The Crystal Structure of the Laves Phase in Mg-Zn-Ag-System. I

Abstract

A semi-binary Laves phase compound 92.5 mol% MgZn₂-7.5 mol% MgAg₂ was examined by the X-ray method. A structural principle of the Laves phases is at first discussed generally on the basis of layer stacking. Although there are six fundamental layers of A, A′, B, B′, C and C′ in the Laves phases, layer sequences can be deduced from the same principle derived by Patterson and Kasper (International Table for X-ray Crystallography, Vol. II) as in the case of the closest packing of spheres.
Using this principle the crystal structure of the compound is determined. It is hexagonal and belongs to the P6₃/mmc space group, with 20 formula units in the cell of the dimensions with a=5.22 and c=42.95Å. The structure is expressed by a 10-layer sequence such as ABC′ BCA′ C′ BC′ B′ among 16 possible sequences. Atomic parameters are given on the basis of the above sequence. Relation between structures previously found and a new one is also discussed.