Theoretical study of the H-induced (1 × 2)-reconstruction at the Ni(110) surface

Surface Science 179 (1987) L91-L101 North-Holland, Amsterdam

SURFACE

SCIENCE

L91

LETTERS

THEORETICAL STUDY OF THE H-INDUCED (1 X 2)-RECONSTRUCTION AT THE Ni(ll0) SURFACE H.-J. BROCKSCH

and K.H. BENNEMANN

Freie Universittit Berlin, Institute for Theoretical Cermtmy Received

2 September

1986; accepted

Physics, Amimallee

for publication

1 October

14, D-1000 Berlin 33,

1986

The local electronic structure of controversial structural models for the H-induced (1 x2)reconstruction of Ni(ll0) is examined via a self-consistent tight-binding scheme including sp-d-band hybridization and all-valence electron hopping interactions among the chemisorbed H and the Ni substrate. Comparison of the results with recent UPS experiments give significantly better agreement for a H-induced pairing-row registry at the Ni surface than for an assumed missing-row configuration. The reconstruction is found to be caused mainly by a H-induced progressive lowering of d-band states for an increasing row-pairing-lie displacement of Ni[liO] rows at the Ni selvedge. This shift of electronic states to negative energies occurs beyond 1 ML coverage of H and is dominant at the topmost Ni layer of the pairing-row model.

Among reconstructed fee transition metal surfaces the H-induced (1 x 2)reconstruction of the Ni(llO) face has recently been the subject of intense experimental investigations. Controversial results with respect to the local geometry of the substrate surface unit cell were reported. It is the goal of this Letter to clear up this controversy from a theoretical point of view. It is well known [l] that for low temperatures and within the submonolayer coverage regime hydrogen adsorbing at Ni(ll0) produces a variety of lattice gas phases without affecting the (1 X l)-geometry of the substrate surface lattice, except for some possible minor changes in the multilayer relaxation pattern of the clean surface. However, upon additional H-uptake a reconstruction of the underlying substrate lattice has been observed. This reconstruction is of the (1 x 2)-type, but presently it is still unclear whether hydrogen will invoke a pairing-row (PR) or a missing-row (MR) formation of the surface lattice. From a Pendry R-factor analysis of their LEED data Jones et al. [2] conclude that there is considerably more evidence for a MR-reconstruction of the Ni(ll0) substrate than for a PR-pattern of the topmost layer. In their dynamical LEED calculations they assume a H coverage of 1 monolayer (Bu = 1 ML) and find the best R-factor for the H atoms located at the three-fold hollow sites (B sites) of the (111) microfacets formed by the topmost half-layer and the first full layer of a MR-reconstructed metal-substrate. It was 0039-6028/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

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f?--J. Brocksch, K. H. Bennemann / H-induced (1 x &reconstruction

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found, however, by several other groups [1,3,4] that a coverage of 8, = 1 ML just marks the onset of the substrate transformation which is completed at OH= 1.5 ML. Regardless of this discrepancy Jones et al. [2] argue that due to the low diffraction power of H scatterers for LEED electrons this should not be in conflict with their major conclusions concerning the substrate geometry. Most recent experiments using proton-induced Auger-electron spectroscopy [5] yielded results also in favour of a MR-registry for the H-covered Ni substrate. Using a variety of surface analysis techniques complementing each other in their resolution power for structural details the Ni(llO)/H-phases were examined by Christmann et al. [3], Griffiths et al. [4], and Rieder and Stocker [l]. Their essential findings are, however, that beyond 8, = 1 ML a PR-transformation of the topmost Ni layer is likely to occur. In order to clear up the situation and to promote the understanding of the physical driving-force for this structural transition we have performed an electronic structure calculation. We used the Recursion Method of Haydock, Heine, Nex [6] and a self-consistent two-band tight-binding scheme applied recently to examine the structural stability of the clean (110) face of the noble metals [7]. According to the bulk of experiments we presupposed that the H-induced reconstruction will be completed for 0, = 1.5 ML and remedied the flaw in the model by Jones et al. [2] by increasing the H coverage of their MR-structure by another 0.5 ML. The additional H atoms were assumed to be located on top of the free Ni[liO] chains of the third substrate layer which would become accessible to chemisorption for a MR-type corrugated surface registry (cf. fig. lb of this Letter and fig. 3b of ref. [2]). Contrary to other conceivable adsorption sites at the topmost half-layer (on top or short bridge) this tentative positioning of the additional half monolayer of H adatoms establishes a MR-model with 19, = 1.5 ML completely equivalent to the respective PR-version of the Ni reconstruction as will become clear below. We calculated then the local surface electronic structure of this MR-model and of the PR-registry proposed by Christmann et al. (cf. fig. lb of ref. [3]) and compared the results for the local density of states (LDOS) obtained to experimental UPS spectra for 8, = 1.5 ML. As our main interest was to look for essential differences in the LDOS structure between both of the models only, we used, for simplicity, the following assumptions concerning the local bonding geometry of H and Ni atoms at the surface region. We neglected any H-induced changes in the multilayer relaxation pattern for atomic layer shifts perpendicular to the surface plane of the clean substrate. As for the Ad1 relaxations of the first three atomic layers of the PR-phase we took the experimental values determined by Adams et al. [S] for the clean (1 x 1)-Ni(ll0) face. In case of the MR-registry we performed a total energy minimization of a fictitious clean MR-structure of Ni(ll0) by means of the same self-consistent calculational scheme applied recently to clean surfaces [7,9]. The relaxations thus obtained are Ad& = - 10.3%, Adj = 4.9%, Ad& = - 1.2% (in percent of

