Energetics of $\mathrm{Sr}$ adatom interactions on the $\mathrm{Mo}\left(112\right)$ surface

First-principles methods are used to investigate the formation and structure of the ordered phases of $\mathrm{Sr}$ atoms adsorbed on the furrowed $\mathrm{Mo}\left(112\right)$ surface. The energetics of various commensurate and incommensurate adatom structures providing information on lateral interactions between adatoms is determined for coverages $0.11⩽\Theta ⩽1$ monolayer. It is found that the binding energy of $\mathrm{Sr}$ atoms decreases with increasing coverage. The experimentally observed $p(8\times 1)$ and $p(5\times 1)$ adatom chains are found to belong to the most favored structures for $\Theta <0.5$. The energetic difference between these two structures amounts to $20\mathrm{meV}$. The experimental work function variation with $\mathrm{Sr}$ adatom coverage is very well reproduced. The energy barriers for $\mathrm{Sr}$ diffusion along the atomic troughs are calculated and discussed.

Figure 1

(a) Top view of the $\mathrm{Mo}\left(112\right)$ surface showing the directions of the atomic furrows and two surface unit cells applied in the calculations. (b) Side view of the seven $\mathrm{Mo}$ layer slab. The numbers to the right show the calculated interplanar separations (in $\mathrm{\AA }$) of the topmost layers. The bottom number is for the bulk interplanar spacing.

Figure 2

Geometry of a single $\mathrm{Sr}$ adatom on the $\mathrm{Mo}\left(112\right)$ surface in the $8\times 1$ and $5\times 1$ unit cells (upper row), and two and four $\mathrm{Sr}$ adatoms in the $4\times 2$ cell. Atoms are adsorbed at the bulk sites in atomic furrows. Adatoms belonging to the same furrow are denoted either by upper or lower case letters. The labeling corresponds to that adopted in Table . The binding energy of the $\mathrm{Sr}$ adatom corresponding to a given configuration is printed below.

Figure 3

(Color online) Calculated work function changes for the $\mathrm{Sr}∕\mathrm{Mo}\left(112\right)$ adsorption compared with the measured data (asterisks) of Ref. 5 For each coverage only the work function values of the most stable chain structures (calculated in different surface unit cells of $n\times 1$ type) are plotted.

Figure 4

Binding energy differences (compare Table ) vs coverage, showing the surface phase diagram of stable (gray) and unstable chain structures for low $\mathrm{Sr}$ coverages.

Figure 5

The $\mathrm{Sr}\text{−}\mathrm{Mo}$ bond lengths (in $\mathrm{\AA }$) for the two $\mathrm{Sr}$ chain structures on $\mathrm{Mo}\left(112\right)$. The larger, black circle represents the position of a $\mathrm{Sr}$ atom and the darker circle in the middle a $\mathrm{Mo}$ atom of the second atomic layer. The measured values (Ref. 5) are given in braces.

Figure 6

Energy barrier for $\mathrm{Sr}$ atom diffusion between two volume (vol) sites along the furrow. The other labels denote hollow (hol), atop of $\mathrm{Mo}$ atom (top), and bridge (bri) sites. The distance is measured in units of surface lattice parameter $a=2.73\mathrm{\AA }$.