LEED structural analysis of Al(111)-K-(√3 × √3 )R30°: Identification of stable and metastable adsorption sites

Abstract

A detailed low energy electron diffraction analysis has been performed to investigate the adsorption of potassium on Al(111) at 90 K and at 300 K. At each temperature a (v 3 x~3)R30' structure is formed, however, the adsorbate positions are different. For adsorption at 90 K the adatoms occupy on-top sites and at 300 K they occupy substitutional sites, both geometries have hitherto been regarded as very unusual. An irreversible phase transition from the former to the latter stable structure occurs on warming to 300 K. The obtained interatomic distances and surface relaxations are discussed in terms of an interplay of the electrostatic repulsion between adatoms and substrate screening as well as coordination number. An approximate scheme for including the effect of anisotropic atomic motions under the in8uence of thermal excitation was applied. It is found that the R factors for the on-top site are signi6cantly improved whereas no improvement is found for the substitutional site.

Theory of Adsorption on Metal Substrates

“…Atoms, which are too big to fit into a bulk vacancy, can still prefer to take a substitutional site, because at the surface bigger atoms can simply sit somewhat above the center of the created surface vacancy. Since 1991 many examples of substitutional adsorption have been reported, as for example K on Al(111) Scheffler, 1992, 1993;Stampfl et al, 1992Stampfl et al, , 1994a, Na on Al(001) (Stampfl et al 1994b), Au on Ni(110) (Pleth Nielsen, 1993), Sb on Ag(111) (Oppo et al, 1993), Co on Cu(111) (Pedersen et al, 1997), Mn on Cu(001) (Rader et al, 1997), Co on Cu(001) (Nouvertné et al, 1999), to name a few. Thus, the phenomenon is not at all exotic, but rather general.…”

Converged properties of clean metal surfaces by all-electron first-principles calculations

“…[7,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53], where it has been found that the majority of transition metal surfaces show a contraction of the first interlayer spacing, i.e., the spacing between the two outermost layers is smaller than in the bulk terminated (unrelaxed) surface. An expansion of the outermost interlayer spacing has been reported, e.g., for Mg(0 0 0 1) [29,33,37,44,45], Al(1 1 1) [11,22,24,28,31,46], and Pt(1 1 1) [21,22,47,50].…”

“…The atomic structure can be obtained with high accuracy using quantitative low-energy electron diffraction (LEED) intensity analysis [20][21][22][23][24][25][26][27][28][29][30][31][32][33]. And theoretically, first-principles calculations also give an accurate description, see for example, Refs.…”

Al(111)-(3×3)R30:On-top versus substitutional adsorption for Rb and K

“…1-3,6,9,14 -16 For example, these calculations clearly showed that 1/3 monolayer of Na on Al͑111͒ prefers the substitutional site: At zero temperature the energy difference between substitutional and on-surface adsorption is 300 meV. For K the calculated adsorption energies for the substitutional and onsurface phase are almost degenerate, 2,6 indicating that the chemical nature of the adsorbate strongly affects the bonding. For Rb on Al͑111͒ first-principles calculations have not been reported so far.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12] Among these structures the ordered Al͑111͒-(ͱ3ϫͱ3)R30:X surface ͑with 1/3 monolayer of X ϭNa, K, Rb, Cs͒ probably has one of the simplest structures. Nevertheless, the system exhibits interesting structural phase transitions.…”