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Determination of (5×5 )R26.7° structures formed on Cu(0 0 1) by coadsorption of Bi and K(Cs): on-top site adsorption of K(Cs)
Surface Science 536 (2003) L415–L422 www.elsevier.com/locate/susc
Surface Science Letters
pffiffiffi pffiffiffi Determination of ( 5 5)R26.7° structures formed on Cu(0 0 1) by coadsorption of Bi and K(Cs): on-top site adsorption of K(Cs) Ming-Shu Chen, Seigi Mizuno, Hiroshi Tochihara
*
Department of Molecular and Material Sciences, Kyushu University, Kasuga Kouen 6-1, Fukuoka 816-8580, Japan Received 24 September 2002; accepted for publication 18 March 2003
Abstract pffiffiffi pffiffiffi By using low-energy electron diffraction (LEED), wepobserved ffiffiffi pffiffiffi a ( 5 5)R26.7° pattern in the coadsorption of Bi and K (or Cs) on Cu(0 0 1) at room temperature. Both ( 5 5)R26.7°-Bi,K and -Bi,Cs structures were determined by a tensor LEED analysis. The two structures are ordered mixed phases and similar to each other: Bi atoms are located at surface fourfold hollow sites and K (or Cs) atoms occupy on-top sites of the substrate surface Cu atoms. Significant (-Bi,K) or 0.19 A (-Bi,Cs) is found in the top Cu layer. It is suggested that the on-top site occupation rumpling of 0.13 A of K or Cs atoms is stabilized by additional bonds with surrounding Bi atoms. The Hamilton-ratio tests suggest that K atoms displace laterally from the exact on-top position, but that Cs atoms are upright on the on-top sites. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Copper; Alkali metals; Bismuth; Alloys; Low energy electron diffraction (LEED); Surface structure, morphology, roughness, and topography; Chemisorption
Surface structure is one of the most important factors determining properties of growing interfaces. So far, structures of alkali metal (AM) atoms adsorbed on metal surfaces have been extensively studied because of their relative simplicity serving as a prototype of chemisorption and also because of their important roles in catalytic reactions. Upon adsorption of AM at room temperature (RT), many reconstructed structures are formed on fcc(0 0 1) and even on fcc(1 1 1) surfaces
*
Corresponding author. Tel./fax: +81-92-583-7130. E-mail address: [email protected] (H. Tochihara).
[1–3], in addition to overlayer formation which was previously believed to be the only form of AM adsorbates on metals. In the reconstructed surfaces, AM atoms kick out substrate surface atoms and occupy substitutional sites. And there are some cases where both substitutional sites and surface hollow sites are taken by AM atoms forming complex ordered structures [2]. In the overlayer structures, AM atoms normally prefer the highest coordination sites, i.e., fourfold hollow sites on fcc(0 0 1) surfaces and threefold hollow sites on fcc(1 1 1) and hcp(0 0 0 1) surfaces [1]. But for large AM atoms such as K, Rb and Cs on the close-packed surfaces, on-top sites are also taken [1]. Thus, it has been found that various sites are
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00598-3
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occupied by AM atoms; nevertheless the rule of site-occupation is not yet clear. Coadsorption studies may give us some valuable insight on the site occupation. There have been many studies on the coadsorption of molecules with dissimilar molecules, where site changes of one kind of molecules have been observed upon coadsorption of the other kind of molecules [3,4]. However, there are only several studies on the coadsorption of two dissimilar metal atoms on metal surfaces. The formation of surface ternary alloys was reported for the coadsorption of Na with larger AM atoms on Al(1 1 1) [5]: Na atoms occupy substitutional sites and larger AM atoms are located on surface hollow sites. On fcc(0 0 1) metal surfaces, the following coadsorptions have been studied: Al(0 0 1)-Li,Na [6], Ag(0 0 1)-Na,K [7], Cu(0 0 1)-Li,Na(K,Cs) [8], Cu(0 0 1)-Mg,Li(K,Cs) [9–11] and Cu(0 0 1)-Mg,Bi [12]. Among them, an interesting interplay between Mg and Li atoms has been demonstrated on Cu(0 0 1): A site-switching of Li atoms from the (2 1) substitutional sites [2], which are occupied in the individual adsorption, to surface fourfold hollow sites is induced bypMg atoms, as evidenced by the determined (2 2 p 2)R45° structure [9–11]. This site-switching of the Li atoms is a result of an indirect interaction between Li and Mg atoms. In the coadsorption pffiffiffi pffiffiffi of Mg and K(Cs), ordered mixed ( 5 5)R26.7° structures were formed [10], in which both Mg and K(Cs) occupy their preferable sites as in the individual adsorption, that is, K atoms are at the surface hollow sites and Mg atoms are at the fourfold substitutional sites. In the present study, we have selected Bi and K(Cs) as a coadsorption system for comparison with the Mg–K(Cs) system [10], because the radius of Bi 1 [13–16] is very similar to that of Mg, and because both Bi [15] and Mg [11,17] atoms occupy
on average The radius of Bi is somewhat uncertain: 1.650 A in f-Bi formed at 90 in a-rhombohedral structure, and 1.645 A for elemental kPa with a bcc structure [13]; Gascoin used 1.54 A was used by Bi [14], while a metallic radius of 1.82 A Meyerheim in discussion [15]. The effective radius of Bi in a c(2 2) structure formed by adsorption of Bi on Cu(0 0 1) at , determined by surface X-ray diffraction RT is 1.63 0.08 A by a tensor LEED [16]. [15], and 1.57 0.05 A 1
fourfold substitutional psites onffiffiffi Cu(0 0 1) at low ffiffiffi p coverages at RT. A ( 5 5)R26.7° structure was formed, as in the Mg–K(Cs) system. pThis ffiffiffi structure, however, is different from the ( 5 pffiffiffi 5)R26.7° structure formed by Mg and K(Cs), as determined by a tensor low-energy electron diffraction (LEED) analysis. It is found that the site changes of both Bi and K(Cs) atoms take place upon the coadsorption: K(Cs) and Bi atoms are located at the on-top sites and hollow sites, respectively, in the coadsorption, while K(Cs) and Bi atoms are at the hollow sites and substitutional sites, respectively, in the individual adsorption. It is suggested that these site changes are induced by a direct, attractive interaction between K (Cs) and Bi atoms. The experiments were carried out in a UHV chamber (base pressure of 1 1010 Torr) equipped with a commercial LEED system. The cleanliness of the Cu(0 0 1) surface was achieved by several cycles of Arþ bombardment (0.5 keV, 1.5 lA) and annealing to 640 °C. K and Cs atoms were evaporated from commercial sources (SAES Getters Inc.), and their coverages were calibrated as 0.25 by the presence of a clear c(4 2) LEED patterns at 130 K. The coverage is defined as the ratio of the atomic density of the adsorbate atoms with respect to that of the substrate atoms in the ideal top layer. Bi atoms were evaporated from a Knudsen cell, and its coverage was calibrated by the appearance of the brightest c(2 2) LEED pattern corresponding to a coverage of 0.5 [16]. The LEED spot intensities (I) were recorded as a function of the incident energy (V ) by a computercontrolled video LEED system. Details of the experiments were similar to the previous study [17]. The sample temperature during measurement of I–V curves was 130 K. A Barbieri/Van Hove symmetrized automated tensor LEED package was used to calculate I–V curves for structure models [18]. Thirteen phase shifts were used to calculate atomic scattering. The search algorithm was directed by minimizing the Pendry R-factor, RP [19]. The real part of the inner potential was determined during the course of the theory–experiment fit. The damping was represented by an imaginary part of the potential, VOi , of )5.0 eV.