H.-J. Brocksch, K.H. Bennemann / H-induced (1x2)-reconstruction

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at N(ll0) surfaces

I33

b)

Fig. 1. Hard-sphere profiles (view is in the [liO] direction) through topmost four layers of (a) PR-model and (b) MR-model for (1 x 2)-Ni(llO)/H with OH = 1.5 ML. Row-pairing is indicated by lateral arrows. Full black balls: H atoms. In the MR-model the positions of the additional H atoms with respect to the models by Jones et al. [2] are marked by vertical little arrows. Note that to improve on the visibility of the H atoms they are shown somewhat displaced from their true position.

the bulk Ni-interlayer spacing f&aO); they show the usual damped oscillatory behaviour and compare well to the results of Adams et al. [8] with respect to a higher corrugation amplitude to be expected for a MR-surface profile. The results of these calculations then served as a prerequisite to the actual calculations for the H-covered surface models fixing in both cases the Ad L substrate relaxations at those values found if the lateral row-pairing was kept to zero. To simulate the H-induced lateral shift Ad” of atomic Ni[liO] chains in the [OOl] direction (cf. fig. 1) the Ni atoms were treated as touching hard spheres, so that each [liO] chain affected by the row-pairing mechanism would perform a revolving motion around a respective neighboring chain belonging to the next deeper layer. For the PR-model the topmost layer is involved in the row-pairing, in the case of the MR-model we assume an equivalent row-pairing to occur in the second layer of the substrate (cf. fig. lb). Our assumption of hard spheres then leads to well defined changes in the degree of lateral and normal relaxation of the substrate selvedge with respect to the initial reference situation (no row-pairing in effect). To simulate the consequent changes in the positions of the adatoms we note that in both of the models 1 ML of H atoms occupies three-fold coordinated sites where the local substrate geometry is that of (111) microfacets (cf. fig. 1). The remaining 0.5 ML of adatoms is located at positions which show local four-fold adsorption geometry similar to H on Ni(lOO) if the row-pairing of the respective substrate layers is complete. Measuring the degree of row-pairing AdI’ in percentage of the lattice constant aO, this is the case for AdI’= 14.6% a,. In the Ni(lOO)/H system Stensgaard and Jakobsen [lo] recently determined the Ni-H bond length to be Rfjz = 2.2 A. This value, representing some average among the results of other authors [ll], was chosen to locate the H positions at the four-fold symmetry adsorption sites for complete row-pairing in both of the models. Similarly, for the other adsorption site we took as reference the system Ni(lll)/H, for which the Ni-H bond length was measured to be R$$ = 1.84 A according to

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H.-J. Brocksch, K. H. Bennemann / H-induced (1 x 2)~reconstruction at N(l IO) surfaces

