Structure of the (010) surface of the orthorhombic complex metallic alloy $\mathrm{T}{\text{-Al}}_3$(Mn,Pd)

The atomic and electronic structures of the (010) surface of the $\mathrm{T}{\text{-Al}}_3$(Mn,Pd) complex metallic alloy is investigated by means of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), x-ray photoelectron diffraction (XPD), and ab initio calculations. While structural imperfections are observed at the surface and out of the various possible terminations, the puckered P2 layer is identified as the only surface termination, thus pointing out the existence of a well-defined minimum in the surface energy landscape. The measured step heights correspond to distances between identical planes along the [010] direction in the bulk model, i.e., $b/2$. A bias dependency of the STM topography is found. The XPD and LEED patterns confirm the pseudotenfold symmetry of the sample. XPS and UPS show a more metallic signature of the $\mathrm{T}$ phase compared to Al-based quasicrystalline phases.

Figure 1

(Color online) [(a)–(c)] The structure of the orthorhombic $\mathrm{T}{\text{-Al}}_3$(Mn,Pd) (Refs. 17, 25) is presented along the [010] direction. Blue (dark gray) spheres correspond to Al atoms, pink (light gray) spheres to Mn atoms, and others to mixed atomic sites. The orthorhombic unit cell is outlined by a rectangle. (a) Representation of the flat layer (f). Connecting the Mn atoms leads to a tiling composed of pentagons and rhombi. (b) The puckered layer (P1) is composed only of Al atoms arranged in almost perfect pentagonal chains. (c) The second puckered layer (P2) is composed of Al, Pd, and Mn atoms and several positions have a mixed occupancy. The blue (black) and white spheres correspond to mixed Mn-Al (0.5/0.5) sites [referred as TM(1) and TM(2) sites by Klein et al. (Ref. 17)], the red (gray) and white spheres correspond to mixed Mn-Al (0.3/0.7) sites [TM(3) and TM(4)], the shaded spheres correspond to mixed Mn-Al (0.15/0.85) sites [TM(5)], and the empty circles correspond to mixed Pd-Al (0.625/0.375) sites [TM(6)]. Mn atoms are organized as a zigzag line while other sites form pentagonal chains. Several H tiles are also drawn. (d) Schematic view along the [010] direction of one column of interpenetrating icosahedral clusters centered at point $x$ marked in (a)–(c). (e) Representation of the stacking of layers perpendicular to the $\mathbf{b}$ axis in the unit cell of the model structure.

Figure 2

[(a) and (b)] LEED patterns (inverted for clarity) recorded on the clean surface at 25 eV and 55.9 eV, respectively. The pseudotenfold symmetry is made evident on both patterns by the presence of decagonal rings consisting of ten darker diffraction spots.

Figure 3

(Color online) (a) STM image $\left(250\times 250{\text{nm}}^2\right)$ showing the step and terrace morphology. (b) STM image $\left(40\times 40{\text{nm}}^2\right)$ showing a narrow terrace on the step edge. (c) The height histogram calculated from a $20\times 20{\text{nm}}^2$ STM image exhibits the three possible step heights observed across terraces. (d) Fast Fourier transform calculated from a $50\times 50{\text{nm}}^2$ high-resolution STM image.

Figure 4

(a) $70\times 35{\text{nm}}^2$ STM image recorded on a terrace for $V_b=2\text{V}$ and $I_t=0.06\text{nA}$. The white frame outlines a patch of homogeneous contrast. [(b)–(e)] STM images $\left(5.1\times 5.1{\text{nm}}^2\right)$ acquired on the same region using a constant current of 40 pA and a bias of (b) 0.8 V, (c) $-0.8\text{V}$, (d) 0.4 V, and (e) $-0.4\text{V}$. The arrows point to a common defect visible on all images. As shown by the circles located at the same position on all images, the contrast varies drastically with the applied bias.

