Morse potential [4] with u = 4.5, (ii) using a Mie poten-
R. Kumar, H.E. GrengajSurface
402
energy anisotropy of’lr
Fig. 3. Iridium after vacuum annealing at 1360 K for 32 min, 15.7 kV.
Table 1 Surface energy anisotropy of iridium at 1360 K Plane
Theoretical anisotropy
Experimental Observed facet size (deg)
Correction term MoIMr X Mo tMk _____
True facet size (deg) __--
Anisotropy
Morse
Mie
Pairwise
khkl
a = 4.5
??l=6 n = 12
l7* = 0.4, 03 = 0.2
1.000 1.019 1.079 _ -
1.000 1.032 1.080 _
1.000 1.050 1.076 1.109 1.103
1.099
1.095
1.112
_____-
(111) (100) (110)
30.02 22.64 17.90
1.546 1.401 1.173
46.41 31.72 21.00
1.000 1.047 1.070
(311) (210) hmax
12.64 12.64 -
1.123 1.013 _
15.61 12.80 -
1.078 1.081 1.088
R. Kumar. H.&T.~re~~a~sur~~ce
energy anisotropy of‘Ir
403
tiaL t4] with m = 6 and IZ= 12, and (iii) using the pairwise bonding theory f5] with o2 = 0.4 and o3 = 0.2. The variable parameters a, m, n, a2 and 03, were selected to give the closest agreement with the experimental anisotropies. The observed anisotropies are in qualitative agreement with those calculated from all three methods, but the best fit appears to be with the pairwise bonding model. Previous studies on fee metals over a range of temperatures have given anisotropy Since the anisotropy depends upon temvalues corresponding to 0 to 14% for h,,,. perature, it is desirable to compare the present results for iridium with those for other metals at one-half the absolute melting temperature, T, . While anisotropy values at this temperature are generally not available, McLean [6] has suggested a useful correlation between maximum anisotropy and temperature which might be used to extrapolate the necessary values. Examination of his plot of m~mum anisotropy versus fraction of the absolute melting temperature reveals the following: if a straight line is drawn through the points for Cu, &Co, y-Fe, Ag and Ni, the deviation is only about + 1% in the anisotropy values at any given temperature, and the extrapolated value for the maximum anisotropy at 0.5 Tn,, is approximately 9%, which agrees well with the experimental value of 8.8% obtained in the present work for iridium. Extrapolation to 0.5 rrr, for copper alone yields a value of approximately 8% for 11%. (Values for other metals, which are represenXmax and for @-Co, approximately ted by single points, cannot be extrapolated.) Again, the value of 8.8% for irridium falls within this range. From table I, it is seen that the expe~ment~ maximum anisotropy and anisotropies of (3 t l> and {2 1O> planes are lower than the corresponding theoretical values, while the anisotropies for { 110) and IlOO) planes are in nearly quantitative agreement with the pairwise bonding model. The following observations indicated that equilibrium had not been completely established and that a closer agreement with theoretical ,values would be expected with longer annealing times. The angular widths of { 11 l}, {1 lo}, and { 100) facets had increased slightly between annealing times of 16 and 32 min at 1360 K, while the angular widths of (3 11) and (2 10) facets remained constant. Anisotropy calculations showed that h,,,,, for (311) and f210 } planes and h max had increased by approximately 1% while Xhkl for { 110) and { lOO} planes had remained essentially constant during the additional annealing time. The increase of 1% in X,, corresponded to a 10% increase in { 111) facet size. Therefore, it appeared that the anisotropy values were approaching those given by theoretical calculations with the pairwise bonding model. An important question is then why the {311} and (210) facets were initially so prominent, in particular why (210) facets were formed at lower temperatures when (110) facets were absent. As stated above, the size of (210) facets did not change between annealing times of 16 and 32 min at 1360 K. Furthermore, the (210) facets observed after annealing for 32 min at 1360 K were only about 5% larger than those observed after 65 min at 1130 K, while the difference in { 111) facet size for these two annealing conditions was approximately 300%. On one specimen for which the heating loop broke during flash cleaning, the { 111) and {IOO) facets were beginning
404
R. Kumar, H.E. Grenga/Surfacc
energ~~ anisotropy of Ir
to form, while the {210} facets were already comparable in size to those cited above. These observations indicated that there is an initially high, but rapidly decreasing, driving force for thermal migration on (210) planes so that these facets reach a limited size, almost independent of annealing temperature. An important factor which influences the chemical potential for diffusion in a manner consistent with the above observations is the radius of curvature. The local radii of curvature for { 11 I }, {loo}, (3 1 1}, and (2 10) regions on the specimen shown in fig. 1, for example, are 1075, 925, 765 and 575 A, respectively. The chemical potential for thermal migration on a curved surface is increased by an amount inversely proportional to the radius of curvature [7]. Faceting on {210} planes would increase the radius of curvature and consequently decrease the driving force for further faceting in this region. The (3 11) region is also more highly curved that the low index regions, but less curved than the {210} region; this could explain the observation of (31 I} facets at higher temperatures only. Therefore, the appearance of (210) and (31 I} facets may be due to highly curved surface regions on the field evaporated specimen, this explanation would account for the lack of observation of these facets by other techniques. 3.3. Facetirg in hydrogen Iridium specimens were annealed in 2.5 x 10m3 torr of hydrogen at 890 K for 32
Fig. 4. Iridium
after hydrogen
annealing
at 1130 K for 65 min, 17.0 kV.