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Fig. pffiffiffi 1.p(a) ffiffiffi A normal incident LEED pattern of the Cu(0 0 1)( 5 5)R26.7°-Bi,K structure at 88 eV, and (b) its schematic illustration. Two domains are distinguished by blank and gray circles.
pffiffiffi pffiffiffi A two-domain ( 5 5)R26.7° LEED pattern (see Fig. 1) was observed on Cu(0 0 1) at RT in the coadsorption of Bi and K with both coverages of about 0.2. The same LEED pattern and similar I–V curves were obtained irrespective of the adsorption order. At a coverage of about 0.25, a (2 2) structure was formed, whose will pffiffiffi structure pffiffiffi be described elsewhere [16]. A ( 5 5)R26.7° pattern was also observed in the coadsorption of
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Bi and Cs of both coverages of about 0.2, but a (2 2) structure was not formed at higher coverages. Seven possible structural p models in Fig. ffiffiffi pffiffishown ffi 2 were tested for the ( p5ffiffiffi 5)R26.7°-Bi,K structure (hereafter denoted 5-Bi,K) under conditions that the relaxations of the adsorbate atoms, the first and second layer Cu atoms were allowed and the displacements of K, Bi and Cu atoms preserve the p4 symmetry. The Debye temperatures used for Cu, K and Bi were 335, 120 and 100 K, respectively. The calculated RP value is indicated at the top of each model in Fig. 2. The number of structure parameters is also shown for each model in parenthesis. Model 3, in which Bi atoms are at surface fourfold hollow sites and K atoms are at on-top sites of surface Cu atoms, gets the smallest RP value of 0.21. Model 4, the counterpart of model 3, was excluded. Model 5, in which K atoms are at on-top sites of substitutional Bi atoms, was examined as the case of strong interaction between coadsorbates, and was excluded. Model 1, in which Bi atoms are at fourfold
pffiffiffi pffiffiffi Fig. 2. Top- and side-views of seven examined models for the Cu(0 0 1)-( 5 5)R26.7°-Bi,K structure. Black spheres are Bi atoms, and blank spheres are K atoms. Gray spheres represent Cu atoms. The numerals above each model are the calculated RP factors, and the number of structural parameters is shown in parenthesis.
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pffiffiffi pffiffiffi Fig. pffiffiffi 3.pTopffiffiffi and side-views of (a) the best fit Cu(0 0 1)-( 5 5)R26.7°-Bi,K structure, and (b) the previously determined Cu(0 0 1)( 5 5)R26.7°-Mg,K [10]. Gray and dotted spheres represent Cu atoms in the first and second Cu layer, respectively. Black, blank, and hatched spheres are Bi, K and Mg atoms, respectively.
substitutional sites and K atoms are at hollow sites, was also excluded, though such an ordered mixed structure was found in the coadsorption of Mg and K on Cu(0 0 1) [10]. Model 2, being the counterpart of model 1, and models 6 and 7 including similar configuration to that of model 5, were all rejected due to obviously large RP values as seen in Fig. 2. Further optimization was carried out on model 3 by using six individual phase shifts for K, Bi, Cu atoms under K atoms, other Cu atoms in the top layer, Cu in the second layer and Cu in bulk layers. The optimized Debye temperatures were 150, 100, 180, 300, 320 and 335 K for the above six elements, respectively. A smaller RP value of 0.19 was achieved. The best-fit structure is shown in Fig. 3(a), and structural parameters pffiffiffi are listed in Table 1. The third Cu layer in the 5-Bi,K was kept bulk like and its perpendicular position (shown as a line A–A0 in Fig. 3(a)) was used as a reference plane for atomic heights listed in Table 1. The error range was obtained from the variance of the RP factor, pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DR ¼ Rm 8jVOi j=DE, where Rm is the minimum RP achieved [19] and DE is the cumulative energy range. The cumulative energy range over 12 inequivalent spots was 1750 eV. In Fig. 4, calculated I–V curves of the best-fit structure under the p4
symmetry (dashed lines) are compared with the experimental ones (solid lines) for 12 beams, and the agreement is good. Here, we emphasize some aspects of the best-fit structure (cf. Fig. 3(a) and Table 1). First, the height of K atom from the Cu atom underneath (Cu2 in Fig. 3(a)), denoted as dt in Fig. 3(a), is 2.89 , corresponding to an effective K radius of 1.61 A . This radius is very near to 1.57 A in the A Ni(1 1 1)-(2 2)-K structure determined by LEED, in which K atoms are located at the on-top sites [20]. Second, the height of Bi atoms above the , corresponding fourfold hollow sites, dh , is 2.04 A . This binding to the Bi–Cu distance of 2.72 A in the distance is obviously shorter than 2.85 A c(2 2)-Bi structure formed in the individual adsorption [16], in spite of that Bi atoms in the two structures are located at the same sites. The shorter binding distance in the ordered mixed structure may be related to a direct, attractive interaction between Bi and K atoms mentioned below. Third, (dr in Fig. there is a significant rumpling of 0.13 A 3(a)) in the top Cu layer, which consists of the vertical displacements of the Cu2 atoms inward to and Cu1 atoms outward by 0.04 A bulk by 0.09 A (see Fig. 3(a)) without obvious lateral displacements. We note that such rumpling is normally
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Table 1 pffiffiffi pffiffiffi Optimum parameters for the best fit Cu(0 0 1)-( 5 5)R26.7°-Bi,K and -Bi,Cs (in parenthesis) structures illustrated in Fig. 3(a) Atom
) Lateral displacement (A
K (Cs) Bi
) Interlayer distance (A
6.41 0.04 (6.63 0.04) 5.69 0.06 (5.64 0.05)
dt ¼ 2:90 (dt ¼ 3:19) dh ¼ 2:04 (dh ¼ 2:01)
3.65 0.02 (3.63 0.02)
0.13 (0.19)
Cu2
3.52 0.07 (3.44 0.06)
1.68 (1.61)
Cu3
1.84 0.04 (1.83 0.05)
0.03 (0.01)
Cu1
Cu4
0.00 0.06, 0.00 0.05 (0.00 0.05, 0.03 0.06)
) Height (A
0.02 0.07, 0.00 0.06 (0.01 0.07, 0.02 0.08)
1.81 0.02 (1.82 0.03)
‘‘Lateral displacement ’’ refers to displacements from the center of the hollow site, and ‘‘Height’’ is measured from the plane A–A0 in Fig. 3(a).