Christmann eL al. [12]. This value was chosen to refer to the other limiting situation with AdI1 = 0 (no row-pairing shift assumed). The He-scattering experiments of Griffiths et al. [4], however, show that the minimum shift of topmost Ni rows in the [OOl] direction is at least 0.1 A giving a lower bound AdI’> 3% a, for the PR-structure. We thus can fix plausible ranges for the variation of the AdI’ parameter: 3% a, < AdI’ < 14.6% a, for the PR-model; 0 < AdI’ < 14.6% a, in case of the MR-structure. For intermediate values of AdI’ the adatoms are assumed to join in the rolling motion of the Ni[liO] chains in a simple way by requiring that the local adsorption geometry be preserved always and that the H-Ni bond lengths change in such a way that each H atom keeps an equal distance to its next nearest Ni neighbors. This provides a simplifying, but plausible and easy-to-handle scheme to deal with the combined adatom-substrate displacements and allows to monitor the essentials of the reconstruction kinematics in both cases on an equal footing and as a function of only one single parameter, viz. the row-pairing parameter AdI’. What confirms our point of view is the fact that H does not form a stable hydride phase with bulk Ni [13] under normal conditions, so that the formation of subsurface H phases is not very likely to occur in the course of the substrate transformation. This in turn means that we do not expect the hydrogen to penetrate into the Ni surfacelattice nor to take on adsorption positions very much different from the chosen ones [14]. (Most recent LEED results by the Munich groups around Ertl and Moritz [15] indicate, however, that the lateral row-pairing is likely to be accompanied by a normal buckling of the layer below, which was not accounted for in our calculations so far.) We briefly outline the essential features of the calculational procedure: the two controversial reconstruction registries for (1 x 2)-Ni(llO)/H were modelled by building up atomic clusters of rectangular shape with approximately 2500 atoms each and a H coverage of 8, = 1.5 ML. As a function of variation of the parameter AdI’ the electronic LDOS structure was calculated at each specific site down to the third Ni layer assuming cluster boundary conditions [6] and using 15 levels in the continued fraction expansion of the local Greens functions which was terminated by the Beer/Pettifor version of the square-root terminator [16]. Band hybridization was taken into account for the Ni substrate between the 5d orbitals and an effective orbital of mixed sp character using spin-averaged hopping parameters from Dempsey et al. [17] up to second nearest neighbour interactions and employing the same distance dependence of hopping integrals as in ref. [7]. According to Ellis et al. [18], Melius et al. [19] and the extended-Htickel results by Fassaert and van der Avoird [20] the Nisp band is believed to play an important role for the bonding and cohesive properties in the Ni/H adsorption system, mainly on grounds of the strong d-band filling of Ni. For this reason we tried to take fully into account the Ni-Ni, the Ni-H, and the

H.-J. Brocksch, K. H. Bennemnnn / H-induced (1 x 2)-reconstrurtion at li(i10) surfaces

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H-H couplings on the same footing within the tight-binding framework. As for the coupling between the Ni 3d bands and the H 1s orbital, the respective hopping integrals were calculated recently by Boudeville et al. [21] in the chemisorptive bond limit in terms of a purely numeric treatment based on Herman-Skillman atomic wavefunctions [22]. In the same approximation the remaining hopping integrals linking adsorbate and substrate s-orbitals were evaluated by us analytically [23] applying the full Roothaan-HartreeFock scheme. Owing to wavefunction resonance effects these hopping parameters for the Ni/H system turned out to range in the same order of magnitude as their respective sd counterparts calculated by Boudeville et al. [21]. This result is in support of the aforementioned importance of s-electron binding in the Ni-H chemisorption system [18-201. To calculate the site-dependent surface-shifts of the atomic levels involved local charge neutrality (LCN) was demanded as the condition of self-consistency at each Ni and each H site [7]: AN,(i)

+ AN,(i)

AN,(k)

= 0,

= 0,

i a Ni site, k

a H site,

(AN means changes in the respective local occupation numbers) thus suppressing any charge-transfer among substrate and adsorbate atoms [24]. As the Ni-H bond is known to be of fairly well covalent character [25,26], this should be a reasonable approximation. To be more specific note that the workfunction change that was measured by Christmann et al. [3] for (1 X 2)-Ni(llO)/H at full coverage is only A+ = 0.5 eV, the major contribution of which should be caused by the r~onst~ction of the substrate [27]. The key results of our calculations can be inferred already from fig. 2, giving the difference in LDOS structures only. A complete account of these results giving full details can be found in ref. [28] and will be published elsewhere [29]. Fig. 2 represents calculated LDOS difference spectra for (a) the (2 X I)-2H-Ni(ll0) phase (0, = 1.0 ML), (b) for the PR-version of reconstructed (1 X 2)-Ni(llO)/H (8, = 1.5 ML) and (c) for the MR-version of that system taking the LDOS structure of the clean (1 x 1)-Ni(ll0) face as reference in all three cases. The curves illustrate the change in LDOS structure upon H chemisorption at a Ni site of the topmost substrate layer, respectively, for the z component of the K vector parallel to the surface normal. Maximum row-pairing Adi1 is assumed in cases (b) and (c). Figs. 2b and 2c are to be compared below with the corresponding UPS result obtained by Christmann et al. [3] which is reproduced here in fig. 3b for the readers convenience. Similarly, fig. 3a gives the experimental difference spectrum at a coverage 8, = 1.0 ML and refers to our fig. 2a. The essential result of our calculations explaining the LDOS structure of fig. 2 can be summarized as follows. Beyond the 1 ML coverage regime H

