THE T-AL-3(MN, PD) QUASI-CRYSTALLINE APPROXIMANT - CHEMICAL ORDER ANDPHASON DEFECTS

The structure of an orthorhombic phase called T-Al-3(Mn, Pd), with the composition Al72.3Pd3.2 Mn-24.5, has been determined by single-crysta l X-ray diffraction. The space group is Pnma and the magnitudes of the cell parameters are a = 14.717 Angstrom, b = 12.510 Angstrom and c = 12.594 Angstrom. The final reliability factor is R = 0.022 and for 273 4 independent reflections. The structure exhibits a strong chemical or dering and is compared to the isomorphic phase T-Al3Mn and to the clos ely related R-Al60Mn11Ni4 phase. The possibility of a phase transforma tion occurring through the motion of phason defects, between the T-Al- 3(Mn, Pd) orthorhombic phase and another orthorhombic phase, isomorphi c to the R-Al60Mn11Ni4 phase, is briefly discussed.