Figure 1

(Color online) (a) LEED image of the clean Pd(110) surface. (b) HRCL spectrum from the $\text{Pd}3d_{5/2}$ level. The topmost atomic layer exhibits a SCLS of $-0.48\text{eV}$.Reuse & Permissions

Figure 2

(Color online) Experimental SXRD structure factors $F$ (data points) as a function of the reciprocal lattice coordinate $L$ perpendicular to the surface ($L=1$ corresponds to $2\pi /a_3$). Downward triangles: $\left(-1,1\right)$ rod, circles: $\left(-2,0\right)$ rod, squares: $\left(-1,0\right)$ rod, and upward triangles: (0,1) rod. The solid lines represent the best fit to the data. Inset: side view of the surface along ${\mathbf{a}}_2$ (along the [001] direction) and definition of the relaxations. The fit result of the DW factors is included. The relaxations are discussed in the text.Reuse & Permissions

Figure 3

In situ LEED experiment at constant temperature $\left(300°\text{C}\right)$. (a) Clean surface. (b) At an oxygen pressure of ${10}^{-8}\text{mbar}$, a $\left(1\times 3\right)$ structure is formed. (c) After seven seconds the fractional order diffraction spots starts to move, (d) resulting in a $\left(1\times 2\right)$ structure, and (e) finally a $c\left(2\times 4\right)$.Reuse & Permissions

Figure 4

(Color online) (a) LEED pattern from the $c\left(2\times 4\right)$ structure. (b) SXRD scan along the broken line in (a) ($H=0$ and $L=0.03$) Inset: the clean surface. (c) Atomic model of the $c\left(2\times 4\right)$ structure as calculated by DFT. Large and small spheres corresponds to Pd and O atoms, respectively. (d) Top: experimental STM image ($U=0.1\text{V}$ and $I=1.5\text{nA}$) Bottom: simulated STM image of the $c\left(2\times 4\right)$ structure. (e) HRCLS from a preparation resulting in a $c\left(2\times 4\right)$ LEED pattern.Reuse & Permissions

Figure 5

(Color online) (a) Oxygen-induced $c\left(2\times 4\right)$ reconstruction projected along ${\mathbf{a}}_1$ (along the $\left[1\overline{1}0\right]$ direction). Small spheres represent oxygen atoms, large spheres Pd atoms. The relaxation parameters are indicated and reported in Table . (b) Structure factors of the $c\left(2\times 4\right)$ reconstructed surface (data points) and best fit (solid lines). Upper panel: CTRs (downward triangles: $\left(1,-1\right)$ rod, squares: (0,1) rod, and upward triangles: (1,0) rod). Lower panel: surface rods: (open circles: $\left(0,-3/2\right)$ rod, filled squares: (0,1/2) rod, open squares: $\left(1,-1/2\right)$ rod, and filled circles: $\left(1,-3/2\right)$ rod.)Reuse & Permissions

Figure 6

(Color online) (a) STM image from a preparation yielding a $c\left(2\times 4\right)$ LEED pattern. The surface exhibits bright stripes along the $\left[1\overline{1}0\right]$ direction which are separated by approximately 5–25 nm in the [001] direction ($U=0.1\text{V}$ and $I=1.5\text{nA}$). (b) Zoom in on one of the bright stripes in (a) ($U=0.1\text{V}$ and $I=1.5\text{nA}$). (c) By adding lines with a $\left(1\times 2\right)$ periodicity, one can see that the double rows constitutes antiphase domain boundaries. (d) Model of the $c\left(2\times 4\right)$ structure structure. (e) Model of a situation where one row has been added. The oxygen atoms are assumed to occupy the same sites on the antiphase domain boundary as in the $c\left(2\times 4\right)$, thereby creating a local $\left(2\times 1\right)-2\text{O}$ structure (yellow unit cell).Reuse & Permissions

