AG-SUBSTITUTED AL-CU-FE ICOSAHEDRAL PHASES

The gradual substitution of Cu by Ag in the Al62Cu25.5-xAgxFe12.5 icos ahedral phase (x less than or equal to 10) is investigated by X-ray di ffraction from the viewpoint of the induced structural changes. The ph onon and random phason disorders, estimated from diffraction linewidth s, increase with x, the latter showing, however, an induction range of Ag concentrations. EXAFS results at the CuK and FeK edges show. for l ow Ag contents, and enhanced disorder in the first coordination sphere , partly relaxed for higher x values. The 6D hypercubic lattice parame ter is found to be independent of the Ag content, despite the large di ameter of monovalent Ag. The latter facts suggest a d-sp hybridization of the Ag bonds to the surrounding Al, taking place above a certain x value.