Surfaces of complex approximant phases.

V. Fournée ¹, A.R. Ross ^2,3, T.A. Lograsso ^2,3, J.A. Barrow ^2,5, M. Shimoda ⁴, A.P. Tsai ⁴

and P.A. Thiel ^2,5.

¹ LSG2M, CNRS-UMR7584, Ecole des Mines, Parc de Saurupt, 54042 Nancy, France.

² Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA.

³ Department of Materials Science and Engineering, Ames, Iowa 50011, USA.

⁴ National Institute for Material Science, 1-2-1 Sengen, Tsukuba-shi, Ibaraki 305-0047 Japan.

⁵ Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA.

In a first part, we will present experimental results on the surface structure and stability of approximant phases. Surface of approximant can be produced "accidentally". This sometime occurs on the 5-fold surface of the Al-Pd-Mn icosahedral quasicrystal (IQC) when the chemical composition is driven out of the stability range of the icosahedral phase during the sputtering and annealing. Structural identification of such crystalline overlayers based on the analysis of STM images will be presented.

In rare cases, it is also possible to grow single crystals of approximant large enough for surface science studies. This is the case for the xÕ-AlPdMn approximant. Two single crystals of this phase were grown : one by the Bridgman method (BG sample), the other by a self-flux technique (FG sample). The surface structure perpendicularly to the pseudo ten-fold axis is investigated by STM and LEED and compared to the bulk structural model. For both samples, the LEED pattern shows the expected pseudo ten-fold symmetry, but the local probe of the STM reveals different morphologies. In particular, high-resolution images reveal motifs with 5-fold or 10-fold symmetry protruding atop the terraces. They are identified as part of 3D cluster units from which the xÕ phase can be described. These cluster sites are preferred over the glue sites during the re-growth of the surface upon sputtering-annealing. This is in contrast with the 5-fold surface of the Al-Pd-Mn IQC where the surface re-growth encompasses not only the cluster sites but also the "glue sites," forming together a relatively dense and flat top layer. This observation raises question about the relative stability of clusters and their influence on surface stability.

In a second part, we will describe the growth and properties of a Pd film deposited on the xÕ approximant surface. This topic is related to the catalytic properties of quasicrystal. Deposited Pd atoms form a rough film with nanosized particles rather than a smooth film. The density of Pd particles is high. They are rather stable and the rough aspect of the film persists even after a short annealing to 920 K. Spectroscopy data are consistent with a reduced density of states at E_F on the clean surface of the xÕ approximant. With increasing film thickness, the surface electronic structure changes from that of a poor metal (with a pseudogap at E_F) to that of a good metal like Pd, and this transition already occurs by 3.5 ML. It follows that the surface area of the active catalyst (Pd) is large, consistent with the high catalytic activity measured for AlPdMn IQC. It is also possible that the catalytic activity derives from a perturbation to the electronic structure, as we observe for the small Pd particles. Such an effect is known to occur for other catalytically-active particles.