observed for large AM atoms located at the ontop-sites on fcc(1 1 1) metals pffiffiffi [1]. The structure of the 5p -Bi,Cs was determined ffiffiffi to be similar to that of the 5-Bi,K. The RP value of 0.19 was achieved under the p4 symmetry. The cumulative energy range over 12 in-equivalent spots was 1730 eV. The Debye temperatures for Cs, Bi, Cu under the Cs atoms, other Cu atoms in the top layer, Cu in the second layer and Cu in bulk layers were optimized to be 90, 80, 180, 380, 350 and 335 K, respectively. Structural parameters are shown in Table 1 in parenthesis, which pffiffiffi are very similar to those obtained from the 5-Bi,K. The comparison of the theoretical I–V curves with the experimental ones is shown in Fig. 5, and the agreement is good. In the best-fit structure, the height of Cs atoms from the Cu atoms underneath , accompanied by the (cf. dt in Fig. 3(a)) is 3.19 A in the top significant vertical rumpling of 0.19 A layer Cu atoms. Such structural parameters give an average interlayer spacing between Cs atoms . The and the top layer Cu atoms, 3.04 0.04 A height of Bi atoms above the fourfold hollow sites , corresponding to a Bi–Cu distance of is 2.01 A 2.71 p ffiffiffi A. This distance is identical with that in the 5-Bi,K. pffiffiffi Here, we consider why the 5-Bi,K(Cs), corresponding to model 3 in Fig. 2, is formed in the coadsorption of Bi and K(Cs). As mentioned at
the introduction, that a similar pffiffiffi wepexpected ffiffiffi structurep toffiffiffi the ( 5 5)R26.7°-Mg,K (hereafter denoted 5-Mg,K and shown in Fig. 3(b)) corresponding to the type of model 1 in Fig. 2 would be formed in the coadsorption of Bi and K(Cs). This expectation is based on the facts below. (1) In the individual adsorption of Bi on Cu(0 0 1), a surface X-ray diffraction study [15] had revealed that Bi atoms are located at fourfold substitutional sites at coverage lower than 0.35, but move up to surface fourfold hollow sites at higher coverage. 2 In the present coadsorption, the coverage of Bi in the 2
We note that such site switching of Bi atoms might be explained by the lateral displacement of the top layer Cu atoms as found in the Cu(0 0 1)–Mg,Li(K,Cs) structures [9–11]. At lower Bi coverages, the top layer Cu atoms can displace laterally when Bi atoms are randomly located at fourfold substitutional sites. As a result, Bi atoms in substitutional sites are embedded deeper to coordinate more effectively with the second layer Cu atoms for achieving a higher binding energy. With increasing coverage, the lateral displacement becomes suppressed, since the substitutional Bi atoms locally form the c(2 2) arrangement. In large c(2 2) domains of the substitutional structure at coverage of near 0.5, the displacement is forbidden due to the symmetry requirement. Therefore, Bi atoms cannot make effective bonds with the second layer Cu atoms. Then, the adsorption energy for Bi atoms at the c(2 2) fourfold substitutional sites would be smaller than that at the c(2 2) hollow sites. That is, the c(2 2) hollow sites become favorable.
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Fig. 4. Comparison between experimental I–V curves (solid lines), and theoretical ones under the p4 symmetry (dashed lines) and p under ffiffiffi pthe ffiffiffi p1 symmetry (dotted lines) for the best fit Cu(0 0 1)-( 5 5)R26.7°-Bi,K structure.
pffiffiffi 5-Bi,K(Cs) is 0.2, so the substitutional sites would be preferable. (2) The size of Bi atoms is similar to Mg atoms. (3) It is well known that K atoms prefer surface fourfold hollow sites on fcc(0 0 1) [1,3,4]. However, model pffiffi1ffi is not realized, so structural features of the 5-Mg,K(Cs) are examined [10]. K(Cs) atoms occupy surface fourfold hollow sites and Mg atoms are located at fourfold substitutional sites. In Fig. 3(b), one K atom is depicted by a transparent circle to show clearly arrows in the first layer Cu atoms. The arrows exhibit the lateral displacement from the ideal positions, and play critical role in the stabilization of this structure [10]. That is, the lateral displacement of the top layer Cu atoms allows Mg atoms to be located at deeper positions in the substitutional sites than in the individual adsorp-
Fig. 5. Comparison between experimental (solid) and theoretical (dashed) under the p4 symmetry for the best fit pffiffiffiI–Vpcurves ffiffiffi Cu(0 0 1)-( 5 5)R26.7°-Bi,Cs structure.
