SYMMETRY RELATIONSHIPS IN THE HOMEOTYPICAL ALUMINUM- RICH STRUCTURES WITH PENTAGONAL CHANNELS.

M. Ellner, U. Burkhardt and Yu. Grin(*), Max-Planck- Institut fur Metallforschung and Max-Planck-Institut fur Festkörperforschung (*), Stuttart, Germany

A significant feature of some homeotypical aluminum-rich structures with transition metals (TM) is the occurrence of finite or infinite pentagonal channels, which is dependent on chemical composition i.e. the stoichiometry of these intermetallic compounds is controlled by both s-, p-electrons of aluminum and d-electrons of TM. A phase bundle occurrs in the aluminum-rich portion of the binary system Co-Al: Co₂Al₅ (hP28, P6₃/mmc), CoAl₃, Co₄Al₁₃(h), m-Co₄Al₁₃ (mC102-7.2, Cm) and o-Co₄Al₁₃ (oP102, Pmn2₁). The homeotypical structures Co₂Al₅, m-Co₄Al₁₃, and o-Co₄Al₁₃ show different arrangement of pentagonal channels.

Moreover, the structures o-Co₄Al₁₃ and m-Co₄Al₁₃ are formed by two flat (AA') and two puckered (BB') layers of atoms in the sequence ABA'B'. These structure types can be described in terms of pentagonal and rhombic clusters with dimensions d=4.7 Å and h=8.1 Å. The edges of the pentagonal clusters are formed by TM atoms. Two TM atoms are situated in the middle of the prisms at y = 0.25 and y = 0.75. Each of them is surrounded by five aluminum atoms in form of puckered pentagons. The basal faces of the prisms are alternate occupied by one aluminum atom or five aluminum atoms forming distorted pentagon.

A regular cyclical tiling of pentagonal clusters yields 2D motives either with pentagonal twins showing the point-group symmetry C₅ or with decagonal twins showing the point-group symmetry C₁₀. The unit cell of one segment of these motives is conform with the unit cell of the m-Co₄Al₁₃ structure, while the twin-interface arrangement corresponds to a partial unit cell of o-Co₄Al₁₃.

Pentagonal and rhombic clusters probably dominate the structure CoAl₃. The structure proposal for CoAl₃ agrees well with the experimental powder diffraction data.