min, at 1130 K for 32 and 65 min and at 1330 K for 16 and 32 min. All specimens, except those annealed at 1130 K for 65 min, revealed facets similar to, but slightly larger than, those found on specimens annealed in vacuum at the same temperature and time. The observed increase in each facet size was 3 to 5%, corresponding to a decrease of approximately 0.1% for the surface energy of each plane. The end form of specimens annealed in hydrogen at 1130 K for 65 min, as shown in fig. 4, was different from that after vacuum annealing at this temperature (fig. 2) and resembled that after vacuum annealing at higher temperatures (fig. 3); that is, (1 lo} and (31 1) facets were also found on this end form. From corrected facet size measurements, it was found that { 111) and { 100) facets on the hydrogen annealed end form were about 4% larger than the corresponding facets after vacuum annealing at 1130 K and were less than 50% as large as the corresponding facets after vacuum annealing at 1360 K for 32 min; however, the (1 lo} and {3 11) facets on the hydrogen annealed end form were approximately 70% as large as the corresponding facets after vacuum annealing at 1360 K for 32 min. The {210} facets were approximately the same size (< 5% difference) on all three end forms. These results indicated that hydrogen increased the growth rates of { 110) and {3 1 1} facets more than those of {1 1 1} and (100) facets. Since no similar preferential effect of hydrogen on these planes was found for other annealing conditions, particularly at higher temperatures where equilibrium was more closely approached, these results at 1130 K indicate an effect on hydrogen on surface diffusion in these regions, rather than an effect on surface energy or a higher hydrogen coverage on these planes. More information than is presently available on hydrogen adsorption on single crystal planes of iridium is required to evaluate any kinetic measurements that might be used to confirm this explanation.
4. Conclusions (i) Annealing iridium FIM specimens above 1300 K produces facets in agreement with those predicted by surface energy considerations. (ii) The observed surface energy anisotropy values for iridium agree best with those calculated from the pairwise binding theory with a2 = 0.4, u3 = 0.3. (iii) Hydrogen appears to have an almost negligible effect on the surface energy anisotropy of iridium; a more significant effect of hydrogen on surface diffusion rates in {l 10) and {31 1) regions is indicated.
Acknowledgement The authors gratefully Foundation.
acknowledge
support of this work by the National Science
R. Kumar,H.E.GrengajSurface
406
energy anisotropy of Ir
References [l] [2] [3] [4] [SJ [6] [7]
A. Muller and M. Drechsler, Surface Sci. 13 (1969) 471. S.S. Brenner, Surface Sci. 2 (1964) 496. M.A. Fortes, Ph.D. Thesis, University of Cambridge, U.K. (1968). M. Drechsler and J.F. Nicholas, J. Phys. Chem. Solids 28 (1967) 2609. H. Sang and W.A. Miller, (Surface Sci. 28 (1971) 349. M. McLean, Acta Met. 19 (1971) 387. W.W. Mullins, in: Metal Surfaces, Structure, Energetics and Kinetics. ASM Seminar (1962) p.17.