tion to make stronger bonds with substrate Cu atoms. Thus, the interaction between Mg and K(Cs) is an indirect one. We consider that there might be a direct, attractive interaction between Bi and K(Cs) atoms on Cu(0 0 1). In fact, there are many stoichiometric compounds for the AM–Bi systems such as A3 Bi, A3 Bi2 , A5 Bi4 and ABi (A ¼ K, Rb, Cs) [21] (no intermetallic phase has been found for the Mg–K(Cs) system). Many of such stoichiometric compounds have higher melting points than pure Bi or AM, implying the formation of stronger Bi–AM bonds than Bi–Bi and AM–AM pffiffiffi bonds. In the present LEED analysis of the 5-Bi,K(Cs), the bonding distances of K–Bi and Cs–Bi were determined to be , respectively. These values are near 4.10 and 4.16 A obtained by the summation of to 3.922 and 4.305 A
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bulk radii of Bi and K, and Bi and Cs, respectively, suggesting that K(Cs) and Bi atoms are located almost within the effective bonding distances. (The is slightly larger than that K–Bi distance of 4.10 A of 3.922 A, so the Bi and K atoms may not be within effective bonding distance. Later, we understand that the K atoms are actually not located at the exact on-top sites, but displace laterally.) On the other hand, in the geometry of model 1, the Bi– K and Bi–Cs distances would be about 4.64 and , respectively, being estimated from the 4.78 A pffiffiffi structural parameters of the 5-Mg,K(Cs) [10]. These distances are too long to form chemical bonds, and hence model 1 is not realized. Both Bi and K(Cs) atoms change the adsorption sites in the coadsorption from those in the individual adsorption: Bi atoms change the sites from the substitutional to hollow sites; K(Cs) atoms change from the hollow sites to the on-top sites. According to pffiffiffithe bonding distances mentioned above, the 5-Bi,K(Cs) is stabilized by the direct, attractive interaction between the coadsorbates at the cost of occupation of unfavorable sites, which causes the site-switching of both Bi and K(Cs). It is noted that a c(2 2) structure formed on Cu(0 0 1) by the coadsorption of Mg and Bi also exhibits the presence of the direct, attractive interaction between the coadsorbates [12]. The on-top site adsorption of AM atoms on fcc(0 0 1) metals is found for the first time. So, it is necessary to perform a more detailed examination of the coordination geometry. The above tensor LEED analysis was carried out under the p4 symmetry, in which K(Cs) atoms had to be set upright at the on-top sites (cf. Fig. 3(a)). Here, we allowed relaxations of K(Cs), Bi and Cu atoms in the first two layers under the p1 symmetry. An obvious improvement of RP was obtained in the p ffiffiffi 5-Bi,K: RP ðp1Þ ¼ 0:151 under the p1 symmetry vs. RP ðp4Þ ¼ 0:190 under the p4 symmetry. The I–V curves under the p1 symmetry are shown in Fig. 4 by dotted lines. On the other hand,pno ffiffiffi significant improvement was obtained for the 5-Bi,Cs: RP ðp1Þ ¼ 0:178 vs. RP ðp4Þ ¼ 0:187. A Hamilton ratio, which is known in X-ray crystallography as the Hamilton-ratio test [22], was introduced to LEED to deal with the variable numbers of fit parameters [23]. The
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Hamilton-ratio test was used to check whether it is really improved or not by using the p1 symmetry. The Hamilton ratio, Hr , in this study is, 2
Hr ¼
RP ðp4Þ RP ðp1Þ RP ðp1Þ2
2
DE=2jVOi j p ; pq
where q and p are the corresponding number of fitting parameters (in the present study, q ¼ 10 and p ¼ 36 for the p4 and p1 symmetry, respectively). Accordingly, the Hamilton ratio should exceed 3 to indicate a real improvement [23]. pffiffiffi The calculated Hamilton ratio is 3.1 for the 5Bi,K in the p1 vs. p4 symmetry, suggesting that the p4 symmetry is too ideal as the structure. Actually, K atoms exhibit a lateral displacement of about 0.3 from the exact on-top position under the p1 A symmetry, accompanied by slight, lateral displacements of Bi and substrate Cu atoms.pSuch ffiffiffi pbehavior ffiffiffi was also observed in the Al(1 1 1)-( 3 3)R30°K structure formed at low temperature [24], in which an obvious improvement of R-factor was achieved by displacing the position of a K atom . In contrast, from the exact on-top position by 0.3 A apsmaller Hamilton ratio of 0.55 is obtained for the ffiffiffi 5-Bi,Cs in the p1 vs. p4 symmetry, and we suggest that Cs atoms are upright at the on-top sites. That K atoms are not located at the exact ontop sites is expected, because the K–Bi distance obtained under the p4 symmetry is larger than the summation of radii of K and Bi atoms as mentioned above. Actually, the K–Bi bond length is in b-K3 Bi structure [25] and 3.92 A in Bi2 K 3.81 A structure [26,28]. On the other hand, the atomic size of Cs is appropriate to form effective bonds with the four surrounding Bi atoms. The Cs–Bi in both Cs3 Bi [28] and Bi2 Cs bond length is 4.03 A pffiffiffi structures [26,27]. Thus, it is clear that in the 5Bi,K(Cs), the coordination geometry shown in Fig. 3(a) is suitable for Cs atoms but slightly inappropriate for K atoms in terms of the bonding distances. pffiffiffi pffiffiffi In summary, the ( 5 5)R26.7° structures formed on Cu(0 0 1) at RT by the coadsorption of Bi and K(Cs) have been determined by the tensor LEED analysis. These are ordered mixed phases, in which Bi atoms are located at surface fourfold hollow sites and K(Cs) atoms occupy on-top sites.
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Both adsorption sites are different from those in the individual adsorptions. The stabilization of K or Cs atoms at on-top sites is caused by additional bondings with four surrounding Bi atoms. The Hamilton test suggests that the K atoms have a considerable lateral displacement from the exact on-top position, but Cs atoms are upright at the on-top sites. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11]
R.D. Diehl, R. McGrath, Surf. Sci. Rep. 23 (1996) 43. H. Tochihara, S. Mizuno, Prog. Surf. Sci. 58 (1998) 1. H. Over, Prog. Surf. Sci. 58 (1998) 249. G.A. Sormorjai, Introduction to Surface Chemistry and Catalysis, A Wiley-Interscience Press, New York, 1994. S.V. Christensen, J. Nerlov, K. Nielsen, J. Burchhardt, M.M. Nielsen, D.L. Adams, Phys. Rev. Lett. 76 (1996) 1892. J.H. Petersen, C. Sondergard, S.V. Hoffmann, D.L. Adams, Surf. Sci. 461 (2000) 45. S. Mizuno, M. Imaki, H. Tochihara, Surf. Rev. Lett. 8 (2001) 653. T. Maeda, S. Mizuno, H. Tochihara, in preparation. M.S. Chen, S. Mizuno, H. Tochihara, Surf. Sci. 493 (2001) 91. M.S. Chen, S. Mizuno, H. Tochihara, Surf. Sci. 486 (2001) L480. M.S. Chen, S. Mizuno, H. Tochihara, Surf. Sci. 514 (2002) 194.