Figure 5

[(a) and (c)] Atomically resolved STM image $\left(4\times 4{\text{nm}}^2\right)$ recorded from the clean surface at $V_{bias}=-0.4\text{V}$ and $I_t=0.36\text{nA}$. This image has been filtered to emphasize the structure. [(b) and (d)] Atomically resolved STM image $\left(4\times 4{\text{nm}}^2\right)$ recorded on a different region of the same sample at $V_{bias}=0.6\text{V}$ and $I_t=0.50\text{nA}$. This image has been rotated by $\pi$ for comparison purposes. [(c) and (d)] A tiling corresponding to the P2 layer has been superimposed on both images. The pentagonal chains arranged in a zigzag manner and individual Mn atoms are clearly discernable. While positioned identically with respect to the tiling, the contrast inside the largest pentagons varies drastically within the same image (c) as a consequence of compositional disorder, and between images (c) and (d) with the reversed sign of the bias voltage.

Figure 6

Determination of the position of the cleavage plane from an ab initio simulation. Here, the mixed Pd-Al sites are replaced by Pd atoms (gray) while the mixed Mn-Al sites are all replaced by Al atoms (white circles). Mn atoms appear as black spheres. The unit cell of the bulk structure (a) is expanded in the [010] direction (vertical direction) by 50% (b). After the structural relaxation, the bulk structure has cleaved between the P2 and $\text{P}{2}^{\ast }$ planes (d). As the unit cell consists of two symmetry-equivalent blocks of atomic planes, two vacuum layers between the blocks are formed. Panel (c) shows an intermediate state when the structure begins to cleave.

Figure 7

(Color online) The valence charge-density distribution at the surface P2 plane in the area $25.19\times 29.43{\text{Å}}^2$ corresponding to $2\times 2\text{unit}$ cells. The global structure is represented by a tiling of squashed hexagons H (black lines). The pentagons of Al atoms are arranged in chains (white pentagons). Between two neighboring Mn atoms (marked by white circles) decorating the vertices of the H tiles there is a large charge-density depression. Its position is approximately marked by a big (red, gray) pentagon.

Figure 8

(Color online) Simulated STM images of the surface P2 plane. Images of the surfaces (a) and (b) are simulated for the bias voltages $-0.4\text{V}$ and $+0.4\text{V}$, respectively. The area corresponds to $2\times 2\text{unit}$ cells. The chains of the small pentagons (black), big pentagons (red, gray), circles (black), and the tiling of squashed hexagons represent the structural features of the surface described in Fig. 7. The bias dependency of the images observed experimentally is also seen on the simulated images.

Figure 9

(Color online) Experimental XPD patterns of (a) $\text{Al}2s$, (c) $\text{Pd}3d_{5/2}$, and (e) $\text{Mn}2p_{3/2}$ core levels measured on the $\mathrm{T}{\text{-Al}}_3$(Mn,Pd) (010) surface with $\text{Al}K\alpha$ (1486.7 eV) x-ray source. Single scattering cluster simulations for (b) $\text{Al}2s\left(E_{Kin}=1369\text{eV}\right)$, (d) $\text{Pd}3d_{5/2}\left(E_{Kin}=1152\text{eV}\right)$, and (f) $\text{Mn}2p_{3/2}\left(E_{Kin}=848\text{eV}\right)$ emissions based on a cluster derived from the bulk model (Ref. 17) described in Fig. 1.

Figure 10

Near $E_F$ UPS valence-band spectra of $\mathrm{T}{\text{-Al}}_3$(Mn,Pd) (solid line) and $i\text{-Al-Pd-Mn}$ (dashed line). The position of $E_F$ is indicated by a dashed line. Spectra are normalized to have the same height at 2 eV binding energy. Inset: valence-band spectrum of the $\mathrm{T}{\text{-Al}}_3$(Mn,Pd) up to 8 eV. The main peak at 4.5 eV corresponds to $\text{Pd}4d$ states and the vertical line is located at 2 eV.

Figure 11

Calculated UPS VB (He I) spectrum for the bulk of $\mathrm{T}{\text{-Al}}_3$(Mn,Pd) (full line) and $i\text{-Al-Pd-Mn}$ (dashed line) samples. The spectra are normalized to have the same height at the energy of $-2\text{eV}$ indicated by a black dot.

Figure 12

$\text{Mn}2p_{3/2}$ core-level spectra (open circles) for (a) $\mathrm{T}{\text{-Al}}_3$(Mn,Pd) and (b) $i\text{-Al-Pd-Mn}$. The fitted spectra (thin solid line), the spectra deconvoluted from experimental broadening (short dashed line), the iterative background (long dashed line) and the residual (dots) are also shown.