SURFACE ENERGY ANISOTROPY
OF IRIDIUM
Rajinder KUMAR and Helen E. GRENGA School of Chemical Engineering and Metallurgy, Georgia Institute of Technology, Atlanta, Georgia 30332, U.S.A. Received 18 February 1975
Field-ion microscopy was used to study the faceting behavior and surface energy anisotropy of iridium in vacuum and in hydrogen. In vacuum below approximately 1300 K the order of faceting and the activation energy for growth of (111) facets agreed with previously published results of FIM studies. The unexpected faceting behavior of (210) planes was examined in terms of the geometry of field evaporated specimens. The observed anisotropy at temperatures above 1300 K was in qualitative agreement with Morse and Mie potential calculations and in nearly quantitative agreement with the pairwise bonding model using 02 = 0.4, 03 = 0.2. The observed maximum anisotropy of 8.8% for iridium at 1360 K fell within the range of extrapolated values for other metals at one-half the absolute melting temperature. Hydrogen appeared to lower the surface energy of each plane by only about 0.1%. An anisotropic effect of hydrogen on the faceting behavior, however, was observed and suggested that surface diffusion rates in (110) and (3 11) regions were preferentially increased in the presence of hydrogen.
1. Introduction Field-ion microscopy has been used previously to study the surface energy anisotropy of tungsten [l] and iridium [2]. In the case of iridium, Brenner observed that thermal faceting in vacuum at temperatures up to 1273 K produced { 11 l}, { 100) and (210) facets. He pointed out that these results were not in agreement with those predicted by surface energy considerations, since {1 lo}, which should have a lower surface energy than (2101, did not facet. Brenner also reported that impurities, notably oxygen, affected the faceting rate and the surface energy anisotropy. The purpose of the present paper is to report additional observations on vacuum faceting of iridium, particularly at temperatures greater than 1300 K where the observed order of faceting and surface energy anisotropies agree with theoretical models. Results on the effect of hydrogen on the surface energy anisotropy are also reported.
2. Experimental FIM specimens were prepared
[3] from annealed high purity (99.99%) iridium wire
400
R. Kurnar, H.E. Grenga/Surfbcc- energy anisotropy of‘lr
of 0.005 inch diameter. Each specimen was further polished in an ammonium carbonate solution and mounted in a conventional bakeable metal microscope. The residual gas pressure after bake-out was approximately 1 X 1O--9 torr. A tungsten wire loop was used to heat the specimen, and two additional tantalum leads were used to measure the temperature from resistivity changes in the loop. The specimen was cleaned by flash heating to about 2200 K in a vacuum of low9 torr and by field evaporation at 78 K in helium. The helium flow and the field were cur off, and the specimen was heated in a vacuum or in hydrogen at selected temperatures for varying periods of time. After each heat treatment, the specimen was reimaged in helium at 78 K, and a series of photographs were taken.
3. Results and discussion 3.1. Faceting in vacuum at 900~-1300 K Figs. 1 and 2, respectively, are typical helium-ion micrographs of specimens before and after annealing in a vacuum of 1OS9 torr at 1130 K for 65 min. It is seen that annealing at this temperature produces {I 1I}, { 100) and (210) facets, in agreement with earlier results by Brenner [2]. The activation energy for (111) facet growth in
Fig. 1. Typical
iridium
specimen
before
annealing,
18.0 kV.
R. Kumar, H.E. GwngajSurface
energy arCsotropj*
oflr
Fig. 2. Iridium after vacuum annealins at 1130 K for 65 min, 16.6 kV. the temperature 45.7 kcal/mole,
range of 900-1200 K was also determined [2] and was found to be again in agreement with Brenner’s earlier results of 45 kcal/mole.
3.2. Fucetirzg in Valium above 1300 K Fig. 3 shows a typical helium-ion micrograph of an iridium specimen which had been heated for 32 minutes at 1360 K in a vacuum of approximately lo-’ torr. This end form was essentially the same as that obtained after 16 min at 1360 K and revealed {ill}, {loo}, {llO}, {3ll},and {210} facets. Specimen blunting did not occur, since the best image potential before and after annealing was the same. The angular width of each facet was measured and corrected [l] for local differences in magnification (correction factor expressed as M”/iVr) and for the top ring contraction effect (correction factor expressed as MO/Mk) to obtain the corrected facet size. These values were then used to determine [ 1,2] the surface energy anisotropy, A,,,, which is the ratio of the surface energy of {hkf} planes to that of { 111) planes. From these results, which are summarized in table 1, it is seen that the order of increasing surface energy is {ll l}, {loo}, (1 lo}, {31 l}, (210). The maximum surface energy anisotropy, &ax 7was calculated [2] from the (111) facet size. Table 1 also gives the theoretical surface energy anisotropies predicted by the following methods: (i) using a Morse potential [4] with u = 4.5, (ii) using a Mie poten-
R. Kumar, H.E. GrengajSurface
402
energy anisotropy of’lr
Fig. 3. Iridium after vacuum annealing at 1360 K for 32 min, 15.7 kV.
