SUPERLATTICE ORDERING IN THE LOW-TEMPERATURE ICOSAHEDRAL PHASE OF AL-PD-MN

The low-temperature icosahedral phase, which forms below approximately 700-degrees-C near the alloy composition Al71Pd21Mn8 has been investigated by means of electron diffraction. Precise analysis of diffraction patterns indicates that the Bravais lattice of the low-temperature phase is P type, different from the formerly proposed F type. The six-dimensional lattice parameter of this new P-type lattice is determined to be a(P) = 20.881 angstrom by powder X-ray diffraction and is related to the high-temperature F-type iscosahedral phase by a(P) = taua(F), where tau and a(F) denote the golden mean and the six-dimensional lattice parameter respectively of the high-temperature phase. The low-temperature transformation can be understood by a small breaking of F symmetry caused by superlattice ordering. A model of the required configuration of atomic surfaces (acceptance regions) is discussed by using the six-dimensional 'diamond' lattice. This diamond lattice has a lattice parameter a(F2) = 2a(F) and is related to the above P-lattice by a scaling transformation.