Structural Investigation of the (010) Surface of theAl13Fe4Catalyst

Abstract

We have investigated the structure of the Al(13)Fe(4)(010) surface using both experimental and ab initio computational methods. The results indicate that the topmost surface layers correspond to incomplete puckered (P) planes present in the bulk crystal structure. The main building block of the corrugated termination consists of two adjacent pentagons of Al atoms, each centered by a protruding Fe atom. These motifs are interconnected via additional Al atoms referred to as "glue" atoms which partially desorb above 873 K. The surface structure of lower atomic density compared to the bulk P plane is explained by a strong Fe-Al-Fe covalent polar interaction that preserves intact clusters at the surface. The proposed surface model with identified Fe-containing atomic ensembles could explain the Al(13)Fe(4) catalytic properties recently reported in line with the site-isolation concept [M. Armbrüster et al., Nat. Mater. 11, 690 (2012)].

Structure of the monoclinicAl13Fe4(010)complex metallic alloy surface determined by low-energy electron diffraction

“…The results indicate that the top layer of the Al 13 Fe 4 (010) surface is similar to a bulk puckered layer but lacks the Fe atoms below the mean plane position and the Al atoms that would surround them. This supports the previous STM and DFT analysis [8]. The incomplete top layer resides approximately 10% closer to the underlying flat layer, compared to a bulk structure, with an oscillating relaxation for the next few deeper layers.…”

“…This is the same model proposed by Ledieu Average interlayer spacing is denoted by dz (see Table II). et al [8]. Further optimization of this termination, relaxing up to four surface layers, produced an R factor of 0.29.…”

“…A combination of both layers on the surface is ruled out by the scanning tunneling microscopy (STM) measurements performed by Ledieu et al; the height of the step between the two consecutive terraces was found to be equal to half the lattice parameter (b/2) along the [010] direction [8]. For the structural determination, the flat and puckered bulk terminations were considered as trial structures, and incomplete variations of them were tested.…”

“…The temperature was measured using an infrared optical pyrometer with the emissivity set to 0.35. This preparation led to a clean and flat surface as measured by x-ray photoelectron spectroscopy and scanning tunneling microscopy (see Ledieu et al [8] for more preparation details). The low-energy electron diffraction (LEED) measurements were performed at 300 K using a three-grid retractable Omicron LEED apparatus equipped with an Ir/Th filament.…”

“…Only the z coordinates (perpendicular to the surface) were varied in the analysis. The analysis was also performed starting from the relaxed unit cell from the earlier density functional theory (DFT) analysis [8], but the difference between lattice parameters was only 0.43%, TABLE I. Initial R factors and the calculated Pendry variances for the trial structures.…”