Table 1 Surface energy anisotropy of iridium at 1360 K Plane
Theoretical anisotropy
Experimental Observed facet size (deg)
Correction term MoIMr X Mo tMk _____
True facet size (deg) __--
Anisotropy
Morse
Mie
Pairwise
khkl
a = 4.5
??l=6 n = 12
l7* = 0.4, 03 = 0.2
1.000 1.019 1.079 _ -
1.000 1.032 1.080 _
1.000 1.050 1.076 1.109 1.103
1.099
1.095
1.112
_____-
(111) (100) (110)
30.02 22.64 17.90
1.546 1.401 1.173
46.41 31.72 21.00
1.000 1.047 1.070
(311) (210) hmax
12.64 12.64 -
1.123 1.013 _
15.61 12.80 -
1.078 1.081 1.088
R. Kumar. H.&T.~re~~a~sur~~ce
energy anisotropy of‘Ir
403
tiaL t4] with m = 6 and IZ= 12, and (iii) using the pairwise bonding theory f5] with o2 = 0.4 and o3 = 0.2. The variable parameters a, m, n, a2 and 03, were selected to give the closest agreement with the experimental anisotropies. The observed anisotropies are in qualitative agreement with those calculated from all three methods, but the best fit appears to be with the pairwise bonding model. Previous studies on fee metals over a range of temperatures have given anisotropy Since the anisotropy depends upon temvalues corresponding to 0 to 14% for h,,,. perature, it is desirable to compare the present results for iridium with those for other metals at one-half the absolute melting temperature, T, . While anisotropy values at this temperature are generally not available, McLean [6] has suggested a useful correlation between maximum anisotropy and temperature which might be used to extrapolate the necessary values. Examination of his plot of m~mum anisotropy versus fraction of the absolute melting temperature reveals the following: if a straight line is drawn through the points for Cu, &Co, y-Fe, Ag and Ni, the deviation is only about + 1% in the anisotropy values at any given temperature, and the extrapolated value for the maximum anisotropy at 0.5 Tn,, is approximately 9%, which agrees well with the experimental value of 8.8% obtained in the present work for iridium. Extrapolation to 0.5 rrr, for copper alone yields a value of approximately 8% for 11%. (Values for other metals, which are represenXmax and for @-Co, approximately ted by single points, cannot be extrapolated.) Again, the value of 8.8% for irridium falls within this range. From table I, it is seen that the expe~ment~ maximum anisotropy and anisotropies of (3 t l> and {2 1O> planes are lower than the corresponding theoretical values, while the anisotropies for { 110) and IlOO) planes are in nearly quantitative agreement with the pairwise bonding model. The following observations indicated that equilibrium had not been completely established and that a closer agreement with theoretical ,values would be expected with longer annealing times. The angular widths of { 11 l}, {1 lo}, and { 100) facets had increased slightly between annealing times of 16 and 32 min at 1360 K, while the angular widths of (3 11) and (2 10) facets remained constant. Anisotropy calculations showed that h,,,,, for (311) and f210 } planes and h max had increased by approximately 1% while Xhkl for { 110) and { lOO} planes had remained essentially constant during the additional annealing time. The increase of 1% in X,, corresponded to a 10% increase in { 111) facet size. Therefore, it appeared that the anisotropy values were approaching those given by theoretical calculations with the pairwise bonding model. An important question is then why the {311} and (210) facets were initially so prominent, in particular why (210) facets were formed at lower temperatures when (110) facets were absent. As stated above, the size of (210) facets did not change between annealing times of 16 and 32 min at 1360 K. Furthermore, the (210) facets observed after annealing for 32 min at 1360 K were only about 5% larger than those observed after 65 min at 1130 K, while the difference in { 111) facet size for these two annealing conditions was approximately 300%. On one specimen for which the heating loop broke during flash cleaning, the { 111) and {IOO) facets were beginning
404
R. Kumar, H.E. Grenga/Surfacc
energ~~ anisotropy of Ir
