5D SUPERSTRUCTURE MODEL OF DECAGONAL Al-Ni-Co QUASICRYSTALS. Akiji Yamamoto, Steffen Weber*, National, Inst. Res. Inorg. Mat. Namiki 1, Tsukuba, 305 Japan, *Institute of Applied Physics, University of Tsukuba, Tsukuba, 305, Japan, An Pang Tsai, Institute for Materials Research, Tohoku University, Sendai, 980 Japan

A superstructure has been found in decagonal Al-Ni-Co quasicrystals (Edagawa, Ichihara, Suzuki and Takeuchi, Phil. Mag. Lett, 66 (1992) 707). This has a unit cell volume 5 times larger than that of the normal phase. We proposed a model for this superstructure (Hiraga, Sun and Yamamoto, Mat. Trans. JIM, 35 (1994) 657) based on HRTEM images, where the clusters are arranged at vertices of a Penrose pattern with edges of about 20 A (3D model). In this paper, we construct the corresponding 5D model of such a superstructure by the section method. In cluster models, we consider small occupation domains which generate the cluster centers. In the present case, these are 4 pentagons situated at (i,i,i,i,z)/5 (i=1,2,3,4) in the 5D decagonal lattice. Atoms around the centers can be obtained by shifting these domains along the external space. It can be shown that all atom positions in the 3D model can be obtained from the occupation domains around 20 points in the 5D unit cell of the superstructure. Its space group is P10₅/mcm. Their site symmetry is 2/m. This means that two of them are independent. Each independent occupation domain consists of several small ones as mentioned above and has an irregular shape with the symmetry of 2/m. Thus the 5D model is quite different from 5D structures of known decagonal quasicrystals. It is however remarkable that the number of independent sites is two and many other positions are related to them by symmetry operations. This model gives reasonable point density and chemical composition.