[12] T. Shirasawa, M.S. Chen, S. Mizuno, H. Tochihara, Surf. Sci. 530 (2003) L307. [13] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, first ed., Pergamon Press, 1984. [14] F. Gascoin, S.C. Sevov, J. Am. Chem. Soc. 122 (2000) 10251. [15] H.L. Meyerheim, H. Zajonz, W. Moritz, I.K. Robinson, Surf. Sci. 381 (1997) L551. [16] M.S.Chen, S. Mizuno, H. Tochihara, in preparation. [17] M.S. Chen, D. Terasaki, S. Mizuno, H. Tochihara, I. Ohsaki, T. Oguchi, Surf. Sci. 470 (2000) 53. [18] M.A. Van Hove, W. Mortiz, H. Over, P.J. Rous, A. Wander, A. Barbieri, N. Materer, U. Starke, G.A. Somorjai, Surf. Sci., Rep. 19 (1993) 191. [19] J.B. Pendry, J. Phys. C 13 (1980) 937. [20] D. Fisher, S. Chandavarkar, I.R. Collins, R.D. Diehl, P. Kaukoasoina, M. Lindroos, Phys. Rev. Lett. 68 (1992) 2786. [21] T.B. Massalski, Binary Alloy Phase Diagrams, second ed., William W. Scott, Jr. Press, USA, 1996. [22] E. Prince, Mathematical Techniques in Crystallography and Materials Sciences, Springer-Verlag, New York, 1982. [23] C.F. Walters, K.F. McCarty, E.A. Soares, M.A. Van Hove, Surf. Sci. 464 (2000) L732. [24] C. Stampfl, M. Scheffler, H. Over, J. Burchhardt, M. Nielsen, D.L. Adams, W. Moritz, Phys. Rev. B 49 (1994) 4959. [25] D.E. Sands, D.H. Wood, W.J. Ramsey, Acta Crystallogr. 16 (1963) 316. [26] G. Gnutzmann, F.W. Dorn, W. Klemm, Z. Anorg. Allg. Chem. 309 (1961) 210. [27] N.N. Zhuravlev, Sov. Phys. Jetp 34 (7) (1958) 571. [28] G. Brauer, E. Zintl, Z. Phys. Chem. B 37 (1937) 323.
Surface Science Letters
pffiffiffi pffiffiffi Determination of ( 5 5)R26.7° structures formed on Cu(0 0 1) by coadsorption of Bi and K(Cs): on-top site adsorption of K(Cs) Ming-Shu Chen, Seigi Mizuno, Hiroshi Tochihara
*
Department of Molecular and Material Sciences, Kyushu University, Kasuga Kouen 6-1, Fukuoka 816-8580, Japan Received 24 September 2002; accepted for publication 18 March 2003
Abstract pffiffiffi pffiffiffi By using low-energy electron diffraction (LEED), wepobserved ffiffiffi pffiffiffi a ( 5 5)R26.7° pattern in the coadsorption of Bi and K (or Cs) on Cu(0 0 1) at room temperature. Both ( 5 5)R26.7°-Bi,K and -Bi,Cs structures were determined by a tensor LEED analysis. The two structures are ordered mixed phases and similar to each other: Bi atoms are located at surface fourfold hollow sites and K (or Cs) atoms occupy on-top sites of the substrate surface Cu atoms. Significant (-Bi,K) or 0.19 A (-Bi,Cs) is found in the top Cu layer. It is suggested that the on-top site occupation rumpling of 0.13 A of K or Cs atoms is stabilized by additional bonds with surrounding Bi atoms. The Hamilton-ratio tests suggest that K atoms displace laterally from the exact on-top position, but that Cs atoms are upright on the on-top sites. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Copper; Alkali metals; Bismuth; Alloys; Low energy electron diffraction (LEED); Surface structure, morphology, roughness, and topography; Chemisorption
Surface structure is one of the most important factors determining properties of growing interfaces. So far, structures of alkali metal (AM) atoms adsorbed on metal surfaces have been extensively studied because of their relative simplicity serving as a prototype of chemisorption and also because of their important roles in catalytic reactions. Upon adsorption of AM at room temperature (RT), many reconstructed structures are formed on fcc(0 0 1) and even on fcc(1 1 1) surfaces
*
Corresponding author. Tel./fax: +81-92-583-7130. E-mail address: [email protected] (H. Tochihara).
[1–3], in addition to overlayer formation which was previously believed to be the only form of AM adsorbates on metals. In the reconstructed surfaces, AM atoms kick out substrate surface atoms and occupy substitutional sites. And there are some cases where both substitutional sites and surface hollow sites are taken by AM atoms forming complex ordered structures [2]. In the overlayer structures, AM atoms normally prefer the highest coordination sites, i.e., fourfold hollow sites on fcc(0 0 1) surfaces and threefold hollow sites on fcc(1 1 1) and hcp(0 0 0 1) surfaces [1]. But for large AM atoms such as K, Rb and Cs on the close-packed surfaces, on-top sites are also taken [1]. Thus, it has been found that various sites are
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occupied by AM atoms; nevertheless the rule of site-occupation is not yet clear. Coadsorption studies may give us some valuable insight on the site occupation. There have been many studies on the coadsorption of molecules with dissimilar molecules, where site changes of one kind of molecules have been observed upon coadsorption of the other kind of molecules [3,4]. However, there are only several studies on the coadsorption of two dissimilar metal atoms on metal surfaces. The formation of surface ternary alloys was reported for the coadsorption of Na with larger AM atoms on Al(1 1 1) [5]: Na atoms occupy substitutional sites and larger AM atoms are located on surface hollow sites. On fcc(0 0 1) metal surfaces, the following coadsorptions have been studied: Al(0 0 1)-Li,Na [6], Ag(0 0 1)-Na,K [7], Cu(0 0 1)-Li,Na(K,Cs) [8], Cu(0 0 1)-Mg,Li(K,Cs) [9–11] and Cu(0 0 1)-Mg,Bi [12]. Among them, an interesting interplay between Mg and Li atoms has been demonstrated on Cu(0 0 1): A site-switching of Li atoms from the (2 1) substitutional sites [2], which are occupied in the individual adsorption, to surface fourfold hollow sites is induced bypMg atoms, as evidenced by the determined (2 2 p 2)R45° structure [9–11]. This site-switching of the Li atoms is a result of an indirect interaction between Li and Mg atoms. In the coadsorption pffiffiffi pffiffiffi of Mg and K(Cs), ordered mixed ( 5 5)R26.7° structures were formed [10], in which both Mg and K(Cs) occupy their preferable sites as in the individual adsorption, that is, K atoms are at the surface hollow sites and Mg atoms are at the fourfold substitutional sites. In the present study, we have selected Bi and K(Cs) as a coadsorption system for comparison with the Mg–K(Cs) system [10], because the radius of Bi 1 [13–16] is very similar to that of Mg, and because both Bi [15] and Mg [11,17] atoms occupy