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H.-J. Brocksch, K.H. Bennemann / H-induced (1x2)-reconstruction

Iw

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t

a

100~ CPS

b

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L91

at N(II0) surfaces

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0

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6

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, 16

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4

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Fig. 3. Experimental UPS difference spectra (with respect to clean (1 X 1)-Ni(ll0) and for normal incidence) by Christmann et al. [3] for (a) (2x1)-2H-Ni(llO)/H phase (0, =l.O ML) and (b) (1 x2)-Ni(llO)/H (flu = 1.5 ML).

chemisorbed at Ni(ll0) invokes both a lowering of occupied d-band states at the Ni-selvedge to negative energies and a pulling down of unoccupied states (predominantly of d-character) below the Fermi level cr. For larger values of the AdI1 parameter this shifting of states increases and is strongest if the atomic row-pairing is complete in both cases. However, important qualitative differences are found between the structural models under consideration. This can be inferred from fig. 4 which gives the shift of the local d-band centers of gravity at the first three Ni layers of each model as a function of Ad for full H coverage (6, = 1.5 ML) with respect to their positions at the (1 X 1)-Ni substrate. The topmost Ni layer of the PR-model is the one by far affected most. For AdI1 = 14.6% a, the shift in d-band self-energy is found to be de,(l) = -0.63 eV for the PR-model, but only de,(l) = -0.19 eV at the corresponding layer of the MR-structure. It was checked that on increasing the AdI’ parameter this lowering of d-band states takes on appreciable values only for the H-covered surface structures which indicates that it is induced via the Ni-H interaction. Another difference in the LDOS behaviour between both of the reconstruction models shows up in the way how the LDOS shape in the region around the Fermi level gets modified upon increasing Ad’/: In the case of the PR-structure the prominent d-band LDOS peak (mainly of t *s character) through which the Fermi level level passes for clean Ni(ll0) (and bulk Ni) gets shifted to lower binding energy in a way that its shape keeps more or less preserved. This feature causes the “double-peak structure” in fig. 2b to

Fig. 2. Calculated LDOS difference spectra (arbitrary units) An(e) with respect to the clean Ni(ll0) surface of the topmost Ni layer for (a) (2x l)-2H-Ni(llO)/H-phase (8, = 1.0 ML); (b) PR (1 x 2)-Ni(llO)/H phase; (c) MR (1 X 2)-Ni(llO)/H phase (z-axis oriented normal to surface plane). In cases (b) and (c) 8, = 1.5 ML and Ad” = 14.6% ae. The Fermi energy en is marked by a straight line. The partial contributions from d- and sp-bands are given by broken and dotted lines, respectively.

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H.-J. Brocksch. K.H. Bennemann

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AEd [eVl

!o~:CS~

E1Z1 MR 12)

PR 121 ‘4

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1

3.0

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10.0

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Fig. 4. d-band self-energy shifts Azd(i) in eV of the first three substrate layers for the PR- and MR-model as a function of row-pairing Ad” (in percentage of a,) with respect to clean surface (0, = 1.5 ML).

emerge. In the case of the LDOS structure of the MR-model, however, the redistribution of spectral d-band weight is much more pronounced below cF than for the PR-model. At AdI’ = 0 the dominant LDOS peak centered at cF still persists, but for larger values of AdI1 it loses spectral weight, gets deformed strongly and merges into a series of new resonance peaks which appear eventually around the band center of gravity. Hence the negative LDOS peak at eF in fig. 2c originates from the clean Ni surface. When it comes to a comparison of the overall qualitative features of the experimental UPS curve (fig. 3b) with our calculated LDOS spectra (fig. 2b for PR, fig. 2c for MR) the result is undoubtedly in favour of the PR-model. However, the H-induced shifts of the prominent LDOS peak orginating from surface-bound d-orbital emission measured by Christmann et al. [3] is about 1.3 eV below the Fermi level which is twice as much we can calculate even for complete row-pairing of the topmost layer of the PR-structure (cf. fig. 4). On the one hand this may be due to the aforementioned simplifying assumptions

