HIGH-RESOLUTION ELECTRON-MICROSCOPY OF THE EPSILON-AL3CO, A MONOCLINIC APPROXIMANT OF THE AL-CO DECAGONAL QUASI-CRYSTAL

High-resolution electron microscopic images of a monoclinic epsilon-Al 3Co phase (space,group P2/m), which was referred to as the primitive t au(2)-Al13Co4 phase in the early publications, are presented. A struct ure image along the b axis shows a periodic tessellation of flattened hexagonal subunits with polarity. The subunit is composed of two joint decagonal motifs, with fivefold cores in the interior, partial superp osed pentagons on the edge and pentagons at the vertices. A structural model of the epsilon-Al3Co phase is proposed and the calculated image s agree well with the image contrasts of the observed ones. The potent ial of deducing the structure of the AI-Co decagonal quasicrystal from the present model is discussed. (C) 1998 Elsevier Science S.A.