Figure 7

(Color online) (a) LEED and (b) SXRD at $L=0.03$. Evolution of the (0,0.5) diffraction spot over a time period of four minutes at ${10}^{-6}\text{mbar}$ of oxygen at $300°\text{C}$.Reuse & Permissions

Figure 8

(Color online) [(a)–(b)] STM images recorded after a preparation yielding a LEED pattern with a split of the (0,0.5) spot corresponding to (2)–(3) in Fig. 7a [$U=-0.05\left(-0.1\right)\text{V}$ and $I=1.5\left(0.4\right)\text{nA}$], respectively. (c) The Pd(110) surface after oxygen exposure in the pressure range $5\times {10}^{-6}–{10}^{-5}\text{mbar}$ and a sample temperature of $300°\text{C}$ $\left(U=-0.3\text{V}I=0.1\text{nA}\right)$. [(d)–(e)] The complex structure ($U=0.01\text{V}$ and $I=0.5\text{nA}$).Reuse & Permissions

Figure 9

(Color online) (a) Model of the complex structure. (b) An oxygen coverage of 1ML could be obtained by forming a $\left(2\times 1\right)-2\text{O}$ structure. However, this structure induces a large compressive stress along the [001] direction. This stress is relieved by displacing pairs of Pd atoms at the end of chains containing 7 (or 9, not shown) atoms in the $\left[1\overline{1}0\right]$ direction.Reuse & Permissions

Figure 10

(Color online) SXRD scan along $K$ with $H=0$ and $L=0.03$, i.e., along the [001] direction. (b) HRCL spectrum taken after exposing the crystal to $5\times {10}^{-3}\text{mbar}$ of oxygen at $300°\text{C}$. (c) STM images taken after a similar preparation as in (b) ($I=0.05\text{nA}$ and $U=2\text{V}$).Reuse & Permissions

Figure 11

(Color online) Detected diffraction intensity in planes perpendicular (a), and parallel $\left(L=0.03\right)$ (b), to the Pd(110) surface. High and low intensity is indicated as red and blue, respectively. The $HL$ plane in (a) is cutting the $HK$ plane along the dotted line $\left(0.92,K\right)$ in (b). All the detected intensity at fractional values of $H$, $K$, and $L$, can be related to planes in the PdO bulk oxide. The reciprocal PdO lattice vectors, $b^{\ast }$ (parallel to PdO[010]), and $c^{\ast }$ (parallel to PdO[001]), are rotated $\pm 7.34°$ with respect to the surface normal and the $HK$ plane, respectively, see (a). The reciprocal PdO sublattice, here rotated $-7.34°$, is indicated in (a) and (b) with white lines. Here the Pd sublattice is described as body-centered tetragonal, resulting in strong Bragg reflections (indicated by white circles), only when the sum of the indices is even or odd. A possible orientation for the PdO growth, as determined from the diffraction data in (a) and (b), is shown in (c).Reuse & Permissions

Figure 12

(Color online) (a) SXRD scans along the negative $K$ direction (with $H=0$, i.e., perpendicular to the closed packed rows, at $L=0.03$. (b) LEED pattern corresponding to the clean surface, the $c\left(2\times 4\right)$, and the $\left(9\times \sqrt{3}\right)$ structure. The broken line in the LEED images corresponds (in plane) to the SXRD scans in (a).Reuse & Permissions

Figure 13

(Color online) (a) Experimental $\left(T,p\right)$ diagram obtained by performing the $K$ scan in Fig. 12a at different partial oxygen pressures and sample temperatures. The dark gray area (bottom) corresponds to the $c\left(2\times 4\right)$ structure, the white area to the $\left(7\times \sqrt{3}\right)$ and $\left(9\times \sqrt{3}\right)$ structures, and the light gray (top) is the PdO bulk oxide. The “split structure” (denoted by a star) refers to the situation in Fig. 8b, where diffraction intensity is detected at approximately $K=-1.4$ and $-1.6$. (b) Theoretical surface phase diagram, displaying the thermodynamically stable phases as calculated by DFT.Reuse & Permissions