The Al-Pd-Mn quasicrystalline approximant ξ′-phase revisited

A detailed investigation of the Fourier space of several Al-Pd-Mn samples with composition Al-72.6 at. %, Pd-22.9 at. %, Mn-4.5 at. % is reported. In the phase diagram of the Al-Pd-Mn ternary alloy, this composition corresponds to the so-called xi phase which was described as an icosahedral quasicrystalline approximant. By re-examining the Fourier space by means of X-ray diffraction (powder patterns and single crystal precession patterns), complex structures in close relation with the xi-phase have been observed. These long-range order complex structures are described as resulting from a periodic perturbation of the xi structure along the c direction. Two states with periodicities c(xi) (3 + tau) and c(xi) (5 + tau) have been observed in this study (tau golden mean). Structural models based on periodic arrangements of "defects" layers separating layers of xi phase are proposed. These two states are certainly intermediate states between the xi phase and the metastable decagonal quasicrystalline phase.