to form, while the {210} facets were already comparable in size to those cited above. These observations indicated that there is an initially high, but rapidly decreasing, driving force for thermal migration on (210) planes so that these facets reach a limited size, almost independent of annealing temperature. An important factor which influences the chemical potential for diffusion in a manner consistent with the above observations is the radius of curvature. The local radii of curvature for { 11 I }, {loo}, (3 1 1}, and (2 10) regions on the specimen shown in fig. 1, for example, are 1075, 925, 765 and 575 A, respectively. The chemical potential for thermal migration on a curved surface is increased by an amount inversely proportional to the radius of curvature [7]. Faceting on {210} planes would increase the radius of curvature and consequently decrease the driving force for further faceting in this region. The (3 11) region is also more highly curved that the low index regions, but less curved than the {210} region; this could explain the observation of (31 I} facets at higher temperatures only. Therefore, the appearance of (210) and (31 I} facets may be due to highly curved surface regions on the field evaporated specimen, this explanation would account for the lack of observation of these facets by other techniques. 3.3. Facetirg in hydrogen Iridium specimens were annealed in 2.5 x 10m3 torr of hydrogen at 890 K for 32
Fig. 4. Iridium
after hydrogen
annealing
at 1130 K for 65 min, 17.0 kV.
min, at 1130 K for 32 and 65 min and at 1330 K for 16 and 32 min. All specimens, except those annealed at 1130 K for 65 min, revealed facets similar to, but slightly larger than, those found on specimens annealed in vacuum at the same temperature and time. The observed increase in each facet size was 3 to 5%, corresponding to a decrease of approximately 0.1% for the surface energy of each plane. The end form of specimens annealed in hydrogen at 1130 K for 65 min, as shown in fig. 4, was different from that after vacuum annealing at this temperature (fig. 2) and resembled that after vacuum annealing at higher temperatures (fig. 3); that is, (1 lo} and (31 1) facets were also found on this end form. From corrected facet size measurements, it was found that { 111) and { 100) facets on the hydrogen annealed end form were about 4% larger than the corresponding facets after vacuum annealing at 1130 K and were less than 50% as large as the corresponding facets after vacuum annealing at 1360 K for 32 min; however, the (1 lo} and {3 11) facets on the hydrogen annealed end form were approximately 70% as large as the corresponding facets after vacuum annealing at 1360 K for 32 min. The {210} facets were approximately the same size (< 5% difference) on all three end forms. These results indicated that hydrogen increased the growth rates of { 110) and {3 1 1} facets more than those of {1 1 1} and (100) facets. Since no similar preferential effect of hydrogen on these planes was found for other annealing conditions, particularly at higher temperatures where equilibrium was more closely approached, these results at 1130 K indicate an effect on hydrogen on surface diffusion in these regions, rather than an effect on surface energy or a higher hydrogen coverage on these planes. More information than is presently available on hydrogen adsorption on single crystal planes of iridium is required to evaluate any kinetic measurements that might be used to confirm this explanation.
4. Conclusions (i) Annealing iridium FIM specimens above 1300 K produces facets in agreement with those predicted by surface energy considerations. (ii) The observed surface energy anisotropy values for iridium agree best with those calculated from the pairwise binding theory with a2 = 0.4, u3 = 0.3. (iii) Hydrogen appears to have an almost negligible effect on the surface energy anisotropy of iridium; a more significant effect of hydrogen on surface diffusion rates in {l 10) and {31 1) regions is indicated.
Acknowledgement The authors gratefully Foundation.
acknowledge
support of this work by the National Science
R. Kumar,H.E.GrengajSurface
406
energy anisotropy of Ir
References [l] [2] [3] [4] [SJ [6] [7]
A. Muller and M. Drechsler, Surface Sci. 13 (1969) 471. S.S. Brenner, Surface Sci. 2 (1964) 496. M.A. Fortes, Ph.D. Thesis, University of Cambridge, U.K. (1968). M. Drechsler and J.F. Nicholas, J. Phys. Chem. Solids 28 (1967) 2609. H. Sang and W.A. Miller, (Surface Sci. 28 (1971) 349. M. McLean, Acta Met. 19 (1971) 387. W.W. Mullins, in: Metal Surfaces, Structure, Energetics and Kinetics. ASM Seminar (1962) p.17.