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

A semi-binary Laves phase compound 92.5 mol% MgZn 2 -7.5 mol% MgAg 2 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 P 6 3 / mmc space group, with 20 formula units in the cell of the dimensions with a =5.22 and c =42.95A. The structure is expressed by a 10-layer sequence such as A B C ' B C A ' C ' B C ' 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.