CHEMICALLY DRIVEN STRUCTURAL-CHANGE IN QUASI-CRYSTALLINE AL-PD-MN ALLOYS

Quasicrystals, including face-centered icosahedral (FCI), simple icosahedral (SI) and decagonal (D) phases with periodicities of 1.6 nm and 1.2 nm along periodic axis, are found to form over a wide composition range in the Al-Pd-Mn system. A very strong composition dependence of the chemical order is seen in icosahedral Al90-xPdxMn10 alloys. A discontinuous change in the lattice parameter, a(R), with the Pd content of the i-phases suggests that the replacement of Al by Pd is selective. The predominant occupation site for the Pd atoms in icosahedral structure varies between the FCI and SI lattices. It is proposed that the FCI phase can be constructed as a two-cell structure, originating from the Al-Pd and Al-Mn binary systems.