H.-J. &ock.wh, K. H. Bennemaan / H-induced (I X2)-reconstruction at N(1 IO) surfaces

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we made in setting up the calculation, concerning the Ni-H chemisorption bond geometry, or the neglect of genuine charge transfer among H and the substrate, although the latter is to be considered small [30]. On the other hand refined current UPS measurements by Christmarm and co-workers [31] seem to indicate that the H-induced shift of the d-band UPS peak is only 0.9 eV below er. This moves the shift closer to our result giving rise to the hope that from a more refined calculation it may be possible to evaluate the amount of atomic row-pairing Ad 11precisely. To check that the H-induced shift of Nid band states becomes appreciable only for 8, > 1 ML and to have a cross-check at hand for the reliability of our calculational procedure we made a similar comparison between theory and experimental result for the (2 x l)-2H-Ni(ll0) phase (8, = 1.0 ML). As can be learned from fig. 3a giving the result of Christmann et al. and from our fig. 2a, the result in this case is satisfactory. The wiggly structure in fig. 2a around the zero-line is the remainder of extensive cancellation between the (2 x l)-2HNi(ll0) LDOS structure and that of the clean Ni surface. This proves that the surface electronic structure of the Ni substrate is affected only to a minor degree upon a H-coverage rate of 1 ML. The H-induced changes we find, however, in the surface electronic structure of Ni(ll0) at 8, = 1.5 ML coverage bears some similarity to the protonic model of bulk hydrides [32,33]. This interpretation emerges from the fact that apart from lowering electronic levels in energy below fr we do not find that H, as a perturbation of the clean Ni-surface structure, introduces considerable changes around the bottom of the Ni 3d band, but that it induces new LDOS structure preferably within the unoccupied parts of the substrate bands [28,29]. Owing to strong hybridization of the H 1s orbitals with the Ni4sp band, the latter gets broadened considerably above eF, and the upper band-edge is shifted upwards in energy by an amount of 5-6 eV. Futhermore we find indications of H-induced surface resonance effects in the upper part of the Ni4sp band which might be detectable by means of inverse photoemission experiments of the kind Goldmann and co-workers [35] have done recently at clean Ni surfaces. According to the APW results of Switendick [32] or the KKR calculations by Kulikov et al. [34] the penetration of H into bulk Ni above the stoichiometric concentration range results in a strong perturbation of the host electronic structure deforming the bulk bands considerably and shifting electronic states by several eV. Our LDOS results show a surface-specific modification of those features where the shifting of levels and the appearance of new LDOS structure depends sensitively on the individual Ni-surface site. Thus according to the simple Stone-criterion of magnetism our results on the PR-structure of (1 x 2)Ni(llO)/H would imply that the topmost substrate layer should show a strong H-induced reduction of the Ni-magnetic moment. This is perhaps worth an experimental test using spin-polarized photoemission. In summary, we showed that from a self-consistent tight-binding examina-

ISo0

H.-J. Brochch,

K.H. Bennemann / H-induced (1 X 2]-reconstruction

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tion of the local surface electronic structure of the (1 X 2)-reconstructed Ni(llO)/H system we were able to discriminate between controversial structural models for the substrate registry and found that the reconstructive tr~sition is driven via a H-induced lowering of d-band states at the selvedge. We are thankful to Professor Christmarm for numerous stimulating discussions and useful hurts concerning details of the experimental data interpretation. This work was supported in part by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 6.