on average The radius of Bi is somewhat uncertain: 1.650 A in f-Bi formed at 90 in a-rhombohedral structure, and 1.645 A for elemental kPa with a bcc structure [13]; Gascoin used 1.54 A was used by Bi [14], while a metallic radius of 1.82 A Meyerheim in discussion [15]. The effective radius of Bi in a c(2 2) structure formed by adsorption of Bi on Cu(0 0 1) at , determined by surface X-ray diffraction RT is 1.63 0.08 A by a tensor LEED [16]. [15], and 1.57 0.05 A 1
fourfold substitutional psites onffiffiffi Cu(0 0 1) at low ffiffiffi p coverages at RT. A ( 5 5)R26.7° structure was formed, as in the Mg–K(Cs) system. pThis ffiffiffi structure, however, is different from the ( 5 pffiffiffi 5)R26.7° structure formed by Mg and K(Cs), as determined by a tensor low-energy electron diffraction (LEED) analysis. It is found that the site changes of both Bi and K(Cs) atoms take place upon the coadsorption: K(Cs) and Bi atoms are located at the on-top sites and hollow sites, respectively, in the coadsorption, while K(Cs) and Bi atoms are at the hollow sites and substitutional sites, respectively, in the individual adsorption. It is suggested that these site changes are induced by a direct, attractive interaction between K (Cs) and Bi atoms. The experiments were carried out in a UHV chamber (base pressure of 1 1010 Torr) equipped with a commercial LEED system. The cleanliness of the Cu(0 0 1) surface was achieved by several cycles of Arþ bombardment (0.5 keV, 1.5 lA) and annealing to 640 °C. K and Cs atoms were evaporated from commercial sources (SAES Getters Inc.), and their coverages were calibrated as 0.25 by the presence of a clear c(4 2) LEED patterns at 130 K. The coverage is defined as the ratio of the atomic density of the adsorbate atoms with respect to that of the substrate atoms in the ideal top layer. Bi atoms were evaporated from a Knudsen cell, and its coverage was calibrated by the appearance of the brightest c(2 2) LEED pattern corresponding to a coverage of 0.5 [16]. The LEED spot intensities (I) were recorded as a function of the incident energy (V ) by a computercontrolled video LEED system. Details of the experiments were similar to the previous study [17]. The sample temperature during measurement of I–V curves was 130 K. A Barbieri/Van Hove symmetrized automated tensor LEED package was used to calculate I–V curves for structure models [18]. Thirteen phase shifts were used to calculate atomic scattering. The search algorithm was directed by minimizing the Pendry R-factor, RP [19]. The real part of the inner potential was determined during the course of the theory–experiment fit. The damping was represented by an imaginary part of the potential, VOi , of )5.0 eV.
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Fig. pffiffiffi 1.p(a) ffiffiffi A normal incident LEED pattern of the Cu(0 0 1)( 5 5)R26.7°-Bi,K structure at 88 eV, and (b) its schematic illustration. Two domains are distinguished by blank and gray circles.
pffiffiffi pffiffiffi A two-domain ( 5 5)R26.7° LEED pattern (see Fig. 1) was observed on Cu(0 0 1) at RT in the coadsorption of Bi and K with both coverages of about 0.2. The same LEED pattern and similar I–V curves were obtained irrespective of the adsorption order. At a coverage of about 0.25, a (2 2) structure was formed, whose will pffiffiffi structure pffiffiffi be described elsewhere [16]. A ( 5 5)R26.7° pattern was also observed in the coadsorption of
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Bi and Cs of both coverages of about 0.2, but a (2 2) structure was not formed at higher coverages. Seven possible structural p models in Fig. ffiffiffi pffiffishown ffi 2 were tested for the ( p5ffiffiffi 5)R26.7°-Bi,K structure (hereafter denoted 5-Bi,K) under conditions that the relaxations of the adsorbate atoms, the first and second layer Cu atoms were allowed and the displacements of K, Bi and Cu atoms preserve the p4 symmetry. The Debye temperatures used for Cu, K and Bi were 335, 120 and 100 K, respectively. The calculated RP value is indicated at the top of each model in Fig. 2. The number of structure parameters is also shown for each model in parenthesis. Model 3, in which Bi atoms are at surface fourfold hollow sites and K atoms are at on-top sites of surface Cu atoms, gets the smallest RP value of 0.21. Model 4, the counterpart of model 3, was excluded. Model 5, in which K atoms are at on-top sites of substitutional Bi atoms, was examined as the case of strong interaction between coadsorbates, and was excluded. Model 1, in which Bi atoms are at fourfold
pffiffiffi pffiffiffi Fig. 2. Top- and side-views of seven examined models for the Cu(0 0 1)-( 5 5)R26.7°-Bi,K structure. Black spheres are Bi atoms, and blank spheres are K atoms. Gray spheres represent Cu atoms. The numerals above each model are the calculated RP factors, and the number of structural parameters is shown in parenthesis.
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pffiffiffi pffiffiffi Fig. pffiffiffi 3.pTopffiffiffi and side-views of (a) the best fit Cu(0 0 1)-( 5 5)R26.7°-Bi,K structure, and (b) the previously determined Cu(0 0 1)( 5 5)R26.7°-Mg,K [10]. Gray and dotted spheres represent Cu atoms in the first and second Cu layer, respectively. Black, blank, and hatched spheres are Bi, K and Mg atoms, respectively.
