Paper AS+BI-TuA9
Structural Investigation of the (001) Surface of Al₅Co₂

Tuesday, October 29, 2013, 4:40 pm, Room 204

Complex metallic alloys (CMAs) like quasicrystals and approximants are being considered as low-cost alternative materials for heterogeneous catalysis [1,2]. It relies on the so-called site-isolation concept in which the catalytic performance of a material is ascribed to small and well-separated atomic ensembles containing an active transition metal (TM) element at the crystal surface. Such atomic ensembles must be stable under reaction conditions, which in turn depend on the chemical bonding and the crystal structure of the intermetallic compound. Al₁₃TM₄ compounds have been identified as promising candidates for the heterogeneous hydrogenation catalysis [2, 3]. Here we focus on a related CMA system, the Al₅Co₂ crystal, which is also considered as a quasicrystalline approximant.

As a first step towards the understanding of the catalytic properties of this new phase, an atomic scale description of its surface is mandatory. First, a single crystal has been grown using the Czochralski method and oriented perpendicular to its [001] axe. The surface structure investigated under ultrahigh vacuum conditions by low energy electron diffraction and scanning tunnelling microscopy (STM) exhibits a √3x√3R30° reconstruction. According to the bulk model, two types of atomic layers are stacked along the [001] direction: either pure Al puckered (P) layers or flat (F) layers containing both Al and Co atoms. The step height measured by STM indicates that only one type of plane appears as surface termination. Atomically resolved STM images show small triangular atomic ensembles separated by 13 Å from each other and consisting of 3 bright protrusions. Such local configurations can only be interpreted as a reconstructed P layer where a fraction of Al atoms are missing. First-principles calculations using density functional theory (DFT) confirm that P layers are preferred terminations compared to F layers. Calculated surface energies for various surface models, along with the corresponding simulated STM images, show that the √3x√3R30° reconstruction is due to a specific set of missing Al surface atoms. Finally, a very nice agreement is obtained between simulated and experimental STM images, thus confirming the surface model. Other surfaces of interest for catalysis are currently being studied, namely the (100) and the (2-10) surfaces, together with their chemical reactivity towards small molecules of interest (O₂, CO, acetylene,...).

[1] T. Tanabe, S. Kameoka, A. P. Tsai, Appl. Catal., A 384, 241 (2010).

[2] M. Armbrüster, K. Kovnir, M. Friedrich, et al. Nat. Mater. 11, 690 (2012).

[3] J. Ledieu, É. Gaudry, L. N. Serkovic Loli, et al. Phys. Rev. Lett. 110, 076102 (2013).