The T-Al3 (Mn, Pd) quasicrystalline approximant: Chemical order and phason defects

The structure of an orthorhombic phase called T-Al 3 Mn, Pd, with the composition Al 72· Pd 3·2 Mn 24·5 , has been determined by single-crystal X-ray diffraction. The space group is Pnma and the magnitudes of the cell parameters are a = 14.717 A, b = 12.510 Å and c = 12.594 A. The final reliability factor is R = 0.022 and for 2734 independent reflections. The structure exhibits a strong chemical ordering and is compared to the isomorphic phase T-Al 3 Mn and to the closely related R-Al 60 Mn 11 Ni 4 phase. The possibility of a phase transformation occurring through the motion of phason defects, between the T-Al 3 (Mn, Pd) orthorhombic phase and another orthorhombic phase, isomorphic to the R-Al 60 Mn 11 Ni 4 phase, is briefly discussed.