substitutional sites and K atoms are at hollow sites, was also excluded, though such an ordered mixed structure was found in the coadsorption of Mg and K on Cu(0 0 1) [10]. Model 2, being the counterpart of model 1, and models 6 and 7 including similar configuration to that of model 5, were all rejected due to obviously large RP values as seen in Fig. 2. Further optimization was carried out on model 3 by using six individual phase shifts for K, Bi, Cu atoms under K atoms, other Cu atoms in the top layer, Cu in the second layer and Cu in bulk layers. The optimized Debye temperatures were 150, 100, 180, 300, 320 and 335 K for the above six elements, respectively. A smaller RP value of 0.19 was achieved. The best-fit structure is shown in Fig. 3(a), and structural parameters pffiffiffi are listed in Table 1. The third Cu layer in the 5-Bi,K was kept bulk like and its perpendicular position (shown as a line A–A0 in Fig. 3(a)) was used as a reference plane for atomic heights listed in Table 1. The error range was obtained from the variance of the RP factor, pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DR ¼ Rm 8jVOi j=DE, where Rm is the minimum RP achieved [19] and DE is the cumulative energy range. The cumulative energy range over 12 inequivalent spots was 1750 eV. In Fig. 4, calculated I–V curves of the best-fit structure under the p4
symmetry (dashed lines) are compared with the experimental ones (solid lines) for 12 beams, and the agreement is good. Here, we emphasize some aspects of the best-fit structure (cf. Fig. 3(a) and Table 1). First, the height of K atom from the Cu atom underneath (Cu2 in Fig. 3(a)), denoted as dt in Fig. 3(a), is 2.89 , corresponding to an effective K radius of 1.61 A . This radius is very near to 1.57 A in the A Ni(1 1 1)-(2 2)-K structure determined by LEED, in which K atoms are located at the on-top sites [20]. Second, the height of Bi atoms above the , corresponding fourfold hollow sites, dh , is 2.04 A . This binding to the Bi–Cu distance of 2.72 A in the distance is obviously shorter than 2.85 A c(2 2)-Bi structure formed in the individual adsorption [16], in spite of that Bi atoms in the two structures are located at the same sites. The shorter binding distance in the ordered mixed structure may be related to a direct, attractive interaction between Bi and K atoms mentioned below. Third, (dr in Fig. there is a significant rumpling of 0.13 A 3(a)) in the top Cu layer, which consists of the vertical displacements of the Cu2 atoms inward to and Cu1 atoms outward by 0.04 A bulk by 0.09 A (see Fig. 3(a)) without obvious lateral displacements. We note that such rumpling is normally
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Table 1 pffiffiffi pffiffiffi Optimum parameters for the best fit Cu(0 0 1)-( 5 5)R26.7°-Bi,K and -Bi,Cs (in parenthesis) structures illustrated in Fig. 3(a) Atom
) Lateral displacement (A
K (Cs) Bi
) Interlayer distance (A
6.41 0.04 (6.63 0.04) 5.69 0.06 (5.64 0.05)
dt ¼ 2:90 (dt ¼ 3:19) dh ¼ 2:04 (dh ¼ 2:01)
3.65 0.02 (3.63 0.02)
0.13 (0.19)
Cu2
3.52 0.07 (3.44 0.06)
1.68 (1.61)
Cu3
1.84 0.04 (1.83 0.05)
0.03 (0.01)
Cu1
Cu4
0.00 0.06, 0.00 0.05 (0.00 0.05, 0.03 0.06)
) Height (A
0.02 0.07, 0.00 0.06 (0.01 0.07, 0.02 0.08)
1.81 0.02 (1.82 0.03)
‘‘Lateral displacement ’’ refers to displacements from the center of the hollow site, and ‘‘Height’’ is measured from the plane A–A0 in Fig. 3(a).
observed for large AM atoms located at the ontop-sites on fcc(1 1 1) metals pffiffiffi [1]. The structure of the 5p -Bi,Cs was determined ffiffiffi to be similar to that of the 5-Bi,K. The RP value of 0.19 was achieved under the p4 symmetry. The cumulative energy range over 12 in-equivalent spots was 1730 eV. The Debye temperatures for Cs, Bi, Cu under the Cs atoms, other Cu atoms in the top layer, Cu in the second layer and Cu in bulk layers were optimized to be 90, 80, 180, 380, 350 and 335 K, respectively. Structural parameters are shown in Table 1 in parenthesis, which pffiffiffi are very similar to those obtained from the 5-Bi,K. The comparison of the theoretical I–V curves with the experimental ones is shown in Fig. 5, and the agreement is good. In the best-fit structure, the height of Cs atoms from the Cu atoms underneath , accompanied by the (cf. dt in Fig. 3(a)) is 3.19 A in the top significant vertical rumpling of 0.19 A layer Cu atoms. Such structural parameters give an average interlayer spacing between Cs atoms . The and the top layer Cu atoms, 3.04 0.04 A height of Bi atoms above the fourfold hollow sites , corresponding to a Bi–Cu distance of is 2.01 A 2.71 p ffiffiffi A. This distance is identical with that in the 5-Bi,K. pffiffiffi Here, we consider why the 5-Bi,K(Cs), corresponding to model 3 in Fig. 2, is formed in the coadsorption of Bi and K(Cs). As mentioned at
the introduction, that a similar pffiffiffi wepexpected ffiffiffi structurep toffiffiffi the ( 5 5)R26.7°-Mg,K (hereafter denoted 5-Mg,K and shown in Fig. 3(b)) corresponding to the type of model 1 in Fig. 2 would be formed in the coadsorption of Bi and K(Cs). This expectation is based on the facts below. (1) In the individual adsorption of Bi on Cu(0 0 1), a surface X-ray diffraction study [15] had revealed that Bi atoms are located at fourfold substitutional sites at coverage lower than 0.35, but move up to surface fourfold hollow sites at higher coverage. 2 In the present coadsorption, the coverage of Bi in the 2
We note that such site switching of Bi atoms might be explained by the lateral displacement of the top layer Cu atoms as found in the Cu(0 0 1)–Mg,Li(K,Cs) structures [9–11]. At lower Bi coverages, the top layer Cu atoms can displace laterally when Bi atoms are randomly located at fourfold substitutional sites. As a result, Bi atoms in substitutional sites are embedded deeper to coordinate more effectively with the second layer Cu atoms for achieving a higher binding energy. With increasing coverage, the lateral displacement becomes suppressed, since the substitutional Bi atoms locally form the c(2 2) arrangement. In large c(2 2) domains of the substitutional structure at coverage of near 0.5, the displacement is forbidden due to the symmetry requirement. Therefore, Bi atoms cannot make effective bonds with the second layer Cu atoms. Then, the adsorption energy for Bi atoms at the c(2 2) fourfold substitutional sites would be smaller than that at the c(2 2) hollow sites. That is, the c(2 2) hollow sites become favorable.