References [l] K.H. Rieder and W. Stocker, Surface Sci. 164 (1985) 55. [2] G.J.R. Jones, J.H. Onuferko, D.P. Woodruff and B.W. Holland, Surface Sci. 147 (1984) 1. [3] K. Christmmn, F. Chehab, V. Per&a and G. Ertl, Surface Sci. 152,053 (1985) 356, and references therein. [4] K. Griffith& P.R. Norton, J.A. Davies, W.N. Unertl and T.E. Jackman, Surface Sci. 152/153 (1985) 374. [S] R. Pfandzelter and C. Vareias, Verhandl. DPG (VI) 21 (1986) 1383. [6] V. Heine, R. Haydock and M.J. Keily, in: Solid State Physics, Vol. 35, Eds. H. Ehrenreich, F. Seitz and D. Turnbull, (Academic Press, New York, 1980). [7] H.-J. Brockscb and K.H. Bennemanu, Surface Sci. 161 (1985) 321. [8] D.L. Adams, LE. Petersen and C.S. Sorensen, 3. Phys. Cl8 (1985) 1753. [9] Local charge neutrality (LCN) was assumed at each Ni surface site. [lo] I. Stensgaard and F. Jakobsen, Phys. Rev. Letters 54 (1985) 711. [ll] Cf. C. Umrigar and J.W. W&us, Phys. Rev. Letters 54 (1985) 1551, and references therein; D.W. BuIlet and ML. Cohen. J. Phys. Cl0 (1977) 2101. [12] K. Christmann, R.J. Behm, G. Ertl, M.A. Van Hove and W.H. Weinberg, 3. Chem. Phys. 70 (1979) 4168. [13] B. Baranowski, in: Hydrogen in Metals II, Topics in Applied Physics, Vol. 29, Eds. G. AIefeld and J. VoIkl (Springer, Berlin, 1978). [14] In the case of Pd, e.g., where hydride formation occurs and subsurface hydrogen has been observed in the reconstruction process {cf. R.J. Behm, V. Pet&a, M.-G. Cattania, K. Christmann and G. Ertl, J. Chem. Phys. 78 (1983) 7486) a simple one-parameter scheme to describe the kinematics of all atomic displacements at the substrate selvedge would obviously be nnreahitic. 1151 K. Christmann, private commmrication. [16] N. -Beer and D.G. Pettifor in: The Electronic Structure of Complex Systems, NATO ASI-Series, Series B: Physics, Vol. 113, Eds. P. Phariseau and W.M. Temmerman (Plenum, New York, 1984). 1171 D.G. Dempsey, W.R. Grise and L. Kleinmann, Phys. Rev. B18 (1978) 1270. [HI] D.E. ElIis, H. Ada&i and F.W. AveriII, Surfa* Sci. 58 (1976) 497. 1191 CF. Melius, J.W. Moskowitz, A.P. Mortola, M.B. Bailhe and M.A. Ratner, Surface Sci. 59 (1976) 279. [20] D.J.M. Fassaert and A. van der Avoird, Surface Sci. 55 (1976) 291, 313; D.J.M. Fassaert, H. Verbeek and A. van der Avoird, Surface Sci. 29 (1972) 501. 1211 Y. Boudeville, 3. Rousseau-Violet, F. Cyrot-Lackmamr and S.N. Khanna, J. Phys. (Paris) 44 (1983) 433.

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[22] F. Herman and S. Skillman, Atomic Structure Calculations (Prentice-Hall, Englewood Cliffs, NJ, 1963). [23] H.-J. Brocksch and L.C. Balbas, J. Phys. Chem. Solids (1986). in press. (241 This simplifying assumption facilitates the iteration process to obtain self-consistency considerably. 1251 D.M. Edwards and D.M. Newns, Phys. Letters A24 (1967) 236; D.M. Newns, Phys. Rev. 178 (1969) 1123. 1261 J.K. Norskov, Phys. Rev. B26 (1982) 2875, and references therein. [27] Christmann’s estimate for the A+-contribution due to polarisation of the Ni-H bond is A+ = 0.2 eV (private communication). [28] H.-J. Brocksch, Thesis, Freie Universitat Berlin (1986). [29] H.-J. Brocksch, K. Christmann and K.H. Bemtemann, to be published. [30] Using the LCN condition for self-consistency instead of global charge neutrality simulates charge transfer effects to some extent via a H-induced enhancement of sp + d charge conversion at a Ni site mediated by band hybridization [31] K. Christmann, private communication. [32] Cf., e.g., the review article by A.C. Switendick, in: Hydrogen in Metals I, Topics in Applied Physics, Vol. 28, Eds. G. Alefeld and J. Valkl (Springer, Berlin, 1978). [33] Although this is by abuse of the language because of assumed local charge neutrality. [34] N.I. Kulikov, Phys. Status Solidi 91(b) (1979) 753; N.I. Kulikov, V.N. Borzunov and A.D. Zvonkov, Phys. Status Solidi 86(b) (1978) 83. [35] A. Goldmann, M. Donath, W. Altmann and V. Dose, Phys. Rev. B32 (1985) 837; A. Goldmann, V. Dose and G. Borstel, Phys. Rev. B32 (1985) 1971.