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Fig. 4. Comparison between experimental I–V curves (solid lines), and theoretical ones under the p4 symmetry (dashed lines) and p under ffiffiffi pthe ffiffiffi p1 symmetry (dotted lines) for the best fit Cu(0 0 1)-( 5 5)R26.7°-Bi,K structure.
pffiffiffi 5-Bi,K(Cs) is 0.2, so the substitutional sites would be preferable. (2) The size of Bi atoms is similar to Mg atoms. (3) It is well known that K atoms prefer surface fourfold hollow sites on fcc(0 0 1) [1,3,4]. However, model pffiffi1ffi is not realized, so structural features of the 5-Mg,K(Cs) are examined [10]. K(Cs) atoms occupy surface fourfold hollow sites and Mg atoms are located at fourfold substitutional sites. In Fig. 3(b), one K atom is depicted by a transparent circle to show clearly arrows in the first layer Cu atoms. The arrows exhibit the lateral displacement from the ideal positions, and play critical role in the stabilization of this structure [10]. That is, the lateral displacement of the top layer Cu atoms allows Mg atoms to be located at deeper positions in the substitutional sites than in the individual adsorp-
Fig. 5. Comparison between experimental (solid) and theoretical (dashed) under the p4 symmetry for the best fit pffiffiffiI–Vpcurves ffiffiffi Cu(0 0 1)-( 5 5)R26.7°-Bi,Cs structure.
tion to make stronger bonds with substrate Cu atoms. Thus, the interaction between Mg and K(Cs) is an indirect one. We consider that there might be a direct, attractive interaction between Bi and K(Cs) atoms on Cu(0 0 1). In fact, there are many stoichiometric compounds for the AM–Bi systems such as A3 Bi, A3 Bi2 , A5 Bi4 and ABi (A ¼ K, Rb, Cs) [21] (no intermetallic phase has been found for the Mg–K(Cs) system). Many of such stoichiometric compounds have higher melting points than pure Bi or AM, implying the formation of stronger Bi–AM bonds than Bi–Bi and AM–AM pffiffiffi bonds. In the present LEED analysis of the 5-Bi,K(Cs), the bonding distances of K–Bi and Cs–Bi were determined to be , respectively. These values are near 4.10 and 4.16 A obtained by the summation of to 3.922 and 4.305 A
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bulk radii of Bi and K, and Bi and Cs, respectively, suggesting that K(Cs) and Bi atoms are located almost within the effective bonding distances. (The is slightly larger than that K–Bi distance of 4.10 A of 3.922 A, so the Bi and K atoms may not be within effective bonding distance. Later, we understand that the K atoms are actually not located at the exact on-top sites, but displace laterally.) On the other hand, in the geometry of model 1, the Bi– K and Bi–Cs distances would be about 4.64 and , respectively, being estimated from the 4.78 A pffiffiffi structural parameters of the 5-Mg,K(Cs) [10]. These distances are too long to form chemical bonds, and hence model 1 is not realized. Both Bi and K(Cs) atoms change the adsorption sites in the coadsorption from those in the individual adsorption: Bi atoms change the sites from the substitutional to hollow sites; K(Cs) atoms change from the hollow sites to the on-top sites. According to pffiffiffithe bonding distances mentioned above, the 5-Bi,K(Cs) is stabilized by the direct, attractive interaction between the coadsorbates at the cost of occupation of unfavorable sites, which causes the site-switching of both Bi and K(Cs). It is noted that a c(2 2) structure formed on Cu(0 0 1) by the coadsorption of Mg and Bi also exhibits the presence of the direct, attractive interaction between the coadsorbates [12]. The on-top site adsorption of AM atoms on fcc(0 0 1) metals is found for the first time. So, it is necessary to perform a more detailed examination of the coordination geometry. The above tensor LEED analysis was carried out under the p4 symmetry, in which K(Cs) atoms had to be set upright at the on-top sites (cf. Fig. 3(a)). Here, we allowed relaxations of K(Cs), Bi and Cu atoms in the first two layers under the p1 symmetry. An obvious improvement of RP was obtained in the p ffiffiffi 5-Bi,K: RP ðp1Þ ¼ 0:151 under the p1 symmetry vs. RP ðp4Þ ¼ 0:190 under the p4 symmetry. The I–V curves under the p1 symmetry are shown in Fig. 4 by dotted lines. On the other hand,pno ffiffiffi significant improvement was obtained for the 5-Bi,Cs: RP ðp1Þ ¼ 0:178 vs. RP ðp4Þ ¼ 0:187. A Hamilton ratio, which is known in X-ray crystallography as the Hamilton-ratio test [22], was introduced to LEED to deal with the variable numbers of fit parameters [23]. The
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Hamilton-ratio test was used to check whether it is really improved or not by using the p1 symmetry. The Hamilton ratio, Hr , in this study is, 2
Hr ¼
RP ðp4Þ RP ðp1Þ RP ðp1Þ2
2
DE=2jVOi j p ; pq
where q and p are the corresponding number of fitting parameters (in the present study, q ¼ 10 and p ¼ 36 for the p4 and p1 symmetry, respectively). Accordingly, the Hamilton ratio should exceed 3 to indicate a real improvement [23]. pffiffiffi The calculated Hamilton ratio is 3.1 for the 5Bi,K in the p1 vs. p4 symmetry, suggesting that the p4 symmetry is too ideal as the structure. Actually, K atoms exhibit a lateral displacement of about 0.3 from the exact on-top position under the p1 A symmetry, accompanied by slight, lateral displacements of Bi and substrate Cu atoms.pSuch ffiffiffi pbehavior ffiffiffi was also observed in the Al(1 1 1)-( 3 3)R30°K structure formed at low temperature [24], in which an obvious improvement of R-factor was achieved by displacing the position of a K atom . In contrast, from the exact on-top position by 0.3 A apsmaller Hamilton ratio of 0.55 is obtained for the ffiffiffi 5-Bi,Cs in the p1 vs. p4 symmetry, and we suggest that Cs atoms are upright at the on-top sites. That K atoms are not located at the exact ontop sites is expected, because the K–Bi distance obtained under the p4 symmetry is larger than the summation of radii of K and Bi atoms as mentioned above. Actually, the K–Bi bond length is in b-K3 Bi structure [25] and 3.92 A in Bi2 K 3.81 A structure [26,28]. On the other hand, the atomic size of Cs is appropriate to form effective bonds with the four surrounding Bi atoms. The Cs–Bi in both Cs3 Bi [28] and Bi2 Cs bond length is 4.03 A pffiffiffi structures [26,27]. Thus, it is clear that in the 5Bi,K(Cs), the coordination geometry shown in Fig. 3(a) is suitable for Cs atoms but slightly inappropriate for K atoms in terms of the bonding distances. pffiffiffi pffiffiffi In summary, the ( 5 5)R26.7° structures formed on Cu(0 0 1) at RT by the coadsorption of Bi and K(Cs) have been determined by the tensor LEED analysis. These are ordered mixed phases, in which Bi atoms are located at surface fourfold hollow sites and K(Cs) atoms occupy on-top sites.
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Both adsorption sites are different from those in the individual adsorptions. The stabilization of K or Cs atoms at on-top sites is caused by additional bondings with four surrounding Bi atoms. The Hamilton test suggests that the K atoms have a considerable lateral displacement from the exact on-top position, but Cs atoms are upright at the on-top sites. References [1] [2] [